U.S. patent application number 10/784686 was filed with the patent office on 2004-12-16 for measurement of air characteristics in the lower atmosphere.
Invention is credited to Martin, Andrew Louis.
Application Number | 20040252586 10/784686 |
Document ID | / |
Family ID | 25646784 |
Filed Date | 2004-12-16 |
United States Patent
Application |
20040252586 |
Kind Code |
A1 |
Martin, Andrew Louis |
December 16, 2004 |
Measurement of air characteristics in the lower atmosphere
Abstract
Sodar systems and methods for acoustically sounding air are
disclosed in which chirps longer than 300 ms--and preferably with
durations of tens of seconds--are used along with matched filter
and/or Fourier processing methods to derive phase signals
indicative of air characteristics in range. A listen-while-transmit
strategy is preferred, the direct signal being removed by
subtracting the phase signals from two or more receivers located
near the transmitter so as to be in the same noise environment. The
resultant differential signals can be related to cross-range wind
with range distance. In one example, apparatus (100) is employed
comprising a reflector dish (102) over which one central
loudspeaker (110) and four microphones (112, 114, 130 and 132) are
mounted, the microphones preferably being located on cardinal
compass points and having their axes (124, 126) slightly angled
with respect to the vertical transmission axis (122).
Inventors: |
Martin, Andrew Louis; (Ferny
Creek, AU) |
Correspondence
Address: |
HOVEY WILLIAMS LLP
2405 Grand Blvd., Suite 400
Kansas City
MO
64108
US
|
Family ID: |
25646784 |
Appl. No.: |
10/784686 |
Filed: |
February 23, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10784686 |
Feb 23, 2004 |
|
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PCT/AU02/01129 |
Aug 19, 2002 |
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Current U.S.
Class: |
367/89 |
Current CPC
Class: |
G01S 15/885 20130101;
G01W 2001/003 20130101; G01N 2291/045 20130101; G01N 29/2456
20130101; G01S 15/582 20130101; G01S 15/87 20130101; G01N 2291/103
20130101; G01N 29/46 20130101; G01S 15/104 20130101; G01P 5/24
20130101; G01N 29/02 20130101; G01N 2291/012 20130101 |
Class at
Publication: |
367/089 |
International
Class: |
G01S 015/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 23, 2001 |
AU |
PR 7203 |
Sep 21, 2001 |
AU |
PR 7832 |
Claims
1. A method for acoustically sounding air over a range that extends
away from an acoustic transmitter and receiver, the method
comprising the steps of: transmitting an acoustic chirp comprising
coded pulses having pulse compression waveforms and having a
duration of at least 300 ms down-range, using the receiver to
detect acoustic inputs and to generate a receiver output that is
representative of said inputs, and processing said receiver output
to generate signal phase data indicative of air characteristics in
the range.
2. A method according to claim 1, wherein said step of using the
receiver to detect acoustic inputs includes detecting echoes
returned by the chirp while the chirp is still being
transmitted.
3. A method according to claim 1 including the steps of: using the
receiver means to detect first acoustic inputs, including echoes
returned in a first direction from the chirp, to generate a first
receiver output related to said first inputs, using the receiver
means to detect second acoustic inputs, including echoes returned
in a second direction from the chirp, to generate a second receiver
output related to said second inputs, generating, using at least
one of Fourier and matched filter techniques, a first phase signal
comprising phase-related components from said first receiver
output, generating, using at least one of Fourier and matched
filter techniques, a second phase signal comprising phase-related
components from said second receiver output, manipulation of said
first and second phase signals to generate data relating air
characteristics in range.
4. A method according to claim 3 wherein said manipulation includes
the step of: adding said first and second phase signals to generate
a first additive phase signal that emphasizes common components of
said first and second phase signals indicative of down range air
movement and to reduce components of said phase signals indicative
of cross-range air movement.
5. A method according to claim 4 wherein said manipulation includes
the step of: subtracting from said first additive phase signal a
reference phase signal indicative of system phase noise.
6. A method according to claim 3 wherein: said first acoustic
inputs include a first direct-chirp signal received direct from the
transmitter having substantially no echo component, said second
acoustic inputs include a second direct-chirp signal received
direct from the transmitter having substantially no echo component,
and said manipulation includes subtracting said first and second
phase signals to generate a phase output signal that is
substantially free of said first and second direct-chirp
signals.
7. A method according to claim 3 wherein said manipulation
includes: removing phase signal components that are common to said
first and second phase signals and that are at least in part due to
system noise and to acoustic noise that is common to said first and
second acoustic signals.
8. A method according to claim 3 wherein: said first and second
directions are inclined substantially equally and oppositely to one
another and fall substantially in a first plane that extends
cross-range, and said manipulation of the first and second phase
signals generates data indicative of cross-range air movement
within or parallel to said first plane.
9. A method according to claim 3 including the steps of: using the
receiver to detect third acoustic inputs, including echoes returned
in a third direction from the transmitted chirp, to generate a
third receiver output related to said third inputs, using the
receiver to detect fourth acoustic inputs, including echoes
returned in a fourth direction from the chirp, to generate a fourth
receiver output related to said fourth inputs, generating, using at
least one of Fourier and matched filter techniques, a third phase
signal comprising phase-related components from said third receiver
output, generating, using Fourier techniques, a fourth phase signal
comprising phase-related components from said fourth receiver
output, and manipulation of said third and fourth phase signals to
generate data relating air characteristics in range.
10. A method according to claim 9 wherein: said third and fourth
directions are inclined substantially equally and oppositely to one
another and fall substantially in a second plane that extends
cross-range, and said manipulation of the third and fourth phase
signals generates data indicative of cross-range air movement in
said second plane, and said manipulation of the third and fourth
phase signals generates data indicative of cross-range air movement
within or parallel to said second plane.
11. A method according to claim 10, wherein said first plane and
said second planes are substantially orthogonal to one another, and
said manipulation including the steps of: differencing said first
and second phase signals to remove phase signals common thereto and
to generate first differential phase components indicative of air
movement in or parallel to said first plane, differencing said
third and fourth phase signals to remove phase signals common
thereto and to generate second differential phase components
indicative of air movement in or parallel to said second plane.
12. A method according to claim 11 wherein said manipulation
includes the step of combining the first and second differential
phase signals to generate phase signals indicative of at least one
of the bearing of cross-range wind relative to the down-range
direction and phase signals indicative of cross-range wind
shear.
13. A method according to claim 11 wherein: the range extends
substantially vertically from the transmitter and receiver means,
which are located near at or near the base of the range, the first
and second planes are substantially vertical, the first plane
extends cross-range in a north-south alignment, the second plane
extends cross-range in an east-west alignment, and said
manipulation of said first, second third and fourth phase signals
generates data indicative of the variation of the compass bearing
and velocity of cross-range air movement.
14. A method according to claim 9 wherein said manipulation
includes the step of: adding said first, second, third and fourth
phase signals to generate a second first additive phase signal that
emphasizes common components of said first, second, third and
fourth phase signals indicative of down range air movement and to
reduce components of said phase signals indicative of cross-range
air movements.
15. A method according to claim 14 wherein said manipulation
includes the step of: subtracting from said second additive phase
signal a reference phase signal indicative of system phase
noise.
16. A method according to a claim 1 wherein the chirp is a positive
or negative linear acoustic signal that has an increasing or
decreasing phase or frequency, or wherein both positive and
negative linear chirps are employed.
17. A method according to claim 16 including the steps of:
transmitting positive and negative chirps in sequence, deriving
respective positive and negative versions of said receiver outputs,
processing said positive and negative receiver outputs to generate
corresponding positive and negative signal phase data, differencing
said positive and negative signal phase data to generate third
differential data indicative of variation of air temperature with
range distance.
18. A method according to claim 16 including the steps of:
simultaneously transmitting positive and negative chirps that do
not employ the same acoustic tones, deriving respective positive
and negative versions of said receiver outputs, processing said
positive and negative receiver outputs to generate corresponding
positive and negative signal phase data, differencing said positive
and negative signal phase data to generate fourth differential data
indicative of variation of air temperature with range distance.
19. A method according to claim 17 including the step of:
differentiating said respective third or fourth differential phase
signal to derive a gradient signal that is indicative of the
variation of air temperature with range distance.
20. A method according to claim 1 wherein said processing step
includes generating ambient signal phase data in the absence of an
air disturbance at a location, and including the steps of:
generating disturbance signal phase data in the presence of the
local air disturbance at said location, and using said ambient
signal phase data to normalize the disturbance signal phase data
and to thereby generate normalized signal phase data.
21. A method according to claim 20 including the steps of:
correlating successive samples of said normalized phase data
against multiple Doppler values to generate data indicative of wind
speed with respect to distance.
22. A method according to claim 1 wherein signal amplitude data is
generated and used together with said signal phase data.
23. A method according to claim 1 wherein the duration of the chirp
is greater than five seconds.
24. A system for acoustically sounding air over a range that
extends away from an acoustic transmitter and receiver: a
transmitter adapted to transmit an acoustic chirp comprising coded
pulses having pulse compression waveforms and having a duration of
at least 300 ms down a range that extends away from the
transmitter, a receiver located near said transmitter and adapted
to detect acoustic signals including echoes of the transmitted
chirp returned from down-range and adapted to generate a receiver
output that is representative of said received acoustic signals,
and a digital signal processor for processing said receiver output
to generate signal phase data indicative of air characteristics in
the range.
25. A system according to claim 24 wherein: said receiver is
adapted to detect a direct non-echo signal from the transmitter
while it is transmitting, said direct signal contributing to said
receiver output.
26. A system according to claim 24 wherein: said receiver is
adapted to detect first acoustic inputs, including echoes returned
in a first direction from the chirp, and to generate a first
receiver output related to said first inputs, said receiver is
adapted to detect second acoustic inputs, including echoes returned
in a second direction from the chirp, and to generate a second
receiver output related to said second inputs, said signal
processor is adapted to receive said first and second receiver
outputs, process said outputs using at least one of a matched
filter and a Fourier processor, generate respective first and
second phase signals, and manipulate said first and second phase
signals to generate data relating air characteristics in the
range.
27. A system according to claim 26 wherein the signal processor,
when manipulating said first and second phase signals, is adapted
to: add said first and second phase signals to generate a first
additive phase signal that emphasizes common components of said
first and second phase signals indicative of down range air
movement and to reduce components of said phase signals indicative
of cross-range air movement.
28. A system according to claim 27 wherein the signal processor,
when manipulating said first and second phase signals, is adapted
to: subtract from said first additive phase signal a reference
phase signal indicative of system phase noise.
29. A system according to claim 26 wherein the signal processor,
when manipulating said first and second phase signals, is adapted
to: subtract said first and second phase signals to generate a
phase output signal that is substantially free of direct-chirp
signal components.
30. A system according to claim 26 wherein the signal processor,
when manipulating said first and second phase signals, is adapted
to: remove phase signal components that are common to said first
and second phase signals and that are inter alia due to system
noise and to acoustic noise that is common to said first and second
acoustic signals.
31. A system according to claim 26 wherein: said first and second
directions are inclined substantially equally and oppositely to one
another and fall substantially in a first plane that extends
cross-range, and the signal processor, when manipulating said first
and second phase signals, is adapted to generate data indicative of
cross-range air movement within or parallel to said first
plane.
32. A system according to claim 26 wherein: said receiver is
adapted to detect third acoustic inputs, including echoes returned
in a third direction from the transmitted chirp, to generate a
third receiver output related to said third inputs, said receiver
is adapted to detect fourth acoustic inputs, including echoes
returned in a fourth direction from the chirp, to generate a fourth
receiver output related to said fourth inputs, said signal
processor is adapted to use at least one of Fourier and matched
filter techniques to generate a third phase signal comprising
phase-related components from said third receiver output, said
signal processor is adapted to use at least one of Fourier and
matched filter techniques to generate a fourth phase signal
comprising phase-related components from said fourth receiver
output, and said signal processor is adapted to manipulate said
third and fourth phase signals to generate data relating air
characteristics in the range.
33. A system according to claim 32 wherein: said third and fourth
directions are inclined substantially equally and oppositely to one
another and fall substantially in a second plane that extends
cross-range, and said manipulation of the third and fourth phase
signals is adapted to generate data indicative of cross-range air
movement in said second plane, and said signal processor is adapted
to manipulate the third and fourth phase signals to generate data
indicative of cross-range air movement within or parallel to said
second plane.
34. A system according to claim 33, wherein said first plane and
said second planes are substantially orthogonal to one another, and
wherein: said signal processor is adapted to: difference said first
and second phase signals to remove phase signals common thereto and
to generate first differential phase components indicative of air
movement in or parallel to said first plane, and difference said
third and fourth phase signals to remove phase signals common
thereto and to generate second differential phase components
indicative of air movement in or parallel to said second plane.
35. A system according to claim 34 wherein said signal processor is
adapted to combine the first and second differential phase signals
to generate phase signals indicative of the bearing of cross-range
wind relative to at least one of the downrange direction and phase
signals indicative of cross-range wind shear.
36. A system according to claim 34 wherein: the range extends
substantially vertically from the transmitter and receiver, which
are adapted to be located at or near the base of the range, the
first and second planes are substantially vertical, the first plane
extends cross-range in a north-south alignment, the second plane
extends cross-range in an east-west alignment, and said signal
processor is adapted to manipulate said first, second third and
fourth phase signals to generate data indicative of the variation
of the compass bearing and velocity of cross-range air
movement.
37. A system according to claim 33 wherein said signal processor is
adapted to add said first, second, third and fourth phase signals
to generate a second first additive phase signal that emphasizes
common components of said first, second, third and fourth phase
signals indicative of down range air movement and to reduce
components of said phase signals indicative of cross-range air
movements.
38. A system according to claim 36 wherein said signal processor is
adapted to subtract from said second additive phase signal a
reference phase signal indicative of system phase noise.
39. A system according to claim 24 wherein the transmitter is
adapted to transmit a chirp comprising a positive or negative
linear acoustic signal that has an increasing or decreasing phase
or frequency, or wherein both positive and negative linear chirp's
are employed.
40. A system according to claim 39 wherein: the transmitter is
adapted to transmit positive and negative chirps in sequence, the
receiver is adapted to generate respective positive and negative
versions of said receiver outputs from acoustic input signals
including said positive and negative chirps and echoes thereof,
said signal processor is adapted to: process said positive and
negative receiver outputs to generate corresponding positive and
negative signal phase data, and difference said positive and
negative signal phase data to generate third differential data
indicative of variation of air temperature with range distance.
41. A system according to claim 39 wherein: said transmitter is
adapted to simultaneously transmit positive and negative chirps
that do not employ the same acoustic tones, said receiver is
adapted to generate respective positive and negative receiver
outputs, said signal processor is adapted to: process said positive
and negative receiver outputs to generate corresponding positive
and negative signal phase data, and difference said positive and
negative signal phase data to generate fourth differential data
indicative of variation of air temperature with range distance.
42. A system according to claim 40 wherein: said signal processor
is adapted to differentiate said (respective) third or fourth
differential phase signal to derive a gradient signal that is
indicative of the variation of air temperature with range
distance.
43. A system according to claim 24 adapted to: generate ambient
signal phase data in the manner claimed in the absence of an air
disturbance at a location, generate disturbance signal phase data
in the presence of the local air disturbance at said location, and
use said ambient signal phase data to normalize the disturbance
signal phase data and to thereby generate normalized signal phase
data.
44. A system according to claim 43 wherein said signal processor is
adapted to correlate successive samples of said normalized phase
data against multiple Doppler values to generate data indicative of
wind speed with respect to distance.
Description
RELATED APPLICATION
[0001] This application is a continuation of international
Application Serial No. PCT/AU02/01129 filed 19 Aug. 2002, published
under PCT Article 21(2) in English; and claiming priority from
Australian patent applications PR 7203 filed 23 Aug. 2001 and PR
7832 filed 21 Sep. 2001, and applicant claims the benefit of
Australian patent applications PR 7203 filed 23 Aug. 2001 and PR
7832 filed 21 Sep. 2001.
TECHNICAL FIELD
[0002] This invention relates to the use of acoustic signals for
atmospheric sounding and is particularly concerned with sodar
techniques for measuring air velocity variation--such as horizontal
wind speed variation, wind-shear and/or turbulence--in the lower
atmosphere. The invention may, however, be applied to measuring
local density variation in the atmosphere, such as may be caused by
temperature gradients, temperature, thermal inversions and
variations in moisture content.
[0003] The apparatus and methods of the intention are also
applicable to wind profiling in the vicinity of airports to enhance
air safety and/or permit higher density air traffic at airports.
The atmospheric sounding techniques of the invention belong to a
class of technology recently dubbed SODAR, or Sound Direction and
Ranging. Sodar is to be distinguished from sounding techniques
using electromagnetic waves, such as RADAR (Radio Direction and
Ranging), LIDAR (Light Direction and Ranging), AERI (Atmospheric
Emittance Radiance Interferometry) and the hybrid RASS (Radio
Acousitc Sounding Systems) in the atmosphere. However, common to
all these techniques in their current form is a concern with
Doppler signals and the use of Fourier transform methods in
processing such signals. While SONAR (Sound Navigation and Ranging)
has not been mentioned because it is employed in liquid media, some
overlap between the exclusively acoustic techniques of sonar and
sodar may be seen because sonar ranging and imaging methods have
been applied in air--as in some camera ranging, non-destructive
testing and medical imaging systems.
BACKGROUND OF THE INVENTION
[0004] Though exclusively acoustic methods for wind profiling and
the like have a long history, Coulter & Kallistratova in their
1999 article "The Role Acoustic Sounding in a High-Technology Era"
[Meteorol. Atmos. Phys. 71, 3-19] show that these methods have not
lived up to their promise. This appears to have been largely due to
an inability to achieve an adequate signal-to-noise ratio
[s/n].
[0005] Sodars for atmospheric sounding have almost universally
employ short (millisecond), single-tone high power pulses, multiple
receivers and simple timing circuits to determine the sequence of
echoes at different receivers needed to deduce the height of
various discontinuities in the atmosphere. U.S. Pat. No. 2,507,121
to Sivian [1950] disclosed a method for measuring the height of
atmospheric discontinuities that involved sending such a pulse
vertically into the atmosphere and, after cessation of the
transmitted pulse, detecting vertically returned echoes using two
similar receivers located near the transmitter. In the embodiment
of most interest, the first receiver is shielded against receiving
echoes but the second is not and the two receivers are connected so
that their outputs are opposed and the net signal can be displayed
on an oscilloscope. In the event of a normally returned echo, a pip
is displayed because only the second receiver detects a signal.
However, in the event of local noise such as a gunshot both
receivers detect the same signal and no pip is displayed.
[0006] U.S. Pat. No. 3,889,533 to Balser [1973] disclosed an
`acoustic wind sensor` in which an acoustic transmitter illuminates
a cylindrical column of air by either CW (continuous wave) or
pulsed signals and remote receivers are pointed at narrow or broad
portions of two or more sides of the side column to detect acoustic
energy scattered laterally therefrom. The Doppler components of
this scattered energy are then extracted to determine wind velocity
at the various heights. In order to observe a portion of the
illuminated column, which is--say--1000 m from the ground, the
receivers need to be spaced from the transmitter by a roughly
similar distance. Also, significant spacing is needed to
sufficiently attenuate the direct signal [sometimes called the
`zero Doppler` signal] from the transmitter. An application of this
system to the detection of persistent vortices near runways was
disclosed in U.S. Pat. No. 3,671,927 to Proudian and Balser.
[0007] U.S. Pat. No. 3,675,191 to McAllister [1972] disclosed the
use of four adjacent arrays of acoustic transducers capable of
being used as speakers and microphones, the arrays being aligned
with the cardinal points of the compass and being shielded from one
another, except at their upper faces. Short acoustic pulses were
transmitted vertically upwards and the relative timing of the
returned echoes at each of the four arrays gave the height and
bearing of wind layers. [It might be noted that the physics of
acoustic sounding was well documented in 1969 by McAllister and
others in "Acoustic Sounding--A New Approach to the Study of
Atmospheric Structure" in Proc. IEEE Vol. 57, 579-587.] U.S. Pat.
No. 4,558,594 to Balser disclosed the use of an acoustic phased
array capable of directing successive pulses in different
directions, the echoes from one pulse being detected by the array
before the next is transmitted. U.S. Pat. No. 5,521,883 to Fage et
al uses a similar phased array to send pulses of different
frequencies in different directions and then listen for all echoes
simultaneously, thereby decreasing the cycle time. The typical
angle of elevation for pulse transmission in the latter systems was
between 20 and 30 degrees. The relatively low elevation angle is to
enhance Doppler components in the returned echoes due to horizontal
(rather than vertical) wind speed in the direction of
interrogation.
[0008] In recent years, radar DSP (digital signal processing)
techniques have been applied to the sodar to achieve improved s/n.
In particular, pulse-compression techniques have been used, in
which the echoes from a phase or frequency coded pulse are
processed with matched filters using Fourier transforms to give the
range resolution normally associated with a shorter pulse with a
much higher peak power. Such coded pulses are said to have
`pulse-compression` waveforms or to be `pulse coded`. For
simplicity, pulses of this type will be called `chirps`. In an
article entitled: "Use of Coded Waveforms for SODAR Systems"
[Meteorol. Atomos. Phys. 71, 15-23 (1999)], S G Bradley recently
reviewed, with simulations, the use of radar pulse compression
techniques to improve amplitude discrimination in sodar. Examples
of the use of pulse compression techniques in radar can be found in
U.S. Pat. Nos. 6,208,285 to Burkhardt, 6,087, 981 to Normat et al,
and 6,040,898 to Mroski et al. Despite the application of such
sophisticated techniques to sodar, a review by Crescenti entitled,
"The Degradation of Doppler Sodar Performance Due to Noise"
[Crescenti, G. H., 1998, Atmospheric Environment, 32, 1499-1509],
found that severe problems remain even at modest ranges of 1500
m.
OUTLINE OF THE INVENTION
[0009] From one aspect, the invention comprises methods and
apparatus for acoustic sounding in air in which echoes from a
transmitted chirp (along with extraneous acoustic inputs) are
detected during transmission of the chirp. In other words, there is
`listening while sending`.
[0010] This avoids the need to limit pulse length to secure
near-range capability, which is essential in known pulsed sodars
that employ the `transmit then listen` strategy. For example if the
pulse length of a conventional sodar is one second, the first 170 m
of range will be lost because the receiver will be turned off for
the first second; a 10 s pulse will lose the first 1700 m of range.
Typically, therefore, pulsed sodars of the art employ pulses of a
few tens of milliseconds. By contrast, our chirps are of at least
300 ms duration and, preferably longer than 10 s; indeed, we have
used chirps of up to 50 s, the duration only being limited by our
current signal processing capacity. Preferably, the duration of the
chirp is at least 5% of the listening time; that is, there is at
least 5% overlap between chirp transmission and echo reception, but
it will be appreciated that listening time depends on the distance
range covered. For ranges up to a few km, we prefer chirp lengths
well over 50% of receive time. As a convenient guide, we listen for
about 6 s longer than the chirp for each km of range. Thus, in a
system with a 1 km range, the chirp/pulse duration might be 15 s
and the listening time 21 s; for a 2 km range, we might use a 31 s
chirp and listen for 43 s. Generally, we start listening at the
commencement of the chirp transmission to obtain data from ground
level up. For some applications however we may not want the ground
level data and choose to start listening some time after the end of
the chirp transmission.
[0011] The longer the chirp, the higher its energy for a given
transmitter power and the better the echoes can be discriminated
using appropriate matched filter and/or Fourier techniques. It is
thus much easier to detect faint echoes behind the direct signal
from the transmitter with long, low power chirps than with
conventional short high-power pulses. The danger of receiver
overload is also mitigated by the use of modem microphones that
have a large dynamic range. Of course, acoustic shielding of the
receiver(s) from the direct transmitter signal is sensible.
[0012] Indeed, the improvement now possible with the use of long
chirps and matched filter techniques is such that, from another
aspect, the invention comprises methods and apparatus for acoustic
sounding in air in which the echoes from a chirp of greater than
300 ms are detected and processed using matched filter and Fourier
techniques. Either the `send then listen` or the `listen while
sending` strategy may be used. As already noted, chirps with
duration in the order of seconds are preferred; with chirp
durations of tens of seconds being favored in may situations.
[0013] From another aspect, the invention comprises methods and
apparatus in which multiple receivers are located near a common
transmitter so that each will receive echoes from each transmitted
chirp. Preferably, the receivers are located close enough to share
a common acoustic and system noise environment and, preferably,
they are arranged so as to receive the same direct signal (In both
frequency spectrum and amplitude). This allows received signal
components (eg, direct signal, ground clutter and noise) that are
common to more than one receiver to be to be efficiently removed by
differencing the signals from two or more receiver locations. Of
course, by `multi-receiver` we mean to include the situation where
a single receiver is moved to multiple receiver locations and where
a separate chirp is transmitted for each receiver location.
[0014] While the removal of the common unwanted direct signal, as
well as common noise components and ground clutter, is highly
desirable, it is very difficult to be done directly on the received
signals for a chirp that lasts tens of seconds. According to
another aspect of the invention, we employ multiple acoustic
receivers with a single transmitter and process the received
acoustic signals in the (Fourier) frequency domain using matched
filter techniques to generate a cumulative phase output for each
receiver signal and then manipulate these outputs to achieve the
appropriate measurement. Subtraction or differencing of the
cumulative phase signals eliminates the direct signal, common
noise, common ground clutter and the common signals due to
variation in vertical wind speed, the residual differential
cumulative phase then represents the variation of wind speed with
range distance. This overcomes a major problem with conventional
sodars, which cannot discriminate between returned Doppler signals
due to vertical wind speed without a direct measurement and those
due to horizontal wind speed.
[0015] However, comparison or differencing of two or more
cumulative phase signals requires a common starting or reference
point in the signals. This is conveniently the start of chirp
transmission, which can be determined by the start of the received
direct chirp or by an electronic signal from the transmitter.
However, many other methods of synchronizing the receiver signals
are possible. Thus, while it is desirable that the receivers are
located in a common acoustic environment in the vicinity of the
transmitter, it is not essential that they by equidistant from the
transmitter in order to ensure that the direct chirp arrives at
each receiver at the same time.
[0016] By transmitting two differently coded chirps (at the same
time, using two transmitters or one after the other using one
transmitter) the cumulative phase outputs can be manipulated to
remove all common signals, and components due to cross-range wind,
to allow generation of a further output that is indicative of
variation of the speed of sound with range and, thus, variation of
temperature with range. Preferably, the two chirps are identical
positive and negative linear phase chirps (eg, the positive one
rising from 800 to 1600 Hz and the negative one descending from
1600 to 800 Hz at the same phase rate.
[0017] Thus the last-mentioned aspect of the invention provides a
further large improvement in s/n, allowing much improved echo
discrimination with respect to the art, despite listening while
sending. Also, simultaneous echo reception and processing by
multiple receivers greatly improves cycle time.
[0018] A convenient arrangement of receivers in a system for
vertical atmospheric sounding is to locate one receiver at each
cardinal compass point around the transmitter and to slightly
incline opposed receivers toward or away from one another. Thus,
the phase components common to the N-S receiver signals are removed
by phase differencing to leave that associated with variation of
the net N-S wind over range distance. Systems of this type are
suitable for vertical sounding in noisy environments such as
airports, power stations and urban areas.
[0019] It will be appreciated that the invention is not limited to
the use of four receivers, or to vertical sounding systems or to
the symmetrical placement of receivers around a transmitter. The
receivers may be arranged in a line, for example across a runway
glide path with one or more transmitters to detect persistent
vortices caused by the passage of large aircraft. The high-speed
localized winds which can make up such vortices are difficult to
quantify because the high Doppler shifts of echoes generated have
considerable ambiguity. In this situation, another aspect of the
invention involves the repeated analysis of recorded echo signals
is for each range point using a matched filter that is fed with a
succession of different reference chirps so that, for each range
point, a reference chirp is found that generates a zero phase
gradient output (in the Fourier domain). That reference chirp is
then indicative of the wind speed at that range point. It is also
useful here (as well as in other applications envisaged by the
invention) to take `readings` in the absence of vortices to record
ambient wind and noise conditions and to subtract the associated
phase signals from those generated when there is a vortex
present
[0020] Whilst listening during sending is not essential for the
implementation of the last described aspects of the invention, it
is certainly desirable because it enables the use of long chirps,
better echo discrimination and the effective elimination of range
dead-zones.
[0021] The transmitted acoustic chirp can be generated by feeding a
commercially available loudspeaker (transmitting acoustic
transducer) with an electrical input signal from the sound card of
a computer (for example), while the echoes can be detected using
commercially available microphones (receiving acoustic
transducers). The loudspeaker and microphone(s) can be mounted with
separate concentrating reflectors (plates, homs, dishes or the
like) or they may be mounted with a common reflector. For example,
four microphones can be arranged in quadrature around a single
loudspeaker above a single reflector dish so that the transmission
axis is substantially axial with respect to the dish. Since the
microphones are then offset with respect to the axis of the dish,
the receiving axes of opposed pairs will be oppositely inclined
toward the transmission axis; that is, each receiver will be most
sensitive to echoes coming from a direction opposite to its
location on the dish with respect to the transmission axis.
[0022] An arrangement where a loudspeaker and multiple microphones
are mounted on a common structure allows the transmission axis to
be conveniently aimed or set as desired by moving the structure. A
system with a transmission axis of low elevation can be used, for
example, to detect or characterize vortices caused by large
aircraft landing or taking off at an airport it is even possible to
use airborne systems of this type to warn pilots of clear air
turbulence (CAT) that is difficult to detect using radar. For
example, a compact transmitter and receiver system could be mounted
in the nosecone of a large aircraft. Alternatively, only the
transmitter need be mounted in the nosecone since the receivers can
be mounted in a row along the leading edge of the wings.
[0023] As already noted, the receiving axis of a receiver may be
inclined with respect to the transmission axis and that, where
multiple receivers and signal differencing are used, it is
desirable that the axes of opposed receivers are equally inclined.
The optimum angle of inclination will depend upon the aperture of
the receivers, the range of the system and the desirability of
locating the receivers in a common acoustic environment. An angle
of 20 degrees in a system with a 3 km range is likely to place the
receivers too far from one another to have a common acoustic
environment, but this may not be so in a system with a range of
only 250 m. Angles of inclination of between about 2 and 10 degrees
have been found suitable, with angles between 4 and 7 degrees
preferred. It will be seen that the point of intersection of a
receiver axis with the transmission axis is not intended to be the
nominal range of the sodar system. Indeed, highly satisfactory
results have been obtained where medium aperture microphones are
located about 1 m from the loudspeaker in a common dish with their
receiving axes angled at about 4 degrees to the transmission axis.
In effect, the receivers of an opposed pair are looking for
wind-Induced Doppler signals from large illuminated areas on
opposite sides of the transmission axis but in the same plane as
the receiving axes and the transmission axis.
[0024] As already noted, it is desirable (but not necessary) to
space multiple receivers equidistant from and near to a common
transmitter so that each will be subject to the same ambient noise
(as well as other common components). Generally, the louder and
less uniform the noise environment, the nearer the receivers need
to be to one another to ensure that each is subjected to the same
environmental noise, as far as practicable. We have found that, in
a noisy environment, the distance between a receiver and the
transmitter should be of the order of meters. In a quiet
environment, it can be of the order of 10 m.
[0025] In general, the transmitted chirp should have a tonal range
(acoustical bandwidth) suited to the object being sounded. We have
found that wind-shear below 3000 m is best sounded at the lower end
of the audible range; for example, 500-5000 Hz, more preferably
between 800 Hz and 3 kHz and most preferably between 1.0 kHz and
2.5 kHz.
[0026] While the tones in a chirp can be modulated in frequency
and/or phase in many ways in conformity with pulse compression
techniques, we have found it convenient to use a linear chirp in
which the frequency increases or decreases smoothly and linearly
from start to finish of the chirp. Ideally, such a chirp has a
uniform rate of phase-shift. The use of positive and negative
linear chirps is of particular value in the reduction of unwanted
phase components in techniques for measuring air temperature
disclosed herein. Linear chirps are also easily generated and their
echoes convenient to process using available DSP and Fourier
techniques implemented using personal computers.
[0027] Although (as already noted) long duration chirps offer the
potential of high system processing gains (lower s/n), long chirps
also result in significant computational demands when using the
high signal sampling rates and the Fourier techniques needed to
achieve such gains. We have found that current readily available
FFT algorithms, DSP chips and PCs set a practical limit on chirp
duration of about 40-50 s at sampling rates of about 96 k Hz. This
typically represents some 1400 samples per m, given a range of 3000
m. Indeed, the computational demands are such that we prefer to
dedicate one PC to each receiver of a multi-receiver system so that
echo analysis for all receiver signals can proceed in parallel to
the point where signal differencing takes place. In the future,
developments in chips, FFT/matched filter techniques and PCs may
allow longer chirps to be processed using a single PC--or, much
faster updating times using the pulse lengths presently
achievable.
DESCRIPTION OF EXAMPLES
[0028] Having portrayed the nature of the present invention,
particular examples will now be described with reference to the
accompanying drawings. However, those skilled in the art will
appreciate that many variations and modifications can be made to
the chosen examples while conforming to the scope of the invention
as outlined above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] In the accompanying drawings:
[0030] FIG. 1 is a series of diagrammatic plan views showing
selected arrangements of transmitters and receivers, the
transmitters (loudspeakers) being shown as small shaded circles and
the receivers (microphones) being shown as small unshaded
circles.
[0031] FIG. 2 is a series of diagrammatic elevations showing
co-located and separately located transmitter and receiver
arrangements.
[0032] FIG. 3 is a diagrammatic sectional elevation showing the
arrangement of the transmitter and receivers of the first system
example.
[0033] FIG. 4 is a schematic plan of the system of FIG. 3 showing
the general manner in which signals to the transmitter are
generated and signals from the receivers are processed.
[0034] FIG. 5 is a block diagram illustrating a system for
extracting east-west phase information from the echoes received by
the east and west microphones of the chosen system example.
[0035] FIGS. 5A, 5B and 5C are block diagrams illustrating
respective parts of a circuit and process by which wind-speed,
wind-bearing and wind-shear information with respect to height can
be generated with the system of the first example.
[0036] FIG. 6 is a histogram depicting a typical digitized acoustic
signal detected by a receiver of the system of the first
example.
[0037] FIGS. 7A and 7B are graphs of typical signals before and
after the low-pass filter of the matched filter of the system of
the first example.
[0038] FIG. 8 is a series of graphs depicting the cumulative phase
information derived from all receivers of the system of the first
example.
[0039] FIG. 9 includes bar charts and graphs showing the
atmospheric wind characteristics output from the system of the
first example.
[0040] FIG. 10 is a block diagram illustrating portion of the
process by which temperature with respect to height is derived
[0041] FIG. 11 is a graphical representation of how phase signals
of positive and negative chirps are differenced to yield an
indication of temperature with respect to range distance.
[0042] FIG. 12 is a plot of temperature with respect to range
distance generated by the use of the disclosed system.
[0043] FIG. 13 is a block diagram illustrating portion of the
process by which vertical wind speed with respect to height is
derived.
[0044] FIG. 14 is a schematic diagram illustrating the detection of
wake vortices left by a large airplane near a runway.
[0045] FIG. 15 is a graph showing the notional variation of excess
phase with respect to range distance, this relationship being used
to compute wind velocity and height in awake vortex.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0046] As illustrated by the simple plan-view diagrams (a) to (i)
of FIG. 1, there are many possible ways in which the transmitter
and receivers of sodar systems envisaged by this invention can be
configured, and many others are possible. Diagram (a) shows a
convenient and economical configuration in which four microphones
10 are spaced within and around a common parabolic reflector dish
12 and a single loudspeaker 14 is located at the central focus of
dish 12. In this way, the amplitude lobe of the transmitted pulse
is vertical but the amplitude lobe each received echo is angled
slightly to towards the axis of dish 12 and loudspeaker 14.
[0047] Diagram (b) of FIG. 1 shows three microphones 16 evenly
spaced in a common dish 18 that also has a centrally located
loudspeaker 20. Diagram (c) shows a dish 22 with a central
loudspeaker 24 and only one offset microphone 26, dish 22 being
mounted so that it can be rotated to successively put microphone 24
in different positions [eg, those illustrated in (a) or (b)].
Diagram (d) shows four microphones 28 mounted in a common receiving
dish 30 that is separate and spaced from the associated loudspeaker
32, which is mounted in its own transmission dish 34. Optionally,
receiving dish 30 may have more or less than four microphones
located therein. Diagram (e) illustrates a configuration in which
each of four microphones 36N, 36S, 36E and 36W has its own
receiving dish 38N, 38S, 38E and 38W (respectively) and a single
loudspeaker 40 has its separate dish 42. Diagram (f) is similar to
(e) except only three microphones 44, 46 and 48 and their
respective dishes 50, 52 and 54 are deployed around a single
microphone 56 and its dish 58. Finally diagram (g) shows a
configuration in which a single microphone 60 and its dish 62 are
mounted so as to be rotatable around a single loudspeaker 64 and
its dish 66, so as to be able to simulate configurations such as
those of (e) and (f).
[0048] FIG. 1(h) is a plan view of a linear array of four pairs of
receivers 70a and 70b, 71a and 71b, 72a and 72b, and, 73a and 73b
arranged in a row with one receiver of each pair positioned on
either side of a single transmitter 74. Transmitter 74 generates a
narrow spherically propagated beam orthogonal to the line of
receivers so that the signals received by each receiver pair can be
processed to remove the direct signal and common noise. In the
linear array of FIG. 1(i), which is also shown in plan view, A row
of transmitters 76 is arranged parallel to a row of receivers 78, a
common signal being fed to all transmitters so as to generate a
linearly propagated sound wave. The arrangements of FIGS. 1h) and
1(i) are suited for arrangement across the glide path of an airport
to detect persistent vortices. [A system of this nature will be
described in more detail below.]
[0049] The diagrams (A), (C) and (E) of FIG. 2 are diagrammatic
elevations of configurations (a), (c) and (e) respectively of FIG.
1.
[0050] The system 100 of the first example, illustrated in FIG. 3,
approximates the arrangement of FIG. 1(a) and FIG. 2(A) and is
adapted for vertical or inclined atmospheric sounding where both
cross-range (horizontal, in this case) and along-range (vertical,
in this case) wind velocities are required. For convenience,
however, it will be assumed that chirps from the transmitter are
directed vertically upwards to illuminate an inverted cone of air
indicated at 101 by broken lines. Also for convenience, the
cardinal points of the compass will be referred to as N, S, E &
W, as well as north, south, east and west where thought
necessary.
[0051] System 100 includes a large main dish 102 and a small
secondary dish 104 mounted directly above the main dish. A
transmitter/receiver module 106 is supported centrally above large
dish 102 (by struts that are not shown) and, in turn, supports
small dish 104 on the top thereof. Module 106 comprises a
sound-adsorbent molding 108, into the bottom of which a central
loudspeaker 110 and four peripheral microphones are fitted.
[0052] The microphones are arranged in quadrature and aligned with
the cardinal points of the compass so that, in the sectional
diagram of FIG. 3, the W microphone is shown at 112 (on the
east/right side of loudspeaker 110) and the E microphone is shown
at 114 (on the west/left side of loudspeaker 110). This apparent
reversal of naming the E and W microphones is convenient because
the microphone on the west side of the loudspeaker is positioned to
be most sensitive to echoes coming from the east, after reflection
and focusing by dish 104, and vise versa. The axis of each
microphone is angled to the vertical at between about 3 and 10
degrees. The loudspeaker 110 and microphones 112 and 114 are
located near the focus of large dish 102. A fifth directional
microphone 116 is located at the focus of small dish 104.
[0053] In the diagram of FIG. 3, a single horizontal reflective
atmospheric discontinuity (such as the nocturnal boundary layer or
other thermal inversion layer--TIL), 120 is shown. Since
loudspeaker 110 is pointed vertically downward, it will generate a
downwardly directed vertical beam that will be reflected vertically
upward by large dish 102 along a central system axis 122. Some
echoes will be returned down axis 122 to microphone 116 on small
dish 104. However, the beam of interrogating pulses will be conical
and will illuminate a significant area of the TIL 120 around axis
122 and echoes will be returned from an area to the west of axis
122 along the axis 124 of west microphone 112 and be most strongly
detected by that microphone (in comparison with the signals
detected by the other microphones). Similarly, echoes from TIL 120
to the east of axis 122 will travel along the axis 126 and be most
strongly received by east microphone 114. While echoes returned
from TIL 120 and detected by W microphone 112 may be centered about
path 124, microphone 112 will pickup echoes from a large area of
TIL 120 in the vicinity of axis 122. Thus echoes from a source near
axis 122 are likely to be picked up by all microphones. Doppler
(phase) components common to echoes detected by all microphones at
much the same time are therefore indicative of the vertical
velocity of TIL 120 in the vicinity of axis 122, and it can be
expected that these common components will be most prominent in the
echoes detected by central microphone 116. If the Doppler
components of echoes received by W and E microphones 112 and 114
are subtracted, the common Doppler (phase) components indicative of
the vertical velocity component will be removed and it can be
assumed that the remaining Doppler (phase) components are due to
net wind speed in the east/west direction. Similar subtraction of
the Doppler components of echoes received by the N and S
microphones will yield the net wind speed in the north/south
direction.
[0054] In practice, of course, there will be many atmospheric
discontinuities at many altitudes within range that generate echoes
and that the time of return of such echoes will be indicative of
range or attitude and the amplitude of the echoes will be
indicative of the magnitude of the respective discontinuities.
[0055] The generation of chirps for transmission and the processing
of received echoes may be implemented in many ways, whether in
software or hardware. The mode of implementation will be influenced
by the desired chirp length, listening time/intended range,
sampling rates, and up-date frequency, since these factors largely
determine computation demand. To provide desired high processing
gain, chirp durations greater than 0.3 s are considered essential,
with durations of tens of seconds desirable. Prior art pulsed
systems using the transmit-then-listen strategy typically have
transmit times in the order of tens of milliseconds and a net
listening time of about 6 s for a range of 1000 m. By contrast, in
the present example, for a range of 1000 m and net listening time
of 6 s, the selected chirp duration is 37 s and the total listening
time is 43 s. For the same total transmitted energy the chirp of
the present example can have a thousand-fold lower peak power than
a 37 m chirp typical of the prior art The total listening time of
the present example is more than seven times that typical of the
art for a 1000 m range, providing much greater opportunity for
processing gain. By using chirps (pulse-compression waveforms) and
matched filter processing, processing gains are further multiplied
many-fold.
[0056] The computation demands of the system of the example are,
however, substantial and, in this case, were thought to justify
dedicating a PC to process the signals from each receiver and using
another PC as a controller. FIG. 4 shows this arrangement in which
the N and S receiver microphones are shown at 130 and 132
respectively, the dedicated N, E, W & S and vertical (V)
calculating PCs are shown at 134, 136, 138, 140, 142 and 143
respectively and the controller PC is shown at 144. Controller PC
144 generates the chirp for transmission by transmitter/loudspeaker
110 and the reference chirp for use in matched filtering by the
calculating PCs. It also collects the results of the computations
of PCs 134-142 for integration, display and reporting.
[0057] In this example, the chirp has a phase/frequency that
increases linearly over the 37 s from 800 Hz to 1600 Hz (the chirp
could just as easily decrease linearly from 1600 Hz to 800 Hz) and
is emitted at an acoustic power of a few hundred milli-Watts that
remains constant for the duration of the chirp. This type of pulse
compression waveform has a small bandwidth (about 800 Hz) and is
simple to generate accurately using a PC sound card and
conventional loudspeaker driver circuits for powers up to many
Watts. It is also one for which a `matched filter` or correlator
can be readily designed and used to extract echoes of the chirp
from received signals having high noise levels and to effectively
compress the energy of each echo into a period of time that is much
shorter than that of the transmitted chirp. The use of
pulse-compression waveforms and matched filters thus yield very
high processing gains.
Measurement of Cross-range Wind Velocity
[0058] System 100 of the chosen example is well suited for
measuring various characteristics of horizontal (cross-range) wind.
The manner in which the net phase difference between the east and
west echoes is effected to derive the E-W wind speed with range
distance will now be described with reference to FIGS. 5A, 5B and
5C. It should be appreciated that the system of FIG. 6A is
duplicated for the extraction of the net phase difference between
the north and south echoes in exactly the same manner. This is
indicated by "W/N" and "E/S" in FIG. 5A. However, the following
description will refer principally to the processing of the E &
W echoes.
[0059] The processes of FIG. 5A are performed in dedicated E &
W PCs 138 and 140 and control PC 144 to (i) extract the cumulative
phase and amplitude information from the noisy received signal
using matched filtering and (ii) difference the E-W cumulative
phase information to eliminate the zero Doppler signal (and other
common noise components) to yield a Doppler phase signals
indicative of net east-west wind speed. The cumulative phase
information and the differenced E-W and N-S phase information are
illustrated graphically in FIG. 8. The E-W and N-S signals are then
used as inputs for the circuit and process of FIG. 5B, from which
overall wind velocity and wind shear information is derived. PCs
138 and 140 again do the processing, the results being collated and
displayed on control PC 144 and illustrated graphical in FIG. 9.
FIG. 5C illustrates in more detail the phase and amplitude
extraction circuit, which includes a matched filter or correlator,
shown in FIG. 5A. The phase extractor is substantially identical
for each receiver and is implemented in the respective receiver PC.
The circuit and process of FIG. 5 are illustrated and described
generally because the techniques employed are analogous to known
signal processing techniques used in radar.
[0060] In FIG. 5A, loudspeaker 110 is shown pointing upwards, the
transmitted chirp is indicated by shaded graph 150, east microphone
112 is shown pointing up on the right side of the diagram to
receive input signals indicated by arrow 152 and west microphone
114 is shown pointing up on the left side of the diagram to receive
input signals indicated by arrow 154. Chirp 150 is generated using
a control PC output 156 (indicated by associated graph 169) to
drive a transmitter circuit 158, which in turn, powers matched
loudspeaker 110 to ensure that the acoustic power of transmitted
chirp 150 is uniform over its whole bandwidth (about 800 Hz). In
this example, the power, is about 0.5 Watt; not enough to cause
annoyance even in urban areas.
[0061] Received east and west signals 152 and 154 are respectively
processed (essentially amplified and band-pass filtered) in
receivers 159 and 160, the outputs of which are sampled at 96 k/s
and digitized by analog-digital (A-D) circuits 161 and 162 to
generate digital receiver signals 163 and 165. A representation of
signal 163 or 165 is shown in FIG. 6.
[0062] Outputs 163 and 165 are fed to phase and amplitude
extraction circuits 166 and 167 that employ matched filters to
correlate signals 163 and 165 in the Fourier domain with a version
of the transmitted chirp, extract or derive echo phase and
amplitude information therefrom. Said version of the transmitted
chirp is provided via line 168 by control PC 144 to each extractor
166/167 and comprises the transmitted chirp shifted down by 800 Hz;
ie, converted to a 0-800 Hz chirp as indicated in nearby graph 169.
It may also be desirable to delay signal on line 168 with respect
to the actual transmitted signal so as to select a desired range
band of the system.
[0063] Extractor circuits 166 and 167 each have two outputs.
Outputs 170 and 171 of extractor 166 are respectively indicative of
the cumulative phase and amplitude of the input 163 after matched
filtering (Fourier processing), the east cumulative phase being
shown in FIG. 8(i), the south cumulative phase being shown in FIG.
8(v) and the east and south magnitudes being shown in the barchart
of FIG. 9(i). Outputs 172 and 173 of extractor 167 are respectively
indicative of the cumulative phase and amplitude of the input 165
after matched filtering, the west cumulative phase being shown in
FIG. 8(ii), the north cumulative phase being shown in FIG. 8(iv)
and the west and north amplitudes being shown in the barchart of
FIG. 9(i). Cumulative phase outputs 170 and 172 contain the
information from which wind velocity, direction and wind shear can
be derived, as well as common phase noise and zero-Doppler
components due to the direct signal and ground clutter. Cumulative
phase outputs 170 and 172 are differenced in circuit 174 to remove
the common components and output on line 176 as the net E-W phase
or cumulative phase of graph (iii) of FIG. 8. These outputs contain
the information from which the E-W velocity and wind shear are
derived.
[0064] As already noted, the circuit and process of FIG. 5 is
applied in an identical manner to the north and south signals to
derive the cumulative north phase illustrated graphically by FIG.
8(iv), the cumulative south phase illustrated by FIG. 8(v) and the
net north-south phase variation with height illustrated by FIG.
8(vi). For convenience, east-west phase difference output from
circuit will be identified as 176 E-W of and the north-south phase
difference will be identified as 176 N-S.
[0065] FIG. 5B illustrates how outputs 176 E-W and 176 N-S are
processed in the chosen example to generate wind speed and bearing
(ie, wind velocity) and wind shear magnitude with range distance
(altitude, in this case). Outputs 176 E-W and 176 N-S are fed to a
combiner circuit 180 from which wind bearing or direction is
derived, as shown in FIG. 9(ii). This is done by determining
whether the E-W signal is positive or negative and whether the N-S
signal is positive or negative in order to place the wind direction
in the correct quadrant of the compass. Thus a positive E-W phase
and a positive N-S phase will indicate that the wind direction is
in the first quadrant, and so on. The angle in the first quadrant
can be determined by computing the relevant vector to give a more
precise indication of bearing.
[0066] By employing the well known square-root of the sum of the
squares algorithm, the magnitude of the phase signal can be
determined to output the wind speed and wind shear magnitudes. This
is done by feeding outputs 176 E-W and 176 N-S to respective
squaring circuits 182 and 184, summing the outputs of these
circuits in adder 186 and deriving the square root in circuit
188.The resultant output on line 190 is indicative of the variation
of wind speed with respect to altitude. This is illustrated by FIG.
9(iii).
[0067] By taking the derivative of the signal on line 190 using
differentiator circuit 192 the magnitude of wind shear with
altitude is output on line 194. FIG. 9(iv) illustrates this. Signal
on line 194 can be further processed to apportion the wind shear
into various magnitude bins using a differencing circuit 196 to
generate a series of outputs 198-202 etc, which are illustrated by
the graphs of FIG. 9(v).
[0068] Referring now to FIG. 5C, the basic operation of each
extractor will now be described. Since the extractors are
substantially identical, only extractor 166 for the east receiver
will be described. Again it will be appreciated that the matched
filter and the phase and amplitude extraction techniques adopted
here are known to those skilled in the radar art Preferably, as
also mentioned before, each extractor is implemented in a separate
PC that is dedicated to one receiver.
[0069] Extractor 166 essentially comprises a matched filter 210
that--in the Fourier or frequency domain--matches a reference chirp
(usually a version of the transmitted chirp) to the confused
time-domain echoes that are included in input signal 163. Each
digitized sample of E input 163 is converted to complex form. The
imaginary part (I) is generated by mixing the input with a
digitized 2000 Hz sine signal 300 supplied by control PC 144 using
multiplier 302, and the real part (Q) is generated by mixing the
input with a digitized 2000 cosine signal 304 (also supplied by PC
144) using multiplier 306. The resultant I and Q signals for every
sample taken during the listening period are fed to a complex FFT
[fast Fourier transform] process 308, where all samples are
presented and processed as an array to generate the frequency
domain outputs I' and Q'. These outputs are low-pass filtered at
310 to remove the upper side band. FIGS. 7A and 7B show the
unfiltered and filtered signal components Q' and Q", respectively.
The filtered signals I" and Q" are then passed to complex
multiplier 312 in which they are multiplied with the down-converted
output signal 168 from the control PC 144. The result of this
multiplication is then subjected to complex inverse FFT at 314 to
generate real and imaginary sample-like outputs I'" and Q'" in the
time domain.
[0070] Each `sample` output I" and Q" from IFFT 314 is then
processed to provide corresponding phase and amplitude outputs 170
and 171. Phase output 170 is generated by implementing the function
Atan2(I"/Q") in process 316 to yield a succession of phase values
between -.pi. and +.pi. on line 318, which is input into an unwrap
process 320. The unwrap process is known and implementations are
available in programs such as MatLab. Essentially, process 316
counts the number of 2.pi. phase shifts to generate an accumulated
phase. For every transition from +.pi. to -.pi. (an increasing
phase) the phase accumulator is increased by 1, and vice versa.
This output can then be displayed as a radian count with respect to
sample number (proxy for time and distance) and displayed
graphically as in FIG. 8. The amplitude output 171 is generated by
implementing the function [I.sup.2+Q.sup.2].sup.1/2 in process 322.
The barchart (i) of FIG. 9 shows the variation of amplitude with
height, each amplitude reading being color-coded to indicate
magnitude. It will be appreciated that a sharp variation in
processed signal amplitude at a given height is indicative of
moisture or temperature change and not of wind shear. However,
significant temperature differentials in the atmosphere will be
accompanied by wind change.
Measurement of Air Temperature
[0071] Using the system of the above example, the variation of air
temperature over range distance can be estimated by the use of
positive and negative chirps and manipulating the cumulative phase
outputs generated. This comprises the second detailed example of
the application of this invention.
[0072] Though not essential, the use of substantially identical
positive and negative linear chirps is highly desirable in the
means for the measurement of temperature. Consistent with the above
example, a positive chirp that rises in frequency from 800 to 1600
Hz and a negative chirp that falls in frequency from 1600 to 800 Hz
over a period of 37 s will be assumed. It will also be assumed that
the positive chirp is transmitted first and that the negative chirp
is transmitted immediately after the listening time of 43 s has
elapsed, there also being a listening time of 43 s after the
negative chirp has been transmitted. In this case, it will be
convenient to separately digitize the acoustic signals from each of
the four receivers (N, S, E & W) for the positive chirp and for
the negative chirp and to then process each in the manner describe
above to extract the respective cumulative phase signal.
[0073] The manner in which the cumulative phase signals are
manipulated will now be described with reference to FIG. 10, in
which the N, S, E & W cumulative phase signals for the positive
chirp are shown as inputs 300 and the N, S, E & W cumulative
phase signals for the negative chirp are shown as inputs 302.
Inputs 300 are added together and divided by four in adder 304 to
remove the horizontal (cross-range) wind components for the
positive chirp, and inputs 302 are added together and divided by
four in adder 306 to remove the horizontal (cross-range) wind
components for the negative chirp. The use of four receivers for
temperature measurement is convenient as the same system can be
used for horizontal wind measurements as well, The temperature
measurement can also be made by using a single receiver pointed
vertically but such a system could not be used for horizontal wind
measurements. The outputs of adders 304 and 306 are then
differenced in process 308 to remove common components due to
direct signal, vertical wind, ground clutter and noise, leaving
temperature related cumulative phase difference. The gradient of
this difference signal is then derived in process 310 and
normalized in process 312, the output of this process being a
measure of the relative change of temperature with altitude
(range). To calibrate this, the actual temperature near ground
level is input at 314 and a chart--indicated at 316--of temperature
variation with altitude can be generated. An actual chart generated
by the means of the second example is appended as Figure.
[0074] The physical basis of temperature measurement in the system
of this example is that the total rate of phase advance with
respect to distance of a positive chirp passing through a layer of
cold air will be slightly less than that for a negative chirp, the
difference being dependent upon temperature, The total rate of
phase advance is made up of the sum of a dominant component due to
the propagation of sound in air (nominally 14,500.pi. radians for a
distance of 1 km) and a minor component due to the internal rate of
phase advance within the chirp (eg, +800.times.2.pi./43 or 18
radians/s for a positive chirp and -800.times.2.pi./43 or 18
radians/s for a negative chirp). In the case of a positive chirp,
the internal rate is positive and is added to the propagation rate;
with a negative chip the internal rate is negative and is
subtracted from the propagation rate. Thus, the rate of increase of
cumulative phase with respect to distance is slightly less for the
negative chirp than for the positive chirp. However, when a cold
layer of a fixed distance is encountered, the propagation rate of
the positive chirp is slowed slightly so that the chirp takes
longer to travel the distance the internal phase advance is
proportionately greater than it was when traveling the same
distance in warmer air. And, since the increased internal phase
advance is positive, it will add slightly to the (now slower)
propagational phase advance. While the cold layer also slows the
propagation (phase rate) of the negative chirp and the internal
(negative) phase advance is also increased as a result, the
marginal increase is not as great due to the slower phase advance
of the negative chirp and therefore results in a smaller total
cumulative rate of phase change, resulting in a divergence of the
rate of phase change with respect to time/distance. This is
illustrated in the diagram of FIG. 11.
Measurement of Down-range Wind Velocity
[0075] There are two ways of estimating down-range (in this case
vertical) wind in accordance with the principles disclosed herein.
The first (comprising the third example) is to use the central
receiver 116 in a send-then-listen mode; the second (comprising the
fourth example) is to use the four N, S, E & W receivers in a
listen-while-sending mode. In both cases, long chirps (greater than
300 ms) are employed as taught herein but, in the first, there is
some sacrifice of low-altitude range and, in the second, there is
some sacrifice of accuracy.
[0076] In the third example, a short chirp of about 0.5 s is sent
by loudspeaker 110 and, immediately after, the incoming signals to
microphone 116 are processed by the phase and amplitude extraction
process described above to generate an corresponding cumulative
phase and amplitude output, from which vertical wind speed
variation can be read or deduced. This method will lose the first
85 m of range and will be subject to errors due to noise and
cross-range wind speed, but the greater resolution offered by the
long chirp will be gained.
[0077] In the fourth example, a chirp of about 5 s can be employed
while all receivers are listening, the acoustic outputs of each
receiver then being processed and the respective phases and
amplitudes extracted as described above. Referring to FIG. 13, the
cumulative phases of the N, S, E & W receivers, indicated at
320, are fed to adder 322 where they are added together and divided
by four, as in the temperature measurement case to remove common
elements due to cross-range wind. System dependent phase shift 323
is then removed in process 324, it being easily removed because it
has a constant gradient The output of process 324 is indicative of
the variation of down-range wind speed, but is degraded by the
(relatively short) direct signal and by ground clutter. These
undesired signal components are manifest in a relatively large
phase signal at the origin (zero distance and time) Since it is
reasonable to assume that the wind speed at ground level is zero
the initial or residual phase can be subtracted from the cumulative
phase in process 326 and normalized in process 328, yielding the
display indicated at 330. Alternatively, the resultant phase can be
scaled and calibrated according to a known wind speed at a given
altitude obtained by other means, such as radio sonde.
Detection of Vortices Near Runways
[0078] In this fourth and final example, an array 400 of receivers
and transmitters--such as indicated in FIG. 1(h) or (i)--is
arranged across the glide path at the end of an airport runway.
FIG. 14 shows the use of an array of the type shown in FIG. 1(h)
having a single central transmitter 402 and multiple receivers 404
extending in a row on each side. The entire array may span 150-200
m. In FIG. 14 transmitter 402 is shown generating a spherically
propagated acoustic chirp 406 and the wake vortices are shown at
408, arrows 410 indicating the direction of rotation of each
vortex. The interaction of chirp 406 with the vortices results in
the backscatter of echoes 412, which are picked up by receivers 404
and fed to respective matched filters in a manner similar to that
described in the first example. Multiple receivers are used here to
provide better horizontal resolution and increase overall receiver
gain.
[0079] The difficulty in this case is, however, that the high
Doppler echoes returned from the vortices creates considerable
ambiguity in the results so that the location, size and speed of
the vortices cannot be measured with sufficient accuracy by using
the system of the first example alone. It is necessary to obtain
measurements of the ambient conditions prior to the arrival of a
large plane and use those measurements to adjust and sharpen those
taken with a vortex present, after the plane has passed. The
ambient measurement provides a reference for the amplitude and
phase of the system as well as for the ambient amplitude and phase.
These results are stored.
[0080] When the echoes from a vortex are being processed, the
signal processing proceeds for each receiver as described in the
first example, except that the return signal is correlated against
multiple different Doppler shifts (positive, zero and negative) for
the internal multiply of the matched filter using a corresponding
series of reference chirps generated by the control PC. Each phase
calculated in respect of the vortex condition for each receiver is
then differenced with the corresponding calculated phase for the
ambient condition for each respective receiver, removing system and
noise phase shifts. This leaves a residual or `excess` phase shift
that can be plotted with respect to distance (time) along with the
corresponding Doppler shifts, as shown in FIG. 15.
[0081] To then estimate Doppler vs. distance (height), it is
necessary to search through each excess phase record for each
Doppler shifted result to find the distance ranges for which the
excess phase is zero. By combining the ranges for which a zero
excess phase is found a graph of Doppler shift vs. distance can be
made. From this it is an easy matter to estimate wind speed vs.
distance. It is to be noted that the excess phase alone cannot be
used to without the additional Doppler processing because the range
ambiguity effect spreads the excess phase result out to the extent
that the location and size of the vortex cannot be sufficiently
located.
[0082] Bradley (cited above) discusses a similar estimation process
for the optimization of amplitude and reference should be made to
that paper. However, using phase is more accurate and stable than
amplitude, allowing a better estimation of Doppler to be
obtained.
[0083] While some examples of the application of the invention have
been described, it will be appreciated that the methods of the
present invention can be applied widely to acoustic sounding and
that many alterations and additions can be made without departing
from the scope of the invention as defined by the following
claims.
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