U.S. patent number RE33,152 [Application Number 07/279,453] was granted by the patent office on 1990-01-23 for radar detection of hazardous small scale weather disturbances.
Invention is credited to David Atlas.
United States Patent |
RE33,152 |
Atlas |
January 23, 1990 |
Radar detection of hazardous small scale weather disturbances
Abstract
The detection and warning of microbursts, low level wind shear,
and other weather disturbances, which are hazardous to aircraft
operations and to the public at large, are accomplished with either
an airport surveillance radar (ASR) or a multi-beam Doppler radar.
ASR Doppler systems normally operate to receive one of two
relatively large vertical fan beams having different elevation
angles but which overlap one another so that they have equal gains
at an elevation angle, called the null, at a relatively low angle,
for example 5.degree.. Below this null, the low beam antenna gain
exceeds that of the high beam, and conversely above it.
Accordingly, by subtracting the high beam Doppler spectrum from
that on the low beam, a Difference Doppler Spectrum (DDS) is
produced which is positive below the null and negative above. The
velocity bounds of the positive portion of the DDS provide the wind
speed components at the null and at heights near the surface. These
wind speed components are then utilized to measure and map radial
and horizontal shear, the boundaries of the disturbance and other
signatures such as vertical shear and turbulence and the rate of
change of all the parameters, thereby permitting the detection of
the location and track of the disturbance. A multi-beam Doppler
radar can be utilized to perform similar functions of measuring the
mean Doppler velocity, Doppler spectral breadth, and reflectivity
simultaneously at all elevations. Both systems provide effective
enhancements in signal to clutter ratio through pattern recognition
and motion detection.
Inventors: |
Atlas; David (Bethesda,
MD) |
Family
ID: |
26959671 |
Appl.
No.: |
07/279,453 |
Filed: |
December 5, 1988 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
796086 |
Nov 8, 1985 |
|
|
|
Reissue of: |
812442 |
Dec 23, 1985 |
04649388 |
Mar 10, 1987 |
|
|
Current U.S.
Class: |
342/26R |
Current CPC
Class: |
G01S
13/951 (20130101); Y02A 90/18 (20180101) |
Current International
Class: |
G01S
13/95 (20060101); G01S 13/00 (20060101); G01S
013/95 () |
Field of
Search: |
;342/26 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
T T. Fujita, "Analysis of Storm-Cell Hazards to Aviation as Related
to Terminal Doppler Radar Siting and Update Rate", Dept. of
Geophysical Sciences, University of Chicago, SMRP Research Paper
204, 1983. .
W. David Zittel, "An Aviation Composite Hazards Product" Second
International Conference on the Aviation Weather System, Jun.
19-21, 1985. .
J. W. Wilson et al., "Microburst Wind Structure and Evaluation of
Doppler Radar for Airport Wind Shear Detection", Journal of Climate
and Applied Meteorology, 1984, vol. 23, pp. 898-995. .
John W. Taylor, "Design of a New Airport Surveillance Radar
(ASR-9)", Proceedings of the IEEE, Feb. 1985, vol. 73, pp. 284-289.
.
Committee Report on "Low Altitude Wind Shear and its Hazard to
Aviation" Nat. Academy Press, Wash. D.C., 1983..
|
Primary Examiner: Tarcza; Thomas H.
Assistant Examiner: Barron, Jr.; Gilberto
Attorney, Agent or Firm: Brady, O'Boyle & Gates
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application .Iadd.is a reissue of U.S. Pat. No. 4,649,388,
issued Mar. 10, 1987 for RADAR DETECTION OF HAZARDOUS SMALL SCALE
WEATHER DISTURBANCES, which .Iaddend.is a continuation-in-part of
prior copending application Ser. No. 796,086, filed Nov. 8, 1985
for RADAR DETECTION OF HAZARDOUS SMALL WEATHER DISTURBANCES, and
now abandoned as of the filing date of this application.
Claims
What is claimed is:
1. A method for detecting hazardous relatively small scale weather
disturbances in an area of surveillance, comprising the steps
of:
radiating at least one Doppler radar beam toward a region of
scatterers and scanning said beam in azimuth;
receiving echo signals in at least first and second vertically
overlapping beams from said scatterers in said first and second
beams from a sequence of range bins, wherein the effective
differential two way beam gain of said first and second beams is of
a first polarity at all angles below a predetermined null level and
of a second polarity at all angles above said null level, wherein
said null level corresponds to the elevation angle at which the
effective two way gain of said first beam equals that of said
second beam, and wherein said first and second polarities may be
mathematically operated upon selectively to provide first and
second parameters equivalent to said first and second
polarities;
determining the respective Doppler velocity spectra from said first
and second beams from said sequence of range bins;
generating a composite Doppler spectrum which is a mathematical
function of the Doppler spectra of said first and second beams,
said composite spectrum defining the Doppler velocity domains
wherein the two Doppler spectra differ in said first and second
polarity senses;
determining two velocity bounds in the region of said composite
Doppler spectrum wherein said composite spectrum is of said first
polarity;
generating signals identifying the first of said two velocity
bounds as a measure of wind speed at said null level;
generating signals identifying the second of said two velocity
bounds as a measure of wind speed at a level between the height of
the null level and the surface of the earth, said wind speed being
defined as the radial component of the near surface wind speed;
measuring the near surface wind speeds in said sequence of range
bins;
selectively determining the range derivative of the near surface
wind speeds, the tangential derivative of the near surface wind
speeds, the difference between the first and second velocity
bounds, the difference between the mean reflectivities in said
first and second beams, and generating output signals corresponding
thereto; and
providing at least one indication of said output signals to thereby
provide an indication of a weather disturbance in said area of
surveillance, particularly the shear of the near surface winds
including horizontal, vertical and tangential shear and the
boundaries at which said shears exceed preset thresholds.
2. The method as claimed in claim 1 wherein said composite Doppler
spectrum comprises the difference Doppler spectrum of the two
Doppler spectra.
3. The method as claimed in claim 1 wherein said composite Doppler
spectrum comprises the ratio Doppler spectrum of the two Doppler
spectra and where the first and second polarity senses correspond
to said first and second parameters greater than unity and less
than unity, respectively.
4. The method as claimed in claim 1 and prior to the step of
generating said composite Doppler spectrum additionally including
the steps of:
normalizing the first beam Doppler spectra by multiplying the first
beam Doppler spectra by the fractional power of the sum of the echo
powers in said first and second beams which is received on said
second beam;
normalizing the second beam Doppler spectra by multiplying said
second beam Doppler spectra by the fractional power received on
said first beam;
and wherein said composite Doppler spectrum is a mathematical
function of the normalized Doppler spectra of said first and second
beams.
5. The method as claimed in claim 1 wherein the echo signals from
said first and second beams are received simultaneously.
6. The method as claimed in claim 1 wherein the echo signals from
said first and second beams are received sequentially.
7. The method as claimed in claim 6 wherein said step of generating
said composite Doppler spectrum further includes the steps of
selectively storing the Doppler spectra or signals from which they
are derived of one of said first and second beams until the Doppler
spectra or signals from which they are derived of the other of said
beams is determined and thereafter after determining the difference
between the first and second Doppler spectra as if said first and
second beams had been received simultaneously.
8. The method as claimed in claim 1 wherein said step of radiating
includes radiating a second beam, wherein said first and second
radiated beams comprise a low beam and a high beam having mutually
different polarizations, and additionally including the steps of
generating an additional parameter of said two velocity bounds of
the first polarity region of said composite Doppler spectrum as a
function of the difference in polarization, the velocity bound
corresponding to said null level in said composite spectrum being
identified with nearly equal power spectral density in the spectra
corresponding to both beam polarizations, and the velocity region
corresponding to the near surface wind speeds having power spectral
density which is polarized predominantly with the polarization of
said low beam, said difference in polarization thereby providing an
additional independent indication that the second bound of said
composite Doppler spectrum is associated with near surface wind
speed.
9. The method as claimed in claim 1, wherein said radiating step
includes radiating a second beam at a different carrier frequency
from that of said at least one beam, the difference in carrier
frequencies of said radiated beams being sufficient to preclude
reception of echo signals of said at least one beam by the second
received beam, and vice versa, thereby assuring that the Doppler
spectra from said first and second received beams are substantially
separated, thereby permitting the second bound of said first
polarity portion of composite Doppler spectrum to be more readily
identified with the near surface winds.
10. The method as claimed in claim 1 and including the further
steps of determining the boundaries of the region in which the
radial shear exceeds an adjustable preset threshold and in which
the tangential shear exceeds an adjustable preset threshold and
combining the signals corresponding to said boundaries on a
combined indicator to depict the essentially complete extent of
said weather disturbance in said area of surveillance.
11. The method as claimed in claim 10 and including the further
steps of storing a sequence of boundary signals of said disturbance
at successive intervals and playing back said stored boundary
signals in a time lapse mode, thereby providing a greatly enhanced
effective signal to noise and signal to clutter ratio even for
weakly reflective hazardous disturbances and further enhancing the
probability of detection and minimizing false alarm through pattern
recognition and motion detection as well as providing indications
of the location, motion, and projected positions of such hazardous
disturbances.
12. The method as claimed in claim 1 and including the further
steps of determining a selected combination of any set of said
output signals, storing said output signals as observed at a
sequence of time intervals, and displaying said sequence of stored
signals in accelerated time lapse mode thereby providing increased
reliability of detection of hazardous weather disturbances,
indications of its past and projected track and evolution, and the
earliest possible precursors of incipient disturbances.
13. The method as claimed in claim 1 and additionally including the
further steps of averaging the near surface wind speeds over
preselected area segments of said area of surveillance and
determining the variances of said wind speeds about the segment
averages as a measure and indication of low level turbulence.
14. A method as claimed in claim 1 wherein the first and second
received beams respectively comprise a low beam and a high beam,
and additionally including the step of determining the breadth of
the low beam Doppler spectrum at all ranges and azimuths,
generating a pattern of said breadth, and utilizing said pattern as
a further indication of a weather disturbance.
15. The method as claimed in claim 1 and additionally including the
steps of determining the average reflectivities in each of said
first and second received beams, determining the difference of said
reflectivities, and determining and indicating the rate of change
of said vertical differences in a vertical direction at a sequence
of times as an indication of the progressive alternation of the
vertical profile of reflectivity thereby providing a precursor of
an incipient hazardous small scale weather event such as a
microburst as well as providing a method of tracking the location
and evolution of such events and projecting their positions.
16. The method as claimed in claim 1 and additionally including the
step of generating a two dimensional indication of the vertical
wind shear and its evolution with time to provide an indication of
a microburst and other weather hazards.
17. The method as claimed in claim 1 wherein said first beam
comprises the lower beam and said second beam comprises the higher
beam of said vertically overlapping beams and wherein said first
polarity comprises a positive polarity and said second polarity
comprises a negative polarity.
18. The method as claimed in claim 1 and additionally including the
steps of:
storing hazard indicating parameters for a plurality of successive
scans;
determining the difference between respective parameters for a
sequence of successive pairs of scans; and
indicating said difference when its magnitude exceeds a prescribed
increment, thereby providing an early warning of incipient
hazardous phenomena as well as enhanced effective signal to noise
and signal to clutter ratios to establish a clear pattern of the
track of said phenomena and the associated evolving patterns of
diverging low level winds, gust fronts, turbulence, shear and the
like.
19. The method as claimed in claim 1 wherein said step of receiving
comprises a receiving at least a third vertically overlapping beam
and providing thereby three null levels therebetween, said first
beam being the lowermost beam, said second beam being the
intermediate beam, and said third beam being the uppermost
beam,
wherein said null levels of the differential gain therebetween
provide radial components of the winds at the respective heights of
said null levels,
wherein said second velocity bound of a first polarity portion of a
composite Doppler spectrum between said first and second beams
provides a measure of the near surface wind speed, and
wherein said second velocity bound of a second polarity portion of
a composite Doppler spectrum between said second and third beams
provides a measure of, the radial component of the winds in a
region above the uppermost null level, thereby providing an
approximate vertical profile of the winds within a predetermined
air space.
20. The method as claimed in claim 1 and additionally including the
step of sequentially varying the elevation of said first and second
beams for varying the height of said null and generating thereby a
vertical profile of the winds within a predetermined air space.
21. A radar method of detecting hazardous weather phenomena
associated with rapidly descending events such as a downburst and
which comprises the steps of:
radiating at least one beam sufficiently large in vertical breadth
to illuminate the aras encompassed by a plurality of vertically
displaced receiving beams from an antenna scanned in azimuth toward
a region of scatterers;
receiving the echo powers from said plurality of beams, said echo
powers being representative of the average reflectivity of the
scatterers in said beams;
determining the measure of the relationship between the echo powers
between said beams at successive elevations at all range bins of a
radar system, said relationship providing information
representative of the vertical profile of average reflectivity of
weather phenomena detected by said radar system;
storing a sequence of said measures for successive scans of said
antenna;
determining the time difference of said measures; and
generating a time history of said measures, thereby providing an
early indication of rapidly changing reflectivities and profiles
associated with a descending downburst and providing a changing
pattern corresponding to an evolving downburst and other weather
hazards.
22. A Doppler radar method of detecting hazardous weather phenomena
comprising the steps of:
radiating at least one beam having a predetermined vertical breadth
covering an area encompassed by a plurality of vertically stacked
receiving beams from an antenna scanned in azimuth toward a region
of scatterers;
receiving the echo power and corresponding reflectivities on a
plurality of vertically stacked receiving beams in a predetermined
number of range gates;
measuring the components of the near surface winds along the
direction of the beam from the mean Doopler velocity on one or more
of the lower beams of said plurality of beams at a sequence of
relatively closely spaced azimuths;
determining the range derivative and the tangential derivative of
said surface wind components from the mean Doppler velocities at
successive ranges and at said closely spaced azimuths as a measure
of the radial and tangential shear, respectively, associated with
hazardous small scale phenomena such as downbursts or microbursts
and tornadoes;
determining the locations at which the measure of the radial and
tangential shear exceed adjustable preset thresholds; and
generating alarms when either of said shears exceed said
thresholds.
23. The method as claimed in claim 22 and further including the
steps of generating boundary locations at which said shears exceed
said thresholds, combining the signals corresponding to said
boundary locations and reproducing said signals on a combined
monitoring device to provide an essentially complete boundary and
increased probability of detection of a hazardous disturbance such
as a microburst.
24. The method as claimed in claim 23 and further including the
steps of storing said combined signals for a sequence of times and
reproducing said stored signals in a predetermined playback mode to
provide further enhanced detectability through motion of a coherent
pattern across a clutter environment, and mapping and tracking the
motion and evolution of said hazardous disturbance and projecting
its future positions.
25. The method as claimed in claim 24 and repeating the above steps
on each of the higher beams to provide a potentially earlier
signature of a descending hazardous disturbance.
26. The method as claimed in claim 22 and further including the
steps of measuring the vertical shear of the horizontal wind
components in corresponding range gates of the stack of vertical
beams, generating a signal corresponding to said vertical shear,
differencing said vertical shear signals at successive time
intervals and triggering an alarm and reproducing the locations at
which the difference between said signals exceeds a preset
threshold to provide a potential precursor of hazardous events.
27. The method as claimed in claim 26 and further including the
steps of storing said vertical shear signals for a sequence of time
intervals and reproducing said sequence of signals in a
predetermined playback mode to provide precursor signatures of
hazardous events, enhanced detectability, tracking the location and
evolution of said events, and projecting their future
positions.
28. The method as claimed in claim 22 and additionally including
the steps of averaging the near surface wind speeds over
preselected area segments of said area of surveillance and
determining the variances of said wind speeds about the segment
averages as a measure and indication of low level turbulence.
29. The method as claimed in claim 22 and additionally including
the step of determining the measure of the relationship between the
echo powers between said plurality of beams at successive elevation
angles and at a plurality of range bins and azimuths of a radar
system, said relationship providing information representative of
the vertical profile of average reflectivity of weather phenomena
detected by a multiple beam Doppler radar system, storing said
measures for a sequence of times, and reproducing said time
sequence as an indication of evolving weather hazards.
30. The method as claimed in claim 22 and further including the
steps of reproducing said Doppler spectral width at a plurality of
elevation angles, range bins and azimuths and triggering an alarm
when said width exceeds a preset threshold.
31. The method as claimed in claim 22 and further including the
steps of reproducing the boundaries of the disturbance as
determined on at least two elevation angles simultaneously from at
least one parameter on a single indicator, coding each of said
boundaries in a respectively distinct manner corresponding to said
elevation and thereby providing a quasi three-dimensional
indication of said disturbance, and monitoring the time sequence of
events on said indicator and thereby providing relatively fast and
unambiguous indication of the vertical displacement of said
disturbance as a precursor of its occurrence at the lower elevation
angles and the surface.
32. The method as claimed in claim 22 and further including the
steps of requiring the simultaneous occurrence of at least two
hazard-indicating parameters on at least one elevation angle within
a predesignated distance from one another and triggering an alarm
when such coincidences occur.
33. The method as claimed in claim 22 and further including the
step of triggering an alarm when at least one of said shear
components including said radial shear and tangential shear exceeds
preset thresholds simultaneously on at least two elevation angles
within a prescribed distance of one another.
34. The method as claimed in claim 33 including the step of
triggering an alarm when at least one of said shear components
exceeds preset thresholds and appears sequentially in a succession
of lower elevation angles within predetermined intervals and within
prescribed distances of one another.
35. Apparatus including a Doppler radar for detecting hazardous
relatively small scale weather disturbances, comprising:
means for radiating at least one Doppler radar beam toward a region
of scatterers in an area of surveillance and scanning said beam in
azimuth;
means for receiving echo signals from said at least one Doppler
radar beam in at least first and second vertically overlapping
beams from said scatterers in said first and second beams from a
sequence of range bins, wherein the effective differential two way
beam gain of said first and second beams is of a first polarity at
all angles below a predetermined null level and of a second
polarity at all angles above said null level, wherein said null
level corresponds to the elevation angle at which the effective two
way gain of said first beam equals that of said second beam, and
wherein said first and second polarities are methematically
operated upon selectively to provide first and second parameters
equivalent to said first and second polarities;
means for generating the respective Doppler velocity spectra from
said first and second beams from said sequence of range bins;
means for forming a composite Doppler spectrum which is a
mathematical function of the Doppler spectra of said first and
second beams, said composite spectrum defining the Doppler velocity
domains wherein the two Doppler spectra differ in said first and
second polarity senses, one of said velocity domains including two
velocity bounds in the region of said composite Doppler spectrum
wherein said composite spectrum has said first polarity;
means for detecting the first of said two velocity bounds as a
measure of wind speed at said null level;
means for detecting the second said two velocity bounds as a
measure of wind speed at a level between the height of the null
level and the surface of the earth, said wind speed being defined
as the radial component of the near surface wind speed;
means for measuring the near surface wind speeds in said sequence
of range bins;
means for selectively determining a measure of the range derivative
of the near surface wind speeds, the tangential derivative of the
near surface wind speeds, the difference between the first and
second velocity bounds, the difference between the mean
reflectivities in said first and second beams, and generating
output signals corresponding to said measure; and
means for providing at least one indication of said output signals
to thereby provide an indication of a weather disturbance in said
area of surveillance, particularly the shear of the near surface
winds including horizontal, vertical and tangential shear and the
boundaries at which said shears exceed preset thresholds.
36. The apparatus as claimed in claim 35 and additionally
including:
means for storing signals of predetermined hazard indicating
parameters for a plurality of successive scans;
means responsive to said hazard indicating parameter signals for
determining the difference between respective parameters for a
sequence of successive pairs of scans and generating difference
signals therefrom; and
means responsive to said difference signals for indicating said
difference at any location when said difference exceeds a
prescribed increment, thereby providing an early warning of
incipient hazardous phenomena as well as a clear pattern of the
track of said phenomena and the associated evolving patterns of
diverging low level winds, gust fronts, turbulence, shear and the
like.
37. Apparatus including a radar system for detecting hazardous
weather phenomena associated with rapidly descending events such as
a downburst, comprising:
means including an antenna scanned in azimuth toward a region of
scatterers for radiating at least one beam sufficiently large in
vertical breadth to illuminate an area encompassed by a plurality
of vertically displaced receiving beams;
means for receiving the echo powers from said plurality of
receiving beams, said echo powers being representative of the
average reflectivity of the scatterers in said beams;
means for determining the measure of the relationship between the
echo powers between said beams at successive elevations at all
range bins of said radar system, said relationship providing
information representative of the vertical profile of average
reflectivity of weather phenomena detected by said radar
system;
means for storing a sequence of said measures for successive scans
of said antenna;
means for determining the time difference of said measures; and
means for generating a time history of said measures, whereby an
early indication of rapidly changing reflectivities and profiles
associated with a descending downburst is provided as well as a
changing pattern corresponding to an evolving downburst and other
weather hazards.
38. Apparatus including a Doppler radar for detecting hazardous
weather phenomena comprising:
means including an antenna scanned in azimuth a region of
scatterers for radiating at least one beam having a predetermined
vertical breadth covering an area encompassed by a plurality of
vertically stacked receiving beams;
means for receiving the echo power and corresponding reflectivities
on said plurality of vertically stacked receiving beams in a
predetermined number of range bins;
means for measuring the mean Doppler velocity and spectral width of
the Doppler spectrum in each of said receiving beams and range
bins;
means for measuring the components of the near surface winds along
the direction of the beam from the mean Doppler velocity on one or
more of the lower receiving beams at a sequence of relatively
closely spaced azimuths;
means for determining the range derivative and the tangential
derivative of said surface wind components from the means Doppler
velocities at successive ranges and at said closely spaced azimuths
as a measure of the radial and tangential shear, respectively,
associated with the hazardous small scale phenomena such as
downbursts or microbursts and tornadoes;
means for determining the locations at which the measure of the
radial and tangential shear exceed adjustable preset thresholds;
and
means for generating alarms when either of said shears exceed said
thresholds.
39. The apparatus as claimed in claim 38 and additionally including
means for combining the signals corresponding to said locations at
which said shears exceed said thresholds and means for reproducing
the combined signals on a monitoring device to provide an
essentially complete boundary and increased probability of
detection of a hazardous disturbance such as a microburst.
40. The apparatus as claimed in claim 39 and additionally including
means for storing the combined signals for a sequence of times and
means for reproducing ssid stored signals in a predetermined
playback mode to provide further enhanced detectability through
motion of a coherent pattern across a clutter environment as well
as mapping and tracking the motion and evolution of said hazardous
disturbance and projecting its future positions.
41. The apparatus as claimed in claim 38 and additionally including
means for measuring the vertical shear of the horizontal wind
components in corresponding range bins of said vertically stacked
receiving beams, means for generating signals corresponding to said
vertical shear, means for differencing said vertical shear signals
at successive times, and means for triggering an alarm and
reproducing the locations when the difference between said signals
exceeds a preset threshold providing thereby a potential precursor
of hazardous events.
42. The apparatus as claimed in claim 41 and additionally including
means for storing said vertical shear signals for a sequence of
times and means for reproducing said sequence of signals in a
predetermined playback mode to provide precursor signatures of
hazardous events, enhanced detectability, tracking the location and
evolution of said events as well as projecting their future
positions.
43. The apparatus as claimed in claim 38 and additionally including
means for reproducing the boundaries of the disturbance as
determined on at least two elevation angles simultaneously from at
least one parameter on a single indicator, means for coding each of
said boundaries in a respectively distinct manner corresponding to
said elevation and thereby providing a quasi three-dimensional
indication of said disturbance, and means for monitoring the time
sequence of events on said indicator, thereby providing relatively
fast and unambiguous indication of the vertical displacement of
said disturbance as a precursor of its occurrence at the lower
elevation angles and the surface of the earth.
44. The apparatus as claimed in claim 38 and additionally including
means for triggering an alarm when at least one of said shear
components including said radial shear and tangential shear exceeds
preset thresholds simultaneously on at least two elevation angles
within a prescribed distance of one another.
45. The apparatus as claimed in claim 38 and additionally including
means for triggering an alarm when at least one of said shear
components exceeds preset thresholds and appears sequentially in a
succession of lower elevation angles within predetermined intervals
and within prescribed distances of one another.
46. The apparatus as claimed in claim 38 and additionally including
means for determining the measure of the relationship between the
echo powers between said plurality of receiving beams at successive
elevation angles and at a plurality of range bins, said
relationship providing information representative of the vertical
profile of average reflectivity of weather phenomena detected by
said radar system.
47. The apparatus as claimed in claim 46 and additionally including
means for storing said measures for successive scans of said
antenna; and
means for retrieving and reproducing said stored measures and
providing therefrom an indication of an evolving hazardous small
scale phenomena. .Iadd.
48. A method for detecting hazardous relatively small scale weather
disturbances in an area of surveillance, comprising the steps
of:
radiating at least one Doppler radar beam toward a region of
scatterers;
receiving echo signals in at least first and second vertically
overlapping beams from said scatterers in said first and second
beams from a sequence of range bins, wherein the effective
differential two way beam gain of said first and second beams is of
a first polarity at all angles below a predetermined null level and
a second polarity at all angles above said null level, wherein said
null level corresponds to the elevation angle at which the
effective two way gain of said first beam equals that of said
second beam, and wherein said first and second polarities may be
mathematically operated upon selectively to provide first and
second parameters equivalent to said first and second
polarities;
determining the respective Doppler velocity spectra from said first
and second beams from said sequence of range bins;
generating a composite Doppler spectrum which is a mathematical
function of the Doppler spectra of said first and second beams,
said composite spectrum defining the Doppler velocity domains
wherein the two Doppler spectra differ in said first and second
polarity senses;
determining two velocity bounds in the region of said composite
Doppler spectrum wherein said composite spectrum is of said first
polarity;
generating signals identifying the first of said two velocity
bounds as a measure of the wind speed component at said null
level;
generating signals identifying the second of said two velocity
bounds as a measure of wind speed at a level between the height of
the null level and the surface of the earth, said wind speed being
defined as the radial component of the near surface wind speed;
measuring the near surface wind speeds in said sequence of range
bins; and
providing at least one indication of the variation of the near
surface winds speeds with respect to range and azimuth as a
signature of the weather disturbances including wind shear.
.Iaddend. .Iadd.
49. A method for detecting hazardous relatively small scale weather
disturbances in an area of surveillance, comprising the steps
of:
radiating at least one Doppler radar beam toward a region of
scatterers;
receiving echo signals in at least first and second vertically
overlapping beams from said scatterers in said first and second
beams from a sequence of range bins, wherein the effective
differential two way beam gain of said first and second beams is of
a first polarity at all angles below a predetermined null level,
and a second polarity at all angles above said null level, wherein
said null level corresponds to the elevation angle at which the
effective two way gain of said first beam equals that of said
second beam, and wherein said first and second polarities may be
mathematically operated upon selectively to provide first and
second parameters equivalent to said first and second
polarities;
determining the respective Doppler velocity spectra from said first
and second beams from said sequence of range bins;
generating a composite Doppler spectrum which is a mathematical
function of the Doppler spectra of said first and second beams,
said composite spectrum defining the Doppler velocity domains
wherein the two Doppler spectra differ in said first and second
polarity senses;
generating at least one velocity parameter which is a mathematical
function of said composite Doppler spectrum and which is indicative
of the radial component of the wind velocity at a height
corresponding to a level up to the level of said null in each of
said sequence of range bins; and
providing an indication of the variation of said at least one
velocity parameter as a function of range in said sequence of range
bins as a signature of the low altitude wind shear. .Iaddend.
.Iadd.50. The method as defined by claim 49 wherein said step of
radiating comprises radiating at least one Doppler radar beam
toward a region of scatterers and scanning said beam in azimuth;
and
wherein said step of providing an indication of the variation of
said at least one velocity parameter comprises providing an
indication of the low altitude wind shear at the sequence of
azimuths obtained by scanning said beam in azimuth. .Iaddend.
.Iadd.51. The method as defined by claim 49 and additionally
including the steps of determining the range derivative of said at
least one velocity parameter and the tangential derivative of said
at least one velocity parameter, and
generating output signals from which at least one indication of
hazardous
wind conditions is provided. .Iaddend. .Iadd.52. The method as
defined by claim 49 and additionally including the steps of
determining the range derivative of said at least one velocity
parameter and the tangential derivative of said at least one
velocity parameter, and
generating output signals from which at least one indication of
hazardous wind conditions is provided. .Iaddend. .Iadd.53. The
method as defined in claim 49 wherein said at least one velocity
parameter comprises the mean velocity in the composite Doppler
spectrum, and
wherein the composition Doppler spectrum comprises the difference
between said first and second Doppler spectra over the velocity
range corresponding to the region below said null level. .Iaddend.
.Iadd.54. The method as defined by claim 53 wherein said velocity
range corresponding to the region below said null level is
determined by the velocity domain in which the difference between
said Doppler spectrum of said first beam and the Doppler spectrum
of said second beam is of a predetermined polarity. .Iaddend.
.Iadd.55. The method as defined in claim 49 wherein said at least
one velocity parameter comprises the route mean square velocity in
the composite spectrum, and
wherein said composite Doppler spectrum comprises the difference
between said first and second Doppler spectra over the velocity
range
corresponding to the region below said null level. .Iaddend.
.Iadd.56. The method as defined by claim 49 wherein said at least
one velocity parameter comprises the sum of the magnitudes of the
mean Doppler velocity of a portion of said composite Doppler
spectrum having a predetermined polarity and additionally including
another parameter which is representative of the width of said
polarity portion of the composite
spectrum. .Iaddend. .Iadd.57. A method for detecting hazardous
relatively small scale weather disturbances in an area of
surveillance, comprising the steps of:
radiating at least one Doppler radar beam toward a region of
scatterers;
receiving echo signals in at least first and second vertically
overlapping beams from said scatterers in said first and second
beams from a sequence of range bins, wherein the effective
differential two way beam gain of said first and second beams is of
a first polarity at all angles below a predetermined null level and
a second polarity at all angles above said null level, wherein said
null level corresponds to the elevation angle at which the
effective two way gain of said first beam equals that of said
second beam, and wherein said first and second polarities may be
mathematically operated upon selectively to provide first and
second parameters equivalent to said first and second
polarities;
determining a set of predetermined parameters which are functions
of the mathematical characteristics of the Doppler velocity spectra
corresponding to said first and second beams from said sequence of
range bins;
operating mathematically upon said set of parameters to produce at
least one parameter which is a measure of the radial wind component
along the direction of the beam and which corresponds to the wind
speed at a height representative of the near surface wind speed;
and
providing at least one indication of the variation of the near
surface winds speed with respect to range and azimuth as a
signature of the weather disturbances including wind shear.
.Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to the detection of weather
disturbances which are particularly hazardous to aircraft during
takeoff and landing and more particularly to the radar detection of
the location and intensity of microbursts and the resulting low
level wind shear, as well as wind gust fronts, tornado vortices and
their antecedent mesocyclones.
Although there are existing and contemplated techniques for radar
detection of potentially hazardous weather disturbances, they are
known to require the use of special dedicated narrow beam Doppler
weather radars. A 1984 publication entitled, "Microburst Wind
Structure And Evaluation of Doppler Radar For Airport Wind Shear
Detection" by J. W. Wilson, et al., which appeared in the Journal
of Climate and Applied Meteorology, Vol. 23, at pp. 898-995,
discloses one such an approach. However, no known solution exists
to date for integrating the detection of the aforesaid weather
disturbances, particularly low level wind shear, into radars used
primarily for airport surveillance which by design have one or more
relatively large vertical fan beams. A typical example of this type
of radar is disclosed in a publication entitled, "Design of a New
Airport Surveillance Radar (ASR-9)" by John W. Taylor, et al.,
which appeared in the Proceedings Of The IEEE, Vol. 3, No. 2,
February, 1985, pp. 284-289.
Accordingly, it is an object of this invention to provide an
improvement in the detection of certain weather disturbances.
It is another object of this invention to provide an improvement in
the radar detection of certain weather disturbances which are
accompanied by relatively violent winds in a small locality.
It is still another object of this invention to provide an
improvement in the radar detection of microbursts, low level wind
shear, wind gust fronts, tornado vortices, and mesocyclones.
It is yet another object of this invention to provide for the
detection and indication of the position as well as determining the
intensity of microbursts and the associated low level wind shear
and turbulence which are particularly hazardous to aircraft during
takeoff and landing.
SUMMARY
Briefly, the foregoing and other objects are accomplished by a
method and apparatus for use in connection with airport
surveillance Doppler radars having at least two relatively wide
vertical fan beams whose patterns overlap one another and wherein
the main lobe of one of the beams is directed at an elevation angle
lower than that of the other and has an antenna gain exceeding that
of the other at all elevations below a prescribed elevation angle
where the gains are equal. Above this equal gain or null level, the
gain of the second or upper beam exceeds that of the lower. The
Doppler velocity spectra on both beams are then measured
essentially simultaneously. In a first method a difference Doppler
spectrum is obtained by subtracting the high beam spectra from the
low beam spectra which will be positive at angles below the null
level and negative above. Accordingly, the Doppler velocities
encompassed by the positive segment of the difference Doppler
spectrum represent the velocities of the wind components at
elevation angles lower than the null level. The velocity which is
found where the difference Doppler spectrum crosses, from positive
to negative is the wind component at the null level. Assuming that
the wind varies monotonically with height, the velocity bounds at
or near the other extreme of the positive portion of the difference
Doppler spectrum corresponds to the speed of near surface level
wind. The radial gradient or derivative of the wind speeds provides
an indication of the wind shear that is associated with a
microburst or similar phenomenon. The wind shear is displayed and
when it exceeds a prescribed level, an alarm can be generated, when
desired. In a second method, the ratio of the low beam spectra to
the high beam spectra is obtained to generate a ratio Doppler
spectrum from which the velocity bounds referred to above are
determined where the ratio Doppler spectrum is equal to or exceeds
unity. In a third method a normalization process of the Doppler
spectra in the beams is effected prior to generating either the
difference Doppler spectrum or the ratio Doppler spectrum.
This invention also provides for the measurement of the vertical
shear of the wind in the lower layer of the atmosphere, the
tangential shear of the near surface wind, the turbulence of the
near surface wind, and the difference between the average
reflectivities in the high and low beams. Inasmuch as each of these
parameters is related to one or more of the small scale weather
hazards of interest, in addition, the time rate of change of these
parameters, as measured during a sequence of antenna scans,
provides a probable precursor signature of the onset of an
incipient hazard. Similarly, the display of the parameters
themselves, or their scan to scan differences on a sequence of
scans provides a clear and unambiguous indication of the location,
track and rate of evolution of the hazardous region.
An alternative method and apparatus involves the use of a plurality
of vertically stacked narrow beams which permits the measurement of
radar reflectivity, mean Doppler velocity, and Doppler spectrum
breadth simultaneously at all corresponding elevations. The mean
Doppler velocity on the lowest beam may be operated upon in every
respect, as was the near surface wind velocity in the wide beam
embodiment to provide the desired detection and warning
capabilities. Moreover, the vertical shear and vertical
reflectivity gradient may be determined more accurately and
employed in a manner similar to that for the wide beam
embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
While the present invention is defined in the claims annexed to and
forming a part of the specification, a better understanding can be
had by reference to the following description when taken in
conjunction with the accompanying drawings in which:
FIGS. 1A and 1B are side and top elevational views
diagrammatrically illustrative of a microburst and the associated
wind shear;
FIG. 2A is a top elevational view of another illustrative example
of the strongly sheared wind field resulting from microbursts,
while FIGS. 2B and 2C are side elevational views of the associated
winds respectively occurring along the lines A-B and C-D of FIG.
2A;
FIG. 3 is a diagram illustrative of a typical gain characteristic
of a dual beam airport surveillance radar system;
FIGS. 4A and 4B are a set of characteristic curves helpful in
understanding the invention;
FIGS. 5A through 5C are another set of characteristic curves
helpful in understanding the invention;
FIGS. 6A and 6B are still another set of characteristic curves
helpful in understanding the invention;
FIGS. 7A and 8A are system block diagrams broadly illustrative of
two embodiments of the subject invention for a monostatic dual beam
Doppler radar;
FIGS. 7B and 8B are partial system block diagrams broadly
illustrative of two embodiments of the subject invention for a
bistatic dual beam Doppler radar in which one beam is monostatic
and the other is bistatic;
FIG. 9 is a block diagram of apparatus utilized in connection with
the embodiments shown in FIGS. 7A-8B for determining the difference
in reflectivity;
FIG. 10 is a flow chart illustrative of the operation of the radar
computer shown in FIGS. 7A-8B for operating on difference Doppler
spectrum data for producing an indication of microbursts and wind
shear;
FIG. 11 is a flow chart illustrative of a method for computing and
displaying turbulence with the radar computer shown in FIGS. 7A-7B;
and
FIG. 12 is illustrative of another aspect of this invention where
more than two overlapping beams of a radar are utilized.
DETAILED DESCRIPTION OF THE INVENTION
This invention discloses a method as well as apparatus by which one
may use a relatively large elevational beam Doppler radar such as
that employed for airport surveillance for the detection of
downbursts, also referred to interchangeably as microbursts, as
well as the detection and mapping of tornado vortices and their
antecedent mesocyclones. Mesocyclones are the larger rotating
vortices of the order of a few kilometers in diameter which often
accompany thunderstorms and precede tornadoes. The radar, however,
should preferably have sufficient sensitivity to detect microbursts
which may have equivalent reflectivities as low as 5 dBZ out to
about 15 Km range. It should be noted that dBZ is a well known unit
of reflectivity of meteorological targets. A reflectivity of 5 dBZ
is about the average for dry microbursts. It is preferable that the
radar be able to detect even smaller reflectivities and measure
their associated velocities. While the present invention is
particularly adapted for use with an airport surveillance radar,
the ASR-9 referred to in the above referenced publication being as
an illustrative example and incorporated herein by reference, it is
not meant to be restricted thereto.
In particular, this invention also pertains to the use of a system
utilizing a multiplicity of narrow beams.
One of the primary Doppler radar signatures to be detected in a
microburst induced low level shear is the radial velocity gradient
.DELTA.V/.DELTA.r. This gradient may reach values as high 40 m/s
(about 80 Knots) in 2 Km and may occur in a region as small as 1
Km. Schematic diagrams of microbursts and associated divergence
patterns generating wind shear are shown in FIGS. 1A-1B and 2A-2C.
This type of phenomenon was described in 1983 by T. T. Fujita in a
publication entitled, "Analysis of Storm-Cell Hazards To Aviation
As Related To Terminal Doppler Radar Siting and Update Rate", Dept.
of Geophysical Sciences, University of Chicago, SMRP Research Paper
204, as well as the aforementioned Wilson, et al. publication. It
is to be noted that the divergence as shown in FIG. 1B is not
necessarily circularly symmetrical so that a single radar beam will
discern only that component of the divergence which is along the
line between the radar and microburst. As has become very evident
of late, undetected low level shear is particularly hazardous to
aircraft in takeoff or landing when the sudden encounter of a wind
shift in the same direction as flight causes the aircraft to lose
lift. The primary Doppler radar signature of a mesocyclone is the
tangential or circumferential shear of the radial velocity as the
radar beam moves across its vortex observing a sharp change in
radial components associated with the rotating winds on either side
of the axis of the vortex. In the case of tornado vortices, a
similar signature will occur with a larger tangential shear when
the vortex diameter is larger than the beamwidth. If not, the
primary signature of the tornado vortex is an extremely broad width
of the Doppler spectrum associated with the simultaneous presence
of both very large receding and approaching velocities in the radar
pulse volume.
A problem which makes low level shear in and around the vicinity of
an airport particularly difficult to detect by radar is that the
maximum horizontal shear often occurs at heights below 100 m above
ground level (AGL). This requires that the radar be sufficiently
close to the microburst that it is within the radar's horizon. It
also implies that the radar be located at or near the airport so
that the radar can provide detection capability at such low levels
out to sufficient radius, e.g. 15 to 20 Km, to assure that a
microburst which occurs off to one side may not propagate into the
runway areas without advance notice.
It is therefore vital that the radar be able to direct sufficient
energy to the weather disturbance at the lowest possible angle to
detect the low level shear. Also, the radar must have excellent
ground clutter rejection for two primary reasons: (1) it must be
able to detect weakly reflecting dry microbursts against ground
clutter in close proximity to the radar and airport; and (2) ground
clutter echoes may bias Doppler velocity measurements severely
thereby causing errors in the measurement of radial shear. Airport
Surveillance Radar Doppler radars fit the requirements stated above
in several important ways: (1) they have excellent sensitivity; (2)
they are usually located at the airport; and (3) they generally
have excellent clutter rejection.
In the case of the weakly reflective dry microburst or other
hazardous weather phenomena, the existing clutter rejection may
still be inadequate. This invention, therefore, provides methods of
attaining effectively greater clutter rejection. In addition the
critical weakness of the present ASR radars, such as the
aforementioned ASR-9 Doppler radar, for detecting microbursts, wind
shear and other violet localized disturbancesis its relatively
large vertical beam having a cosecant square radiation pattern
which is designed to detect and track aircraft in an airspace below
25,000 ft. and within 40 to 60 nmi of the airport. Since this broad
vertical beam also views storm systems at all angles up to about 35
degrees, the Doppler spectrum will be relatively very broad. Even
with a uniform wind speed, v, across the depth of the beam, the
measured doppler velocities will range from approximately v to v
cos 35.degree. or from v to about 0.8 v. Add to this particle fall
speeds, wind shear, turbulene, and other spectrum broadening
factors, and the weighting of the echoes by both the beam
illumination pattern and the vertical reflectivity distribution,
makes the Doppler spectrum exceedingly broad. Under these
circumstances, the radial variation of mean Doppler velocity may be
a meaningless measure of the radial shear near the surface.
The present invention overcomes the above noted limitations in its
preferred form by measuring the Doppler velocities associated with
targets near the surface, the "near the surface" being defined as
heights below about 1 Km and determining these velocities in the
Doppler spectrum from the difference in the Doppler spectra on two
separate beams which receive echoes simultaneously or consecutively
and whose radiation patterns 10 and 12 overlap in the vertical as
shown in FIG. 3.
This process illustrated schematically in FIGS. 4A and 4B where the
Doppler spectra associated with the low (L) and high (H) beams,
S.sub.L (v) and S.sub.H (v), respectively, are plotted and which
are shown by the curves 14 and 16. This description assumes that
the reflectivity is constant with height across the two beams. Note
that the null level 18 i.e. where S.sub.L (v)=S.sub.H (v) occurs at
a specific velocity v(0). In the simplest case where the wind
varies monotonically with height, the velocity axis also
corresponds to a height axis. Accordingly, S.sub.L (v)=S.sub.H (v)
at an elevation angle or height at which the beam gains of the two
beams are equal, i.e. G.sub.L (h)=G.sub.H (h), where h=height, as
shown by reference numeral 11 in FIG. 3. Switching from low beam to
high beam, those velocities which disappear or at which the
spectral densities are reduced must be associated with the
altitudes below the null where the low beam gain, G.sub.L (h),
exceeds that of the high beam G.sub.H (h). In FIG. 4A, it can be
seen that the velocities at which S.sub. L (v) exceeds S.sub.H (v)
are large. This is confirmed also by the fact that the mean Doppler
velocity v.sub.L on the low beam exceeds that on the high beam,
i.e. v.sub.H. Accordingly, the velocities which occur below the
null level are those to the right of or larger than that of the
null. In this case, the largest velocity in the low beam spectrum
14 is that at the right hand bound v* of S.sub.L (v). With velocity
varying monotonically with height, v* would be the radial component
of the wind speed near the surface. In FIG. 4B, analogous reasoning
shows that the spectral densities are larger in the low beam at
velocities smaller than that at the null leading to the conclusion
that the radial velocity component is increasing with increasing
height, as confirmed by the fact that the means Doppler velocity
v.sub.L in the low beam is less than that v.sub.H in the high beam.
In this case, the near surface wind velocity v* is at the left hand
bound of the low beam spectrum 14.
For a microburst the low level wind shear is in the radial shear of
the near surface wind as determined from the gradient of v*
observed at a series of adjacent range bins. Other signatures of v*
and v(0) which are related to microbursts and low level wind shear
also provide a way in which vertical wind shear, tornado vortices
and mesocyclones may be detected. The following discussion shall
clarify this process and describe the methods by which it may be
achieved.
The process may be explained mathematically and graphically as
follows. Let Z(h) be the vertical distribution of reflectivity with
height, h. At any range, the vertical illumination pattern may be
expressed in terms of its gain function. Accordingly, the echo
power returned to the antenna from any height increment between
height h and h+dh and from any range bin can be expressed as:
This function, however, is not measurable with a wide vertical
beam. All that can be measured is the total echo power ##EQU1##
where the limits on the integral are the surface height h=0 and
echo top, h(t). It is implicit that the echo power within the
horizontal width of the beam is also integrated. One can also
measure v and .sigma..sub.v.sup.2 ; however, these values have very
little physical significance with wide beams. While S(h) cannot be
measured with a wide beam radar, the Doppler spectrum as a function
of Doppler frequency shift, f=2v/.lambda., where v is the target
Doppler velocity and .lambda. is the wavelength, can be determined.
If the target velocity is a single valued function of height, h,
i.e.
then the height coordinate, h, may be transformed to the Doppler
velocity coordinate, v, through Eq.(3). By employing the
equality
Eq. (1) can be stated as:
and comprises the Doppler spectrum which can be determined. It
should be noted that the factor .vertline.dh/dv.vertline. simply
transforms the height scale so that the magnitude of the echo power
which is returned from the height interval h to h+dh appears in the
velocity interval v to v+dv. Thus if the wind varies linearly with
height, then dh/dv is constant and the Doppler velocity scale is
simply a constant times the height scale. On the other hand, if the
wind varies non-linearly with height, then the velocity scale is
compressed or expanded variably along the height scale.
Since echo power is always a positive quantity, the quantity
.vertline.dh/dv.vertline. is stated as an absolute value. This is
important because one of the crucial aspects of this invention is
that it permits a determination of whether the wind is stronger at
low levels than at high levels, in spite of the fact that the sign
of (dv/dh) may be positive or negative; i.e. increasing upward or
downward, respectively.
If a relative narrow vertical beam were utilized such that
G(h)=G.sub.0 (h), where G.sub.0 (h) is essentially the axial gain
of the antenna, then it would make sense to measure the mean
Doppler velocity as the beam scanned vertically. In that case, the
variation of mean Doppler velocity with scan angle or height would
provide the vertical profile of v with h given by Eq. (3).
Similarly, the echo power at each height would be given by Eq. (2)
from which one could recover the reflectivity profile Z(h). As it
is, however, all that is available is the measured Doppler spectrum
and knowledge of the antenna pattern. This leaves two unknowns in
Eq. (5), i.e. Z(v) and .vertline.dh/dv.vertline., or v=f(h), thus
precluding the determination of either the desired wind velocities
or their heights.
In the preceding and subsequent discussion it should be noted that
when a radar is used in a monostatic manner, the antenna gain
function G(h) simply refers to the standard radiation pattern of
the antenna, where h=R tan(EL), (EL) being the elevation angle
above the horizon and R the range. However, when the radar is used
bistatically such that transmission occurs on one beam and
reception on another, then G(h) is the effective antenna gain
defined by G(h)=[G.sub.1 (h)G.sub.2 (h)].sup.1/2 where the
subscripts 1 and 2 refer to the transmitting and receiving beams,
respectively. In the case of the ASR-9, for example, transmission
occurs on the low beam and reception is selectively either on the
low beam or the high beam. Thus the effective gain function for the
low beam is simply G(h)=G.sub.1 (h), since G.sub.2 (h)=G.sub.1 (h).
This is shown as curve 12 in FIG. 3. On the other hand, the
effective gain for the high beam is the square root of the product
of the low and high beam gain functions as described above and
shown as 10 in FIG. 3.
In the case of the wide beam airport surveillance radar such as the
ASR-9, the two effective beams are as shown, for example, in FIG.
3. The axis of the effective high beam 10 is peaked at about
6.degree. while that of the low beam 12 is peaked at about
2.5.degree.. The gains are equal at about 5.0.degree.. Below this
level 11, and low beam 12 has considerably greater gain than the
high beam 10. At 2.5.degree. elevation, this difference is about 12
db. Above 6.degree. the gain of the high beam 10 is typically 3 to
3.5 db greater than that of the low beam 12. In this invention, the
exact differences are of little consequence since it is only
important that the two beams 10 and 12 have significantly different
gains and that the differences be of opposite sign at low and high
elevations, and that the equal gain angle not be more than a few
degrees above the surface.
With the foregoing in mind, and utilizing a dual beam Doppler radar
having antenna beam characteristics like that shown in FIG. 3, a
first method of this invention calls for forming the difference in
the Doppler spectra on the two beams. Using Eq. (5) for each of the
beams, for example, beams 12 and 10, the following expression
results:
where the subscripts L and H represent low and high beams,
respectively. Hereafter, the term G.sub.L -G.sub.H is referred to
as the "differential gain" (DG). Since reflectivity is positive,
the difference spectrum is exactly zero at the height, i.e.
elevation angle, at which G.sub.L =G.sub.H. This is the null level
as shown by reference numeral 11 in FIG. 3. At low altitudes, below
the null level, DG is positive and the difference spectrum is also
positive; and conversely for heights above the null level, the
difference spectrum is negative. Since DG can only be positive at
low elevation angles, one may simply identify or associate the
Doppler velocities in the positive portion of the difference
spectrum with low altitudes. For example, in the case of the ASR-9
radar, this would be below 5.0.degree. elevation. Of course, one
may adjust or design the beams so that the null level occurs at
lower elevation angles. Indeed, this would be preferable for the
detection of low level winds and wind shear. However, such an
alteration might compromise the aircraft detection and tracking
function of the radar. In any event, a method now becomes available
for measuring radial velocities of winds in a storm cell within a
prescribed altitude region with a radar having two large beams
having a relatively wide beamwidth.
Alternatively and in accordance with a second method of this
invention, the ratio of the low beam spectrum to that on the high
beam is formed instead of the difference. Whether the reflectivity
is constant or varies with height, the ratio of the two Doppler
spectra, is defined as the Ratio Doppler Spectrum (RDS). RDS will
exceed unity whenever G.sub.L >G.sub.H, will equal unity where
G.sub.L =G.sub.H (i.e. the null level), and will be less than unity
where G.sub.L <G.sub.H. In short, the same objective may be
achieved by forming either the Difference Doppler Spectrum (DDS) or
the Ratio Doppler Spectrum (RDS) between the two beams.
Considering now the effect of the reflectivity profile Z(h) in
equations (1) and (6), Z(h) appears as a multiplier of the antenna
gain difference pattern (DG). Accordingly, if the profile of Z(h)
increases downward, then it will enhance the positive DG portion;
however, if Z(h) increases upward, it will degrade this difference.
In the case in which Z(h) approaches zero in the zone below the
null, both S.sub.L (v) and S.sub.H (v) will approach zero and the
DDS will also approach zero. Then, the positive portion of the DDS
may become negligibly different from zero. On the other hand, if
Z(h) simultaneously becomes larger above the null, it will enhance
the negative portion of the DDS, thus sill permitting one to
identify the null.
In order to maintain a relatively constant ability to identify the
null and the positive region of the DDS, a third method of this
invention involves the concept of normalizing the spectra of the
two beams. This involves multiplying the low beam spectra by the
fractional power, i.e. that portion of the sum of the two spectra
which resides in the high beam, multiplying the high beam spectrum
by the fractional power in the low beam. This technique, to be
described below, compensates in large part for any vertical
reflectivity gradient. It also maintains the levels of the DDS
within a reasonable dynamic range. However, it is not necessary to
the performance of this invention and accordingly may be resorted
to if it fits the requirements of the user as an alternative
operational mode.
The process by which one normalizes the spectra in the two beams is
as follows. First the total power from each beam is measured as
given by Eq. (2). The quantity G(h)Z(h) in Eq. (2) i the
reflectivity-weighted vertical radiation pattern, and the Doppler
spectrum in each beam is given by Eq. (5). The integral of (5) over
the entire vertical extent of each beam is the area under each
Doppler spectrum and is the total power. Thus the vertical gradient
in the reflectivity profile is normalized by multiplying the low
beam Doppler spectrum S.sub.L (v) by the ratio P.sub.H /(P.sub.L
+P.sub.H) and the high beam spectrum S.sub.H (v) by P.sub.L
/(P.sub.L +P.sub.H). These are called the normalized Doppler
spectra S.sub.Ln (v) and S.sub.Hn (v) such that: ##EQU2## and
##EQU3## The subscript n designates the term being "normalized".
Accordingly, the normalized spectra are such that the low beam
spectrum is enhanced by the fractional power of the two beams which
is found in the high beam, and conversely. If reflectivity does not
vary with height, then the power weighting factors F.sub.H and
F.sub.L are equal and a situation similar to that previously
discussed obtains.
In any case, it should be noted that the area of concern is with
short ranges and altitudes below the differential gain null level
which for the ASR-9 occurs at about 5.0.degree.. At ranges less
than about 15 Km, the height of this null is less than 1.3 Km. Thus
Z would have to increase sharply upward to substantially degrade
the methods just described even without normalization.
In order to clarify the inventive concepts further, reference is
now made to FIGS. 5A through 5C. FIG. 5A is a graph illustrative of
the vertical profile of the antenna gain difference characteristic
of a high and low beam pattern such as shown in FIG. 3. The actual
pattern is not critical as it is only important that the
differential gain be as large as possible on both sides of the null
11 (FIG. 3) and that its vertical derivative near and across the
null be large. In FIG. 5A, the DG is plotted versus elevation
angle. In FIG. 5B an arbitrary altitude scale ranging from the
ground up to 1 Km is shown corresponding to a specific range. Also
in FIG. 5B, three velocity profiles of three different wind
conditions 1, 2 and 3 are shown. The velocity profiles 1 and 2
decrease upwards while profile 3 increases upwards. Profile 3,
moreover, has the same velocity range as profile 2 but with
opposite shear. In order to compute the difference Doppler spectrum
(DDS) it has been assumed that the reflectivity profile is constant
with height and has a value of 10 Log Z= 0 dBZ, or Z=1.
FIG. 5C is a plot of the difference Doppler Spectrum S.sub.L
(v)-S.sub.H (v) of the three velocity profiles. Note that the
ordinate in FIG. 5C is the velocity scale which is a simple linear
transformation of the height scale because v is linear with height
in all three cases.
For profile 1, the corresponding difference Doppler spectrum (DDS)
is shown by the solid curve 1' in FIG. 5C. It can be seen that
positive values of the difference Doppler spectrum occur between 30
and 40 m/s because these velocities occur where the differential
gain (DG) is positive. However, the entire DDS is compressed
between 20 and 40 m/s because this is the range encompassed by
velocity profile 1 in FIG. 5B. Curve 2' shown by the dot-dash line
corresponds to the wind profile 2 in FIG. 5B and also exhibits a
positive DDS at the larger velocities, i.e. in excess of 35 m/s,
because these also occur below 1 Km where the differential gain is
positive. Now, however, the upper bound of the DDS is 60 m/s, thus
indicating that this is the greatest wind speed and that it occurs
near zero height which is the surface of the earth. Considering
curve 3' shown by the dashed line in FIG. 5C, it is positive at low
velocities and negative at the high velocities indicating that the
high speeds occur aloft where differential gain is negative above
the null level. In this case the near surface wind velocity
component v*=10 m/s corresponds to the non-null bound of the
positive portion of the DDS.
Accordingly, it can be seen that one Doppler velocity in the
differential Doppler spectrum can be identified as that occurring
at DG=0 or null level. This velocity has already been defined as
v(0). If the wind varies monotonically with height, then the other
(non-null) bound of the positive DDS region is automatically the v*
near the surface and represents the wind velocities which are
sought to be determined.
In the event that the velocity profile is not monotonic with
height, then the bounds of the positive region of the DDS will be
v(0), that at the null level, and v(max) or v(min) at the level
where the wind speed is a maximum or a minimum, respectively. As
long as this occurs, in the positive DDS region, the level of
v(max) or v(min) is known to be below that of v(0). Since short
ranges typically less than 15 Km are of consideration and where the
null level is at heights less than 1.3 Km, then v(max) or v(min) is
known to occur somewhere below the null v(0). Thus there is little
danger in designating v(max) or v(min) as the near surface wind
velocity, v*. Indeed, it is exactly for this reason that v* has
been called the "near surface" wind velocity; i.e. there are
circumstances in which it will represent the wind at some level
above the surface, but never higher than the height of the null.
Also, the use of v* as the near surface wind will be advantageous
in those situations where the microburst begins to diverge aloft
rather than at the surface.
In summary, it is to be noted that: (1) the velocity v(0) which
occurs at the level at which the DDS=0 or goes from positive to
negative is the velocity at the null level; (2) the velocity v* at
the other bound of the positive DDS region is a measure of the near
surface wind velocity at some level below the null level; (3) the
DDS is a compressed image of the DG pattern if the reflectivity Z
is constant with height and v increases linearly with height, and
the image is inverted if v decreases linearly with height; and (4)
an approximate measure of the vertical wind shear is the velocity
range encompassed by the positive side of DDS, v*-v(0).
There are a variety of means by which the Difference Doppler
Spectrum (DDS) between the two beams, for example 10 and 12 (FIG.
3), can be obtained. If one can receive on only one beam at a time
and has a single receiver and Doppler processor, one may switch the
signals alternately from each beam into the processor, store the
spectra, and subtract. However, this may not be desirable for a
rapidly scanning antenna such as that of the ASR-9. Instead, both
beams may be received simultaneously in which case each beam would
have its own receiver and Doppler processor so that both the
individual beam spectra and the DDS may be obtained simultaneously
in real time. Two embodiments of such apparatus are disclosed in
FIGS. 7 and 8 and will be considered subsequently.
One of the problems which needs to be addressed is the accuracy
with which one may obtain the individual Doppler spectra and
Difference Doppler Spectrum. Errors in estimating the spectral
power density on each beam will be reflected in the DDS and its
velocity bounds.
In the case of a discretely sampled waveform such as that available
in a pulsed Doppler radar, one may obtain spectral estimates at M
frequencies each of which is a multiple of the lowest frequency
f.sub.0 and which may be expressed by the relationship:
where M is the number of samples in the sequence and T.sub.S is the
interpulse period. In other words, the frequency resolution is
simply the inverse of the total dwell time, MT.sub.S. Since
f=2v/.lambda., the velocity resolution .DELTA.v can be stated
as:
The maximum unambiguous frequency or Nyquist interval is
The corresponding maximum unambiguous Doppler velocity is then
and the velocity resolution is simply
which is equivalent to Eq. (10).
In the ASR-9 radar, for example, there are eighteen pulses sampled
per beamwidth. Then pulses are transmitted at a high PRF and eight
at a lower PRF. Ten digital filters are used during the first burst
of ten pulses and eight during the second burst of eight pulses.
This dual PRF prevents blindness to Doppler frequencies close to
integral multiples of the average PRF. In any case, at the high PRF
of 1255 pulses per sec., the maximum unambiguous velocity interval
is v(max)=33.6 m/s. With ten pulses and ten filters, the velocity
resolution is thus 6.7 m/s. A separate filter bank of eight filters
is used at the low PRF of 976 pulses per sec. with v(max)=26.1 m/s
and .DELTA.v=6.5 m/s.
If a velocity resolution of 2 m/s (or approximately 4 Knots) is
desired, the dwell time must be increased typically by a factor of
about three. This may be achieved readily by sampling three times
as long. A side effect of this is to increase the effective
horizontal beamwidth from 1.4.degree. to 4.2.degree.. However, this
is not too serious since at ranges less than 15 Km, the beamwidth
would still be less than 1.2 Km. Since the effective Doppler
velocity resolution and horizontal beam resolution are inversely
related for a fixed PRF and scan rate, one may compromise by
utilizing a factor of two. In this case the velocity resolution
would be approximately 3.3 m/s and effective beamwidth would be
2.8.degree.; i.e. less than 0.8 Km at ranges less than 15 Km.
Whatever known form of the Doppler processor takes in the radar
system, e.g. analog filters, digital filters, a Fast Fourier
Transform of Discrete Fourier Transform, the number of equivalent
filters would have to be increased by the same factor as the dwell
time.
Because the boundes of the positive portion of the DDS must be
determined with some precision, the Doppler spectra must be
smoothed as much as possible with the allowable dwell time. This
can be accomplished in several ways, namely: (1) average spectral
estimates over several pulses widths either by decreasing the pulse
width or by decreasing the net range resolution; (2) average
spectral estimates from the same position on successive antenna
scans; or (3) decrease the antenna scan rate to increase the dwell
time. Unfortunately, the last approach is the least permissible
because it tends to degrade the update cycle for aircraft
tracking.
Velocity ambiguities or aliasing may occur in either or both the
low and high beam Doppler spectra. However, except in the case in
which the spectra are so wide as to cover the entire unambiguous
velocity range given by Eq. (12), there should be little difficulty
in resolving such ambiguities in the Difference Doppler Spectrum
because the DDS will still have the proper polarity even where
aliased. Accordingly, the aliased portion of the DDS may be
properly located in the velocity domain.
As can be seen with reference to FIGS. 1A and 1B when a microburst
downdraft approaches the surface, it generally spreads out or
diverges in all directions, although the diverging flow may not be
circularly symmetrical. This means that the Doppler velocity will
generally change signal upon crossing the axis of the microburst.
However, this is not a necessary condition if the microburst is
embedded in a strong background wind field. In that case, the
outflow which opposes the wind direction will simply decrease the
magnitude of the wind; however, the change in speed from one side
of the microburst to the other, or the gradient, will be the same
as that in the absence of a background wind. In short, a critical
signature of the microburst and its low level wind shear is the
gradient of speed as indicated by the radial change in the near
surface Doppler velocity, the latter being determined in the manner
previously described.
Referring back briefly to FIGS. 2A, 2B and 2C, there is depicted a
microburst as observed by Wilson, et al. as set forth in the
aforementioned publication. A dual Doppler radar system was used
which permitted the determination of the vector velocity at each
point in the area observed essentially simultaneously by both
radars. Note that these vector winds have been computed from the
Doppler observed winds after subtracting the mean wind speed in the
environment. Also the horizontal cross-section at the top (FIG. 2A)
corresponds to heights between 50 and 100 m above the ground. The
vertical cross-section below, (FIG. 2B) corresponds to the section
AB in FIG. 2A, while that in FIG. 2C corresponds to section CD. It
can be seen that the maximum downdrafts of about 12 m/s are located
in section AB at a position of about 14 Km east of the CP-2 radar
and a height of 1 Km. Near the surface the horizontal speed is zero
on the axis of the downburst, and increases to about 6 m/s directed
oppositely on either side of the axis at a distance of about 1 Km.
Thus a radar beam directed along the vertical cross section A-B of
FIG. 2A would observe a v* signature such as that shown
schematically by curve 20 in FIG. 6A. Without subtraction of the
mean horizontal wind, the signature would simply be raised or
lowered; however, the gradient between the receding and the
approaching peaks would be preserved. In this case, it measures
approximately 6 m/s per Km. It should be noted that the velocity
profile (curve 20) in FIG. 6A actually corresponds to the mean
Doppler velocity observed with a narrow beam radar. With a wide
beam system such as an ASR-9, the radar beam might encompass the
entire 1.2 Km depth seen in vertical sections A-B (FIG. 2B) and C-D
(FIG. 2C). In such cases, one must employ the methods previously
discussed to estimate v*, the Doppler velocity near the
surface.
In addition, the Doppler spectrum breadth or the velocity width of
the positive portion of the difference Doppler spectrum would
generally be narrow on the axis of the microburst where the air
velocity is downward and perpendicular to the radar. On either side
of the axis one would find a broad Doppler spectrum as the air
velocity vectors turn from downward with zero radial component to
horizontal with near maximum radial component. With the typical
flow pattern believed to exist in a microburst, the maximum breadth
of the Doppler spectrum would be expected to occur on both sides of
the axis at a position closer to the axis than the peak Doppler
velocities shown by curve 20 in FIG. 6A.
A schematic signature .sigma..sub.v.sup.2 of the expected variation
in Doppler spread across the microburst is shown by the dashed
curve 22 in FIG. 6A. The combination of a sharp radial gradient
with a maximum Doppler breadth on either side of the axis is
expected to be an excellent signature of the microburst and low
level shear. Since strong flows cannot remain laminar near the
surface, one should expect some variations from the idealized
curves in FIG. 6A.
The breakdown of the high velocity outward flows near the surface
would be manifested in turbulent variations of wind with range and
azimuth and this low level turbulence could also be hazardous to
aircraft. It is for this reason that the present method also
includes a measurement of the root means square variations in v* in
both range and azimuth. In addition, there will be statistical
fluctuations superimposed upon both curves because weather echoes
are naturally noisy and their moments also fluctuate. However,
since radars which have narrow pulse lengths of about 1 microsec
are being hypothetically utilized, about 7 estimates of each of the
parameters per kilometer will be obtained. Thus the idealized
curves of FIG. 6 should be regarded as a 2 to 4 point running mean
of the unsmoothed estimates. Since ASR radars rotate rapidly, one
may also obtain new parameter estimates every 4 or 5 sec and
combine these with the previous estimates to provide more reliable
profiles. However, this process should not be extended for more
than 2 or 3 scans since the microburst may diverge very
rapidly.
It is clear that a variety of algorithms can be employed to
identify either or both signatures in FIG. 6A automatically. For
example, one alternative method is to take the radial derivative of
curve 20 shown in FIG. 6A. This is depicted as curve 24 in FIG. 6B.
This comprises a direct measure of the low level shear across the
microburst. If it exceeds a preset threshold, then an alarm may be
triggered. If the derivative is not yet strong enough to constitute
a hazard, a caution alarm may be initiated, thereby indicating the
possibility of a developing microburst. Similarly an auxiliary
algorithm may be established for the Doppler breadth signature. If
there are two peaks in the latter which exceed prescribed
thresholds and are located on either side of the microburst axis,
as indicated by the shear, then one may also set off an alarm.
Alternatively, one may require both conditions to be met before the
alarm is sounded.
Strong low level winds are often associated with turbulent
fluctuations of other winds which are potentially hazardous, e.g.
hurricanes, chinook or foehn winds, strong extratropical cyclones,
etc. The present invention therefore also contemplates methods to
measure variability of the near surface wind, v*, which is
associated with turbulence. This will be described
subsequently.
Additional signatures of a microburst include: (1) the difference
in the local vertical shear of the horizontal wind from that in the
surrounding environment; and (2) the rapid time changes in this
parameter as an incipient microburst develops and approaches the
surface. This is based on the observation that microbursts often
transport the horizontal momentum associated with the winds aloft
down toward the surface. Therefore, in addition to the observation
of the horizontal shear of the near surface wind associated with
the diverging microburst, there should generally be a sharp
difference in the vertical shear from that in the surrounding
environment. Since the quantity v(0)-v* is an approximate measure
of the vertical shear, a display of this quantity provides another
indicator of a downburst. Moreover, because the vertical shear of
the horizontal wind is likely to change rapidly during the
development of an incipient microburst, a method of monitoring the
time changes in the vertical shear is also included as part of this
invention, thereby providing an earlier warning of an imminent
downburst than may be available from the horizontal shear of the
low level wind itself.
Block diagrams of four embodiments of apparatus for implementing
the method described are shown in FIGS. 7A-8B. In FIG. 7A, there is
disclosed a single transmitter 30 delivering radiant energy to two
feeds 32 and 34 to form two beams 36 and 38 simultaneously on a
single antenna 40. Both beams are broad but one beam 38 has a peak
gain at a low elevation angle and the other beam 36 at a higher
elevation. One key of this invention is that the low beam 38 has
higher gain than the high beam 36 at low elevation angles and
conversely at the higher elevation angles so that there is an angle
of equal gain G.sub.H =G.sub.L in the vicinity of 3.degree. to
6.degree. of elevation. Each beam returns its signals to respective
receivers 42 and 44, and separate Doppler processing chains.
The two receivers 42 and 44 are identical, are linear and have wide
dynamic range to encompass meteorological targets over a
reflectivity range of at least 70 dBZ. The entire
transmitter-receiver chain, moreover, is coherent so that one may
detect the phase changes corresponding to moving targets at the
output of the IF amplifier, not shown, in the receivers and produce
inphase (I) and quadrature (Q) components of the complex signals by
IF quadrature video detectors 46 and 48 which are converted to
digital signals by the analog to digital converters 50 and 52. The
I and Q components corresponding to each and every range bin of the
high and low beams 36 and 38 are sent to Doppler processors 54 and
56 for signal processing. Illustrative outputs are the first and
second moments of the Doppler spectrum corresponding to the mean
Doppler velocity v.sub.L and v.sub.H and the spectrum variance
.sigma..sup.2 v.sub.L and .sigma..sup.2 v.sub.H. The most important
outputs of Doppler processors 54 and 56, however, are the entire
Doppler spectra S.sub.L (v) and S.sub.H (v) of the two beams 36 and
38. A Doppler processor is well known to those skilled in the art
and may take a variety of forms, a typical example being that of
the ASR-9 radar. An important feature of the Doppler processor
used, however, is that it must have a sufficient number of
equivalent filters to permit the spectrum to be determined with
adequate velocity resolution. This requires that the duration or
dwell time of the sequence of complex signals be sufficiently large
as discussed above. Further as shown in FIG. 7A, the Doppler
spectra S.sub.L (v) and S.sub.H (v) from the two Doppler processors
54 and 56 are fed to a computational block 58 where the difference
or ratio of the two Doppler spectra for each range bin is computed
and the difference Doppler spectrum (DDS) or ratio Doppler spectrum
(RDS) generated are thereafter fed into digital computer apparatus
60 for further processing. The data relating to the mean Doppler
velocities V.sub.L and V.sub.H and spectrum variance
.sigma..sub.vL.sup.2 and .sigma..sub.vH.sup.2 are also fed to the
computer 60. The computer 60, among other things, operates to
determine the measure of the radial gradient or derivative of the
resulting spectrum output of the block 58 to provide, for example,
a measure of wind shear. A suitable display 62 and alarm apparatus
64 are coupled to the computer 60 to provide an indication on a PPI
display and running account of the location of a storm cell
including microbursts, wind shear and the like, as well as the
associated intensities. Since information is available concerning
the mean Doppler velocities V.sub.L and V.sub.H, the computer 60
also computes the difference therebetween to provide an approximate
measure of the vertical shear between the reflectivity weighted
boresight axes of the beams 36 and 38 as well as providing a
confirmatory indication as to whether or not v* is greater or less
than v(0) and thus aids in determining whether the wind increases
or decreases as a function of altitude.
In order to compensate for a vertical gradient in reflectivity, for
example, the embodiment shown in FIG. 7A includes means for
implementing a normalization of the beam spectra S.sub.L (v) and
S.sub.H (v) mentioned above. Shown are means 66 and 68 for
measuring the low and high beam echo powers which are coupled to
video detectors 46 and 48, or alternatively to the outputs of the
A/D converters 50 and 52, through switches S1.sub.a and S2.sub.a.
The power outputs P.sub.L and P.sub.H are fed to an adder 70 which
generates an output of P.sub.L +P.sub.H. This output is fed to a
divider computational block 72 along with P.sub.L and P.sub.H to
provide power normalizing factors F.sub.L =P.sub.L /(P.sub.L
+P.sub.H) and FH=P.sub.H /(P.sub.L +P.sub.H). A first multiplier 74
couples to the S.sub.L (v) output of Doppler processor 54 along
with the output P.sub.H /(P.sub.L +P.sub.H) to form a normalized
output of S.sub.Ln (v) which is coupled to the computational block
58 by way of the switch S1.sub.b which is also ganged with switches
S1.sub.a as well as S1.sub.c. In one position of the ganged
switches S1, S.sub.L (v) is fed to the difference/ratio block 58
while in the other position, S.sub.Ln (v) is coupled thereto.
In the same manner, a second multiplier 76 couples to the S.sub.H
(v) output of Doppler processor 56 along with the P.sub.L /(P.sub.L
+P.sub.H) output from the divider 72. The multiplier 76 generates a
normalized spectrum output S.sub.Hn (v) which is coupled to the
block 58 which switch S2.sub.b is closed. Switch S2.sub.b is ganged
with switches S2.sub.a and S2.sub.c and operates together with
switches S1.sub.a, S1.sub.b and S1.sub.c so that the computational
block 58 either receives unnormalized or normalized spectra
depending on the position of the ganged switches which are set in
accordance with the desired operational mode.
Since it is important for system operation that it be possible to
distinguish the Doppler spectra observed with two radar beams, it
should be pointed out that this can be enhanced when desirable by
transmitting different microwave carrier frequencies while
radiating the beams simultaneously as shown in FIG. 7A or, when
desirable, different polarizations can be utilized for the high
beam 36 and low beam 38. For an application where different carrier
frequencies are to be utilized, it can be accomplished in a variety
of ways, one of which may be the use of a second transmitter shown
by reference numeral 66 and which is shown in FIG. 7A as a phantom
element. Another method could include transmitting a pair of pulses
in rapid sequence from the single transmitter 30 such that the two
pulses are radiated at different frequencies. Also a single
transmitter 30 may be utilized wherein the RF carrier frequency is
switched after a sequence of pulses. In any event, it is understood
that the frequency of the local oscillator with which the receive
signals are mixed, must be switched synchronously with the
frequency shifting of the transmitter. Where different
polarizations are utilized, the feeds 32 and 34 are polarized
differently. This technique is well known to those skilled in the
art.
Where, however, the two radar beams are radiated sequentially,
neither polarization or carrier frequency differentiation of the
two beams is required. Accordingly, a second embodiment is shown in
block diagrammatic form in FIG. 8A and is illustrative of apparatus
whereby the transmitter and receiver are alternately switched from
low beam to high beam as the antenna scans slowly and where fast
scanning antennas are utilized the switching is done on alternate
antenna rotations. Referring now to FIG. 8A, a three pole, two
position switch is included wherein switch section S3.sub.a is
coupled between a single transmitter 70 being alternately coupled
to the feeds 72 and 74 of an antenna 76 where, for example, the
feed 72 generates a high beam 78 while the feed 74 generates a low
beam 80. A single receiver and Doppler processor shown by reference
numeral 82 is shown coupled to a buffer storage 84 through the
switch section S3.sub.b and to an echo power measuring circuit 86
which has its output alternately coupled to a high beam storage 88
and a low beam storage 90 through the switch section S3.sub.c.
Further as shown in FIG. 8A, the buffer storage 84 which includes a
section 92, for the spectra of the high beam and a section 94 for
the spectra of the low beam, have their respective outputs S.sub.H
(v) and S.sub.L (v) coupled either to a difference or ratio
computational block 96 or a pair of multipliers 98 and 100 in a
normalizer section 102. The connection is made through two
sections, S4.sub.a and S4.sub.b of a four pole, two position switch
which also includes sections S4.sub.c and S4.sub.d which couple the
outputs from the multipliers 98 and 100 to the difference/ratio
computational block 96. It can be seen that in the first position
of the switch S4, the high and low beam spectra outputs are coupled
directly to the computational block 96, whereas in a second
position of the switch, the spectra are coupled to the multipliers
98 and 100 which also receive as inputs the multiplication factors
F.sub.H =P.sub.H /(P.sub.L +P.sub.H) and F.sub.L =P.sub.L /(P.sub.L
+P.sub.H) from a divider 104 which receives the inputs P.sub.H and
P.sub.L from the storage elements 88 and 90 and an adder 106 which
performs an addition of the values of P.sub.H and P.sub.L.
Thus where the beams 78 and 80 are operated sequentially, the
outputs of the Doppler processor 82 are switched to the low and
high beam sections 94 and 92 of the buffer memory 84 in synchronism
with the RF switch between the transmitter and the high and low
beams. The power measurement apparatus 86 alternately measures the
low and high beam echo powers depending upon which beam is
operating. These echo powers are alternately stored in the memory
sections 88 and 90 from which the adder and divider blocks 106 and
104 provide the power normalizing factors F.sub.H and F.sub.L in a
manner previously described with respect to the embodiment shown in
FIG. 7A. The difference/ratio computational block 96 provides
either a difference Doppler spectra (DDS) or ratio Doppler spectra
(RDS) for each range bin which are further operated upon by a
computer unit 108 which includes sufficient storage and processing
capabilities to identify the v* Doppler velocities at low levels
near the surface, display them in a sequential range format to
depict the radial shear and also to display the breadth of the
Doppler spectra as determined either by the Doppler processor or
the DDS, i.e. v(0)-v*. Accordingly, suitable display apparatus 110
and alarm devices 112 are coupled to the computer 108 in the same
manner as that shown in FIG. 7A.
Block diagrams of two alternative embodiments of apparatus for
implementing the method are shown in FIGS. 7B and 8B. In FIG. 7B,
transmitter 30 radiates its energy through only the low beam 38
when switch S5a is closed. However, there is sufficient energy
radiated by the low beam toward scatters at high elevation angles
that echoes from targets in the high beam may be received
simultaneously on both high and low beams. In this case the high
beam is operating in the bistatic mode such that its effective gain
function is as previously described.
In contrast, when switch S5.sub.b is closed and S5.sub.a is open,
the high beam 36 acts as a transmitting and receiving beam in a
monostatic mode and the low beam 38 receives in a bistatic mode. In
the latter mode, the gain of the high beam will be enhanced with
respect to the effective gain of the low beam. This will increase
the magnitude of the negative differential gain above the null and
decrease that of the positive differential gain below the null.
Accordingly, switching the transmitter 30 from low to high beam
will act predominantly to reduce the magnitude of the positive
portion of the DDS and increase that the negative portion. This
modulation of the DDS with switching can be used to enhance the
confidence with which one may identify the particular bounds of the
DDS in which one is interested. However, if the radar is to be used
simultaneously for aircraft surveillance, the preferred mode is to
transmit through the low beam. In either case, those echoes
returned via the low beam 38 and high beam 36 are channeled to
their associated receivers 42 and 44, respectively, and the
remainder of the embodiment operates as shown in FIG. 7A.
Corresponding to the sequential switching and signal storing
embodiment in FIG. 8A is the alternative approach shown in FIG. 8B.
There, energy from transmitter 70 is channeled either to the high
beam 78 or low beam 80 through switch S6 in a manner corresponding
to the embodiment of FIG. 7B. The echoes received via these beams
are then switched alternately to the receiver and Doppler processor
82 via switch S3.sub.a. Switch S3.sub.b which is ganged to S3.sub.a
then channels the corresponding spectral information to the high
beam and low beam buffer stores 92 and 94, respectively. The
outputs of the buffer store 84 are then processed further as was
illustrated in FIG. 8A.
Referring now to FIG. 9, there is shown a means which can be
utilized with either of the two embodiments shown in FIGS. 7 and 8
for measuring the differences in average reflectivity in the two
beams and monitoring the rate of change thereof as an
implementation for detecting a possible precursor of a downburst.
As depicted, a pair of multipliers 114 and 116, respectively, are
coupled to the echo power P.sub.H and P.sub.L along with a range
square signal R.sup.2 from a squarer computational block 118. The
output of the two multipliers 114 and 116 respectively comprises
the average reflectivities Z.sub.L =CP.sub.L R.sup.2 and Z.sub.H
=CP.sub.H R.sup.2, where C is a radar constant and R is range,
which are fed to apparatus 120 for determining the difference DZ
therebetween. The difference in the average reflectivities is
furthermore shown coupled to display apparatus 122 as well as a
pair of storage units 124 and 126 which are operable to store the
magnitude of DZ in successive scans N and N+1. The difference in
reflectivities stored are then fed into means for determining the
time difference as shown by reference numeral 130 whereupon the
time difference is fed to additional display apparatus 132 as well
as a threshold circuit 134 which is coupled to an alarm circuit
136.
Attention is now directed to FIG. 10, where there is shown the
sequence of steps in the method for computing the difference
Doppler spectrum (DDS). It should be noted, however, that the same
method is utilized for computing the ratio Doppler spectrum (RDS)
and thus can be substituted for DDS throughout the description to
follow. Step 140 simply shows the DDS exists for each range bin. In
step 142 the portion of the DDS which is positive and greater than
zero is determined. This identifies v(0), the velocity at the null.
This assumes that the high beam is subtracted from the low beam.
The converse procedure will also work except that the step 142
condition requires that one then finds the portion of the DDS which
is negative. Next in step 144 the other bound v* of the positive
portion of the DDS is determined. This is associated with the wind
speed near the surface. At the same time as step 114 the width of
the DDS corresponding to elevations below the two beam null is
determined as shown by reference numeral 146. As noted earlier, the
quantity v(0)-v* is an approximate measure of the vertical shear of
the horizontal wind between the surface and the height of the null.
The switch 148 at the output of step 146 indicates that the low
beam Doppler spread may also be used as an alternative measure of
this shear. Next steps 150 and 152 indicate that the values v*
(positive or negative) at all range binds for each beam on the N
and (N+1) scans are stored. Assuming that the scans are not
displaced more than a few seconds so that the observed phenomenon
does not change excessively between scans, the v* values on scans N
and N+1 for the same ranges and azimuths are averaged per step 154
to provide a smoother range profile of v* than is likely to be
available on a single scan. Following this, step 156 provides
additional smoothing by taking a running average of v* over a few
range bins under the condition that the range resolution after
averaging remains better than about 300 m. It should be noted that
narrower transmitted pulses may be used to provide more independent
measurements of v* and reasonable range smoothing while retaining
high range resolution. Either steps 154 or 156 may be omitted if
the signal dwell time is sufficient and there are a sufficient
number of equivalent Doppler filters in the Doppler processor to
provide high quality Doppler spectra. The radial derivative of v*
or the shear is next provided per step 158 followed by a display of
the radial shear as shown by reference numeral 160 on a contour
mapped or coded display such as a plan position indicator (PPI) or
equivalent display thereby allowing an observer to determine
quickly where the radial shear is excessive. Step 162 submits the
radial shear to a preset threshold and triggers an alarm when the
threshold shear is exceeded as shown by 164. Simultaneously, it
cause the position of the large shear to be displayed digitally in
R, .theta. or equivalent coordinates in step 166. To allow for the
occurrence of a number of microbursts simultaneously, the alarm
display must have the capacity to display a multiplicity of such
positions. The alarm positions are communicated automatically for
example, to all pertinent airport surveillance displays and to the
air traffic controllers in the tower. When desirable, the display
apparatus may be replaced by or include machine controlled
algorithms; e.g. artificial intelligence systems.
The Doppler width signal is also presented on a PPI or equivalent
display per step 168 and is indicative of either the vertical shear
of the horizontal wind in the low levels, or tornado vortices or
turbulence intensity when the latter are present. Because all these
phenomena affect both v(0)-v* and the low beam Doppler spread, the
Doppler width display is ambiguous and requires additional
signatures or human interpretation. Nevertheless since tornado
vortices and intense turbulence are also hazards, when the Doppler
spread exceeds a predetermined threshold (step 170) an alarm (step
172) is also triggered. The positions of these alarms are also
indicated in R, .theta. or equivalent coordinates on all relevant
displays per step 174.
It will become clear at a later date whether either the radial
shear alarm or the Doppler width signature alarm is sufficient in
and of itself. If the simultaneous occurrence of both enhances the
probability of detection and minimizes false alarms, then it is
evident that one may include an additional step (not shown) which
requires coincidence of the two alarms within a prescribed R,
.theta. window.
Although it has not been mentioned up to this point, the
availability of the near surface winds at all ranges and azimuths
also permits the detection of the azimuthal or tangential shear of
the radial wind. This is also indicative of microbursts, of
mesocyclones, and of tornado vortices. Since these are also hazards
both to flight safety and the general public, the flow chart of
FIG. 10 further includes step 175 which calls for a buffer memory
to store the v* values on adjacent beams, J and J+1, and if
desired, on still other additional beams. Following step 175, the
tangential derivative of v* is determined by step 176 and displayed
on a PPI or equivalent display as shown by step 177. Appropriate
signature recognition algorithms may also be utilized to identify
the nature and intensity of the phenomenon responsible for the
observed shear.
In the case of a microburst, the measured radial shear will be
maximized along the direction of the radar beam which coincides
with the axis of maximum divergence as shown by the long axis of
the elliptically shaped outline in FIG. 1B. Along this direction
and immediately adjacent thereto, there will be small tangential
shear. In contrast, on the left side of the microburst boundary
shown in FIG. 1B, the wind component will shift from V.sub.L which
is normal to the major axis, to the environmental wind outside the
microburst. This will also occur on the right side. Accordingly,
these lateral boundaries will display minimal radial shear and
large tangential shear to a radar which is located some distance
away along the direction of the major axis of the ellipse.
Accordingly, the display which is highly likely to depict the
nearly complete boundary of the microburst gust front is that which
plots the sum of the signals corresponding to the radial shear
which exceeds a threshold and the tangential shear which exceeds a
threshold. Both thresholds should be adjustable until experience
demonstrates the levels at which one obtains the most complete and
coherent boundary which is discernable from noise.
It should be noted that the combination of the two shear exceedence
thresholds will also provide one of the most reliable displays of
any frontal zone regardless of the relative orientation of the
frontal boundary to the radar.
Based upon the above description, the output of the tangential
shear is therefore passed to a threshold step 178. The output of
step 162 corresponding to the regions in which the radial shear
exceeds the threshold 162 is then combined with step 178 after
having determined the corresponding boundaries to provide the
combined shear boundary display 179.
The combined shear boundary display may be either a storage display
in which the sequence of successive positions of the microburst
boundary as seen at successive intervals may be depicted, or it may
be a storage and playback device in which the successive positions
of the expanding boundary or other frontal zones may be played back
in accelerated time lapse mode or animation.
It is emphasized that the process of displaying a pattern such as
the ellipse-like boundary of the microburst produces a significant
enhancement of the effective signal to clutter ratio which was a
problem of concern earlier with regard to the detection of the
weakly reflective dry microbursts. Pattern recognition permits the
analyst to detect the phenomenon even though the region being
mapped may contain a distribution of strong clutter echoes. This is
particularly relevant in the case of the areas immediately
surrounding airport runways which must be clear of obstacles and
are therefore free of major ground clutter.
Furthermore the process of either displaying the sequence of
evolving microburst boundaries or the rapid time lapse playback of
their successive positions enhances the effective signal to noise
and signal to clutter ratio even more dramatically. Thus the
methods just described should be exceedingly powerful in detecting
even weakly reflecting hazardous small scale phenomena with
confidence, in minimizing false alarms, and in projecting the paths
of the disturbances. It should be noted that pattern recognition
may involve either human cognitive capabilities or artificial
intelligence systems.
FIG. 11 also indicates additional steps by which turbulence of the
near surface wind speed v* can be identified. Step 180 calls for
storing all the values of v* over an entire scan. In step 182, v*
in all R+.DELTA.R, .theta.+.DELTA..theta. windows are averaged
where the size of the window is determined by the number of range
bins N and number J of azimuths averaged. Next the RMS variation of
v* over the selected sub-sectors as a measure of the turbulence
intensity is computed as indicated by step 184. This is followed by
step 186 which turbulence intensities in PPI form or the equivalent
are displayed. Steps 188 and 190 comprises the steps associated
with selecting and triggering the desired turbulence intensity
threshold and alarm.
As a microburst, mesocyclone, or tornado develops and builds
downward, the variations with time of the radial shear, the
tangential shear, and spectral breadth, and the turbulence will all
change rapidly at the location of the phenomenon in question.
Similarly, the vertical shear of the low level wind as determined
from the bounds of the positive portion of the DDS will also vary
rapidly. Thus, the early warning of incipient hazards will depend
upon algorithms, displays and alarms which indicate rapid changes
in the above listed parameters during a sequence of scans partly as
already described above. Although not shown, another set of steps
in the method by which such changes may be detected with time are
to store the full plan map of each parameter on scan M and to
compare it to the values of the same parameter on scan M+1. A new
plan map is then plotted and displayed indicating only those
positions at which the parameter has changed by an amount exceeding
a prescribed increment, either positive or negative. Since there
will be both natural and statistical fluctuations in this process,
the process may be repeated on scans M+2, M+3, etc., thereby
mapping only those places at which the change is persistent and in
excess of a prescribed value.
The parameters which should be examined in this way are v*, the
radial and tangential gradients of v*, the vertical shear v(0)-v*,
or low level Doppler spread, the turbulence intensities, and the
difference in the echo powers in the two beams. It should be noted
that in the case of a descending and evolving microburst, an
expanding ellipse corresponding to the diverging front of the
microburst should be discernible and should appear in most of the
above listed parameters as described earlier for the combined shear
display. An increased measure of confidence in sounding an alarm
may be provided by requiring the time changes of two or more of the
parameters to occur essentially simultaneously and in the immediate
proximity of one another.
In the case of downbursts, the axis of the downburst generally has
been found to be coincident with a reflectivity maximum. In the
case of wet downbursts, i.e. those accompanied by rain, its onset
will tend to be marked by reflectivity increasing with height and
by a rapid descent of the high reflectivity zone aloft. Thus, any
rapidly descending region of high reflectivity is a potential
indicator of the downburst. Accordingly, the echo power may be used
as a proxy for the beam weighted reflectivity; alternatively the
echo power may be normalized according to the square of the range,
i.e. PR.sup.2 which is proportional to the beam weighted or
equivalent reflectivity Z.sub.e. One may form the difference
between Z.sub.e in the high and low beams and store the vertical
difference parameter (Z.sub.eL -Z.sub.eH) on scan M and compare it
with the successive echoes on scans (M+1), (M+2), etc. as described
in the previous section for all the other parameters. Then only the
time changes in this parameter are displayed as previously
described. The rapid change of this parameter should provide an
early indication of an incipient downburst, especially when
accompanied by one or more of the signatures previously described.
Of course, either a flashing marker or an audible alarm may be used
to indicate time changes exceeding a preset threshold.
As noted earlier, another approach toward the measurement of low
level wind speed with a dual wide beam Doppler radar is to utilize
differential polarization to discriminate the echoes on the lower
beam from those on the upper beam. For example, if horizontal
polarization is used on the low beam and vertical polarization on
the high beam, it is possible to obtain approximately 15 to 20 dB
of isolation between the two beams depending upon the polarization
purity of the antenna and the shape and orientation of the
scatterers. In any case, the Doppler spectra on each of the beams
can be obtained by detecting only those signals which are
appropriately polarized. The difference between the Doppler spectra
will then be similar to that expressed by Eq. (6), with the
exception that the reflectivity factor may be polarization
dependent. Eq. (6) therefore becomes
where it is understood that the low and high beams are polarized
differently and the subscripts L and H on Z indicate the
corresponding beams and polarizations. Of course, it is not
necessary that the polarizations be linear vertical and horizontal.
They may be chosen to be of any suitable form which provides
discrimination and separation of the high and low beam signals upon
reception.
The crucial advantage of differential polarization on the two beams
is that it provides an additional means to distinguish the echoes
on the low beam from those on the high beam. Because rain has a
slightly larger reflectivity for horizontal polarization than for
vertical polarization, it is desirable to use horizontal
polarization on the low beam and vertical polarization on the high
beam.
In stratiform storms, the melting layer also generally shows higher
reflectivity for horizontal polarization. However, in general, one
does not expect to find a well defined melting layer in convective
storms which produce most of the hazardous phenomena with which we
are concerned. Thus, it is likely that the use of horizontal
polarization on the low beam will enhance the effective DG between
the two beams. At all heights below the null, the ratio of S.sub.L
(v) to S.sub.H (v) should exceed unity (DDS>0) simply because
the upper beam is insensitive to horizontal polarization and
conversely. Moreover, at the null level where G.sub.L =G.sub.H, the
spectral density in the two differentially polarized beams should
be roughly equal unless a layer of oriented particles as in the
melting layer is present. In that case, one would expect the echoes
from that layer to enhance the horizontally polarized lower beam if
that beam encompasses the melting zone at the ranges of interest.
Therefore, the use of differential polarization in conjunction with
differential gain enhances the methods of detection previously
described. In the event that the beam discrimination of
precipitation and other meteorological targets is accomplished by
differential polarization, it would not be possible to use circular
polarization simultaneously on both beams unless one is left hand
circular and the other is right hand circular. It is also possible
to discriminate between the low and high beams by use of
polarization modulation in a distinctive code on a series of
pulses.
It should be apparent that differential polarization should not be
used when either the high or low beam is used in a bistatic mode as
in FIGS. 7B and 8B. If this were done, then the receiving beam
would be virtually insensitive to the polarization of the
transmitting beam.
The method of identifying the range of wind velocity components
above and below the null of a two beam Doppler system may
furthermore be extended to three or more beams of the use of two
beams which are sequentially stepped in elevation angle to provide
more explicit information concerning the vertical profile of winds.
The essence of this approach is illustrated in FIG. 12. In FIG. 12,
three beams 1, 2 and 3 are depicted. Alternatively beams 2 and 3
may be regarded simply as beams 1 and 2 which have been elevated.
Whether just two beams are employed and elevated in sequence or
three or more beams are used simultaneously depends mainly upon the
speed with which one wishes to accomplish the measurements;
otherwise the principles are the same.
It is to be noted that three nulls exist at which the gains of the
various pairs of beams are equal. Using the two beam method
previously described, three null velocities v.sub.12 (0), v.sub.23
(0), and v.sub.13 (0) can be determined where the subscripts
indicate the corresponding number of the beams involved. Using
beams 1 and 2, v* from the bounds of the corresponding DDS can also
be determined where as noted earlier, v* is the near surface wind
component. Similarly, by using the negative portions of the DDS
between beams 2 and 3, the velocity range at heights above the null
2/3 can be determined. In addition, the non-null bound of that
negative portion, designated as v** which is the wind velocity at a
height near the upper reaches of beam 3 can also be determined. In
this way, an approximate profile of the radial wind components can
be generated. It is obvious that the method may be extended to more
beams or to more heights by using just two beams in the sequential
stepping mode.
Because of the many advantages of the rapid update cycle of a fast
scanning antenna system, including enhanced detection confidence
through pattern recognition and time lapse playback, and the
further benefits of obtaining 3-dimensional information such as the
vertical shear and reflectivity profile, it becomes clear that one
may enhance the overall system performance even more by the use of
a vertical stack of narrow pencil beams. A set of several beams
(e.g. 3 to 6), each with its own receiver could cover the lowermost
5.degree. or 6.degree. of elevation angle. The transmitting beam
could be a single vertical fan beam which just envelopes the stack
of receiving beams. Alternatively, the power may be distributed
among the plurality of narrow beams, or each may use its own
transmitter. In any case, the use of a set of narrow beams would
allow the determination of the mean Doppler velocity, the Doppler
spectral breadth, and the reflectivity at all elevations and ranges
simultaneously. Then the mean Doppler velocity on the lowermost
beam can be utilized in every instance in lieu of the near surface
velocity v* obtained by the previously described methods. At the
same time, the vertical shear would be determined from the profile
of mean Doppler velocities on all the beams and similarly for the
reflectivity profile. One may operate on these parameters in the
manner previously described.
Some of the major advantages of the latter approach are: (1) to
achieve more accurate measurement of the surface air velocity and
all the shear components with the narrow beams and the longer dwell
time which is available because a single narrow beam is not
required to scan a 3-dimensional volume; (2) to be able to reduce
antenna side lobes and enhance clutter rejection; (3) to permit use
of the time changes of the more accurate vertical shear and
reflectivity gradients as precursors of hazardous events; (4) to
enhance effective signal to clutter ratios through pattern
recognition and a time lapse playback; and (5) to increase overall
sensitivity. Except for the increased costs involved in replicating
a plurality of beams, receivers, and data processing systems, the
multiple stacked narrow beam approach would be a desirable mode if
one were not required to use an existing radar or otherwise
constrained.
Accordingly, what has been shown and described is a method aimed
primarily at the detection and mapping of the position and
estimating the intensity of microbursts and the associated low
level wind shear which are particularly hazardous to aircraft in
takeoff and landing as well as detecting the position and intensity
of other weather phenomena such as gust fronts, mesocyclones and
tornado vortices which are associated with significant changes in
which velocity with range and/or azimuth.
Although this invention has been disclosed with a certain degree of
particularity, it should be noted that the same has been made by
way of illustration and not limitation. Accordingly, all
modifications, alterations, and changes coming within the spirit
and scope of the invention as defined in the following claims are
herein meant to be included.
* * * * *