U.S. patent application number 10/160907 was filed with the patent office on 2003-12-04 for radar imaging system and method.
Invention is credited to Krikorian, Kapriel V., Rosen, Robert A..
Application Number | 20030222808 10/160907 |
Document ID | / |
Family ID | 29419737 |
Filed Date | 2003-12-04 |
United States Patent
Application |
20030222808 |
Kind Code |
A1 |
Krikorian, Kapriel V. ; et
al. |
December 4, 2003 |
RADAR IMAGING SYSTEM AND METHOD
Abstract
An imaging system and method. The invention provides an
intra-pulse repetition interval (PRI) agile beam technique for
enhanced resolution that can be used at aspect angles near the
velocity vector of a host vehicle. It is particularly useful at
small scan angles where beam sharpening array times become large.
At these scan angles, the bandwidth of the clutter is narrower than
at higher scan angles and allows large PRIs without degradation
from Doppler ambiguities. In accordance with the present teachings,
sequential illumination is performed within a PRI to multiple beam
locations using an agile beam. The interleaving of beams reduces
map formation times compared to conventional techniques using
sequential arrays. The inventive system is adapted for use with an
electronically scanned (e.g., synthetic aperture array radar)
antenna. The inventive method includes the steps of activating the
antenna to generate a beam of electromagnetic energy; causing the
beam to scan over a predetermined scan volume consisting of a
predetermined range of scan angles relative to a reference vector;
and generating multiple simultaneous beams of electromagnetic
energy over a subset of the predetermined range of scan angles.
Inventors: |
Krikorian, Kapriel V.; (Oak
Park, CA) ; Rosen, Robert A.; (Simi Valley,
CA) |
Correspondence
Address: |
PATENT DOCKET ADMINISTRATION
RAYTHEON SYSTEMS COMPANY
P.O. BOX 902 (E1/E150)
BLDG E1 M S E150
EL SEGUNDO
CA
90245-0902
US
|
Family ID: |
29419737 |
Appl. No.: |
10/160907 |
Filed: |
June 3, 2002 |
Current U.S.
Class: |
342/25R ;
342/190; 342/191; 342/195; 342/74; 342/81 |
Current CPC
Class: |
G01S 13/426 20130101;
G01S 2013/0245 20130101; G01S 13/89 20130101; G01S 13/90
20130101 |
Class at
Publication: |
342/25 ; 342/74;
342/81; 342/190; 342/191; 342/195 |
International
Class: |
G01S 013/90 |
Claims
What is claimed is:
1. An imaging system comprising: first means for generating a beam
of electromagnetic energy; second means for causing the first means
to scan the beam over a predetermined scan volume consisting of a
predetermined range of scan angles relative to a reference vector;
and third means for causing the first means to generate multiple
simultaneous beams of electromagnetic energy over a subset of the
predetermined range of scan angles.
2. The invention of claim 1 wherein the first means includes an
electronically scanned antenna.
3. The invention of claim 1 wherein the second means includes a
processor.
4. The invention of claim 3 wherein the processor includes a radar
timing and control circuit.
5. The invention of claim 1 wherein the third means includes means
for determining azimuth beam positions for the scan volume.
6. The invention of claim 5 wherein the third means further
includes means for computing a dwell time for beam sharpening for
each of the azimuth beam positions.
7. The invention of claim 6 wherein the third means further
includes means for computing a maximum pulse repetition interval
for each of the beam positions.
8. The invention of claim 7 wherein the third means further
includes means for computing a minimum pulse repetition interval
common to all of the beam positions.
9. The invention of claim 8 wherein the third means further
includes means for selecting a pulse repetition interval for each
beam position as the greatest multiple of the minimum pulse
repetition interval which is less than the maximum pulse repetition
interval.
10. The invention of claim 9 wherein the third means further
includes means for interleaving azimuth beam positions within the
scan volume, which allow for a synchronization of pulses.
11. The invention of claim 10 wherein the third means further
includes means for generating the multiple beams at the interleaved
beam positions in accordance with the dwell times.
12. A radar system comprising: a synthetic aperture array antenna;
a radar timing and control system coupled to the array antenna; a
processor coupled to the radar timing and control system; and
software running on the processor effective to cause the processor
to generate signals to the antenna via the radar timing and control
system to cause the antenna to scan a beam of electromagnetic
energy over a predetermined scan volume consisting of a
predetermined range of scan angles relative to a reference vector
and for causing the antenna to generate multiple simultaneous beams
of electromagnetic energy over a subset of the predetermined range
of scan angles.
13. An imaging method including the steps of: generating a beam of
electromagnetic energy; causing the beam to scan over a
predetermined scan volume consisting of a predetermined range of
scan angles relative to a reference vector; and generating multiple
simultaneous beams of electromagnetic energy over a subset of the
predetermined range of scan angles.
14. The invention of claim 13 wherein the generating step includes
the step of determining azimuth beam positions for the scan
volume.
15. The invention of claim 14 wherein the generating step includes
the step of computing a dwell time for beam sharpening for each of
the azimuth beam positions.
16. The invention of claim 15 wherein the generating step includes
the step of computing a maximum pulse repetition interval for each
of the beam positions.
17. The invention of claim 16 wherein the generating step includes
the step of computing a minimum pulse repetition interval common to
all of the beam positions.
18. The invention of claim 17 wherein the generating step includes
the step of selecting a pulse repetition interval for each beam
position as the greatest multiple of the minimum pulse repetition
interval which is less than the maximum pulse repetition
interval.
19. The invention of claim 18 wherein the generating step includes
the step of interleaving azimuth beam positions within the scan
volume which allow for a synchronization of pulses.
20. The invention of claim 19 wherein the generating step includes
the step of generating the multiple beams at the interleaved beam
positions in accordance with the dwell times.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to imaging systems. More
specifically, the present invention relates to radar imaging
systems.
[0003] 2. Description of the Related Art
[0004] Imaging techniques are well known and widely used in the
art. Certain imaging technologies are better suited for particular
applications. For example, radar imagery is widely used for
surveillance and reconnaissance as well as target tracking and
identification. For radar and other imaging technologies, the
ability to clearly resolve and discriminate targets may be
essential in meeting objectives specified for a particular
application.
[0005] One such application involves `real beam ground mapping.`
Real beam ground mapping involves scanning an area, e.g., the
earth's surface, using a scanning antenna or an electronically
scanned antenna. Returns from an illumination of the surface are
then examined for `back-scatter` or reflections therefrom. As the
beam is scanned in azimuth, information is collected with respect
to the range direction. At each beam position, the distance of
various scatterers may be ascertained for each range cell. This
information may then be displayed in a real beam ground mapped
image.
[0006] While range data may be resolved with adequate resolution,
currently, resolution of azimuth data with comparable resolution
has proved to be problematic. This is due to the fact that azimuth
resolution is limited to the width of the antenna beam and the
corresponding cross range resolution degrades as a function of
range. Accordingly, the poor resolution of conventional real beam
mapping systems limits the ability of the system to discriminate
scatterers.
[0007] Conventional doppler beam sharpening (DBS) or Synthetic
Aperture Radar techniques may be used to improve the azimuth
resolution, but these require excessive frame times if the coverage
includes regions close to the velocity vector.
[0008] "Super resolution" techniques are widely used to sharpen the
radar imagery. However, the quality achieved is scene dependent and
is not robust.
[0009] Hence, a need remains in the art for an improved system or
method for providing ground mapped images, in a timely manner, that
include regions near the velocity vector. Specifically, a need
remains in the art for a system or method for providing enhanced
cross-range (azimuthal) resolution with a frame time similar to
that of a real beam ground mapping radar system.
SUMMARY OF THE INVENTION
[0010] The need in the art is addressed by the imaging system and
method of the present invention. The inventive system is adapted
for use with an electronically scanned (e.g., synthetic aperture
array radar) antenna. The inventive method includes the steps of
activating the antenna to generate a beam of electromagnetic
energy; causing the beam to scan over a predetermined scan volume
consisting of a predetermined range of scan angles relative to a
reference vector; and generating multiple simultaneous beams of
electromagnetic energy over a subset of the predetermined range of
scan angles.
[0011] In a specific illustrative embodiment, the inventive method
further includes the steps of determining azimuth beam positions
for the scan volume; computing a dwell time for beam sharpening for
each of the azimuth beam positions; computing a maximum pulse
repetition interval for each of the beam positions; computing a
minimum pulse repetition interval common to all of the beam
positions; selecting a pulse repetition interval for each beam
position as the greatest multiple of the minimum pulse repetition
interval which is less than the maximum pulse repetition interval;
interleaving azimuth beam positions within the scan volume which
allow for a synchronization of pulses; and generating the multiple
beams at the interleaved beam positions in accordance with the
dwell times.
[0012] Thus the present invention provides an intra-pulse
repetition interval (PRI) agile beam technique for enhanced
resolution, which can be used at aspect angles near the velocity
vector of a host vehicle. At these scan angles, the bandwidth of
the clutter is narrower than at higher scan angles and allows large
PRIs without degradation from Doppler ambiguities. In accordance
with the present teachings, sequential illumination is performed
within a PRI to multiple beam locations using an agile beam. The
interleaving of beams reduces map formation times compared to
conventional techniques using sequential arrays.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1a is a side view of an aircraft and flight provided to
illustrate the longitudinal axis and velocity vector thereof.
[0014] FIG. 1b is an end view of the velocity vector of the
aircraft showing an illustrative application of real beam ground
mapping in accordance with conventional teachings.
[0015] FIG. 1c is a diagram which shows an illustrative beam
pattern resulting result from a beam ground scan.
[0016] FIG. 2 is a generalized block diagram of a radar system
implemented in accordance with the teachings of the present
invention.
[0017] FIG. 3 is a flow diagram showing an illustrative embodiment
of a method implemented in accordance with the teachings of the
present invention.
[0018] FIG. 4 is a flow diagram that shows an illustrative
implementation of certain steps of FIG. 3.
[0019] FIG. 5 shows a timeline of illustrative interleaved beams
for a wide area mapped in accordance with the teachings of the
present invention.
[0020] FIG. 6 is a diagram that shows a magnifying the view of the
scan of FIG. 5 at small scan angles.
DESCRIPTION OF THE INVENTION
[0021] Illustrative embodiments and exemplary applications will now
be described with reference to the accompanying drawings to
disclose the advantageous teachings of the present invention.
[0022] While the present invention is described herein with
reference to illustrative embodiments for particular applications,
it should be understood that the invention is not limited thereto.
Those having ordinary skill in the art and access to the teachings
provided herein will recognize additional modifications,
applications, and embodiments within the scope thereof and
additional fields in which the present invention would be of
significant utility.
[0023] The present invention is adapted for use on a vehicle such
as an aircraft moving with a velocity vector such as that shown in
FIGS. 1a-c.
[0024] FIG. 1a is a side view of an aircraft and flight provided to
illustrate the longitudinal axis and velocity vector thereof. As
shown in FIG. 1a, the velocity vector V of the aircraft 1 is
coincident with the longitudinal axis 2 thereof.
[0025] FIG. 1b is an end view of the velocity vector of the
aircraft showing an illustrative application of real beam ground
mapping in accordance with conventional teachings. Real beam ground
mapping is effected by scanning an antenna beam 3 back and forth in
the azimuthal direction around the velocity vector of the vehicle
over a surface 4.
[0026] FIG. 1c is a diagram which shows an illustrative beam
pattern resulting from a beam ground scan. In accordance with the
present teachings, azimuthal, cross range resolution is improved by
increasing the dwell time of an electronically scanned synthetic
aperture array radar antenna during a predetermined portion of the
scan thereof. The increase in the dwell time leads to a longer
coherent integration time and allows for a sharpening or narrowing
of the resolution relative to the antenna beamwidth. Near the
velocity vector of the vehicle, e.g., 1-30 degrees off the velocity
vector, coherent integration may be used to implement multiple
beams. This is made possible by the fact that at angles close to
the velocity vector of the vehicle, the clutter bandwidth is
narrow. This allows for the use of very low pulse repetition
frequencies (PRFs) which is equivalent to very long pulse
repetition intervals (PRIs) as PRF=1/PRI. This allows for the
creation of multiple simultaneous agile beams and the individual
steering of same within the same scanning interval.
[0027] FIG. 2 is a generalized block diagram of a radar system
implemented in accordance with the teachings of the present
invention. Those skilled in the art will appreciate that although
the present teachings are disclosed with reference to an
illustrative radar system implementation, the invention is not
limited thereto. The present teachings may be applied to a variety
of image processing applications without departing from the scope
thereof. The system 100 includes a receiver/exciter and transmitter
101 of conventional design and construction. As is known in the
art, the receiver/exciter and transmitter 101 includes an
exciter/waveform generator 102 which generates, in response to
commands from the processor, a novel and advantageous waveform as
discussed more fully below. The radar signal is upconverted by an
upconvert stage 104 and filtered, amplified and transmitted by a
transmitter stage 108 in response to a signal from a modulator 106.
The transmit signal is radiated by an electronically steered radar
antenna 110 as a beam of electromagnetic energy.
[0028] The antenna beam pointing is also under processor control
and, in accordance with the invention, is coordinated with the
waveform generator to achieve overlapping beams which provide
sharpened resolution without excessive frame times.
[0029] In an illustrative real beam ground mapping application,
scatter returns of the transmit beam as it is reflected from the
ground or other surface are received by the antenna 110 and applied
to a radar receiver stage 112.
[0030] The receiver 112 amplifies, filters and down converts the
scatter return in a conventional manner. The amplified, filtered
and down converted scatter returns are digitized by an
analog-to-digital converter stage 114 and fed to a processor 116.
The processor 116 de-interleaves the pulses of each beam. The
processor 116 then coherently combines the pulses within each beam,
as in conventional DBS, to obtain a sharpened image.
[0031] FIGS. 3 and 4 are flow diagrams showing an illustrative
embodiment of a method implemented in accordance with the teachings
of the present invention. As shown in the FIG. 3, the method 200
includes the step 202 of determining azimuth beam positions for
each scan volume. The total number of beam positions is equal to
the ratio of the scan range or scan volume to the beamwidth. For
example, with a scan range of .+-.50 degrees the total coverage
area is 100 degrees wide. With the beamwidth of 3.5 degrees, the
total number of beam positions is 100/3.5=29. Thus, in the
illustrative embodiment, at step 202, 29 beam positions are created
at intervals of 3.5 degrees from the start of the scan.
[0032] Next, at step 204, the system 100 computes the array time
for beam sharpening at each azimuth beam position. In accordance
with conventional teachings, the array time is computed as follows:
1 T az = 0.6 V sin az res ( 1 )
[0033] where T.sub.az is the array or dwell time,
.theta..sub.az=azimuth angle from the velocity vector,
.theta..sub.res=desired angular resolution, .lambda.=wavelength,
and V=vehicle velocity. Thus, T.sub.az represents the dwell time
necessary to achieve a desired (e.g., 20:1) improvement in
resolution in the azimuth direction.
[0034] Next, at step 206, in accordance with the present teachings,
the method 200 computes the maximum pulse repetition interval
PRI.sub.max for each beam position. Recall that the clutter
bandwidth becomes narrower at angles close to the velocity vector.
The clutter bandwidth is:
B.sub.c=(2vsin.theta..sub.az/.lambda.).theta..sub.bw (1a)
[0035] and this represents the minimum value of the PRF to prevent
Doppler foldover. The PRI is the reciprocal of the PRF, so the
maximum PRI for each beam position is: 2 PRI max = 2 V sin az bw (
2 )
[0036] where .theta..sub.bw=3 dB beamwidth.
[0037] Next, at step 208, the minimum PRI is computed using the
following relation: 3 PRI min = 2 R max C + PW + T s ( 3 )
[0038] where R.sub.max=maximum range; PW=radar pulse width (pulse
length in the down range direction); C=speed of light; and
T.sub.s=beam switching time.
[0039] Since the clutter width is very narrow for beam positions
near the velocity vector, the PRI can be very long near the
velocity vector. As discussed above, this allows for the creation
of multiple simultaneous interleaved beams in accordance with the
teachings of the present invention.
[0040] Accordingly, at step 210, a pulse repetition interval for
each beam position (PRI.sub.j) is selected. In the illustrative
embodiment, PRI.sub.js is chosen as the greatest multiple of
PRI.sub.min which is less than PRI.sub.max. In other words:
PRI.sub.js=nj.multidot.PRI.sub.min (4)
[0041] where n.sub.j is an integer and
nj.multidot.PRI.sub.min.ltoreq.PRI.- sub.max.
[0042] Next, at step 212, starting with the maximum pulse
repetition interval at each beam position (PRI.sub.s) beam system
100 and method 200 interleaved azimuth beam positions which allow
synchronization of pulses (as illustrated in the timeline of FIG. 5
below).
[0043] At step 214, based on the required array or dwell time at
each beam position (T.sub.az), the method 200 of the present
invention replaces each beam with completed beams which can be
synchronized with the ongoing beams. At step 216, this process is
repeated until all beams are scheduled.
[0044] FIG. 4 is a flow diagram, which shows an illustrative
implementation of steps 210 through 216 of FIG. 3. As shown in FIG.
4, at steps 218 and 219, for each beam position `j`, the system
selects a pulse repetition interval that is a multiple of
PRI.sub.min:
PRI.sub.j=nj.multidot.PRI.sub.min
[0045] where:
nj=floor(PRI.sub.max/PRI.sup.min) (5)
[0046] and number of pulses:
Nj=round(T.sub.az/(nj PRI.sub.min)) (6)
[0047] Note that (the functions "round" and "floor" are commonly
used: "round" rounds to the nearest integer, "floor" truncates the
fractional part.)
[0048] Next, at step 220, the system initializes a list of used
time slots as follows: q.sub.i=0 where `q.sub.i` is a flag
indicating whether time slot `i` has been assigned and i=0, . . . ,
round (max frame time/PRImin). Next, at step 222, the system 100
loops over the index `j` in descending order of array time. At step
224, the system searches for a minimum value of an index `k` such
that q.sub.k+m nj=0 where m=0, . . . , N.sub.j-1. At step 226, the
system assigns slots k+m.multidot.nj to beam j. Then, at step 228,
the system sets q.sub.k+m.multidot.nj=1 and loops back to step 222
until all the beam positions have been assigned.
[0049] FIG. 5 shows a timeline of illustrative interleaved beams
for a wide area mapped in accordance with the teachings of the
present invention. FIG. 5 shows a scan volume in which pulses are
sharpened by integration over a scan range of plus .+-.50 degrees
in azimuth. As is typically the case with prior art systems, at
scan angles of .+-.30 degrees to .+-.50 degrees in azimuth, the
integration times are relatively short. This is due to the fact
that the integration time (also referred to herein as the `dwell
time` or `array time`), represented by the length of each line
segment in FIG. 5, is inversely proportional to the sine of the
scan angle in azimuth of the beam relative to the velocity vector
of the vehicle. Hence, with a scan angle of 90 degrees, the sine
value is equal to 1, and this represents the smallest integration
time possible for a particular level of resolution. However, as the
scan angle approaches the velocity vector e.g. at scan angles
between .+-.30 degrees, the sine function diminishes and the
integration time increases to achieve the desired resolution. As
illustrated in FIG. 5, at very small scan angles, e.g., between
.+-.5 degrees, the integration time becomes substantially longer.
At these angles, the clutter bandwidth is narrower allowing for a
lower PRF. The lower PRFs are equivalent to longer PRIs.
[0050] In accordance with the present teachings, during these
longer pulse repetition intervals, additional overlapping beams are
generated as illustrated at scan angles between .+-.30 degrees in
FIG. 5. Thus, as illustrated in FIG. 5, in the illustrative
embodiment of the present teachings, at azimuth angles between
.+-.10 degrees, six beams 136 are generated simultaneously. With
conventional coherent integration techniques, these beams may be
sharpened with a ratio as high as 20:1. Note that notwithstanding
the generation of multiple beams, where the dwell times are longer,
the total time to cover the entire scan volume (.+-.50 degrees in
azimuth) has not increased. On the contrary, were it not for the
generation of multiple overlapping beams, it would not be possible
to complete the entire scan volume within the 1.2 second timeframe
illustrated in FIG. 5.
[0051] FIG. 6 is a diagram that shows a magnified view of the scan
of FIG. 5 at small scan angles. As illustrated in FIG. 6, each beam
line is created by a series of pulses.
[0052] The intra-pulse repetition interval (PRI) agile beam
technique for enhanced resolution of the illustrative embodiment of
the present invention can be used at aspect angles excluding a
section of .+-.3.5 degrees near the velocity vector of the vehicle.
It is particularly useful at small scan angles (e.g., 3.5 degrees
to 25 degrees) where beam sharpening array times become large. At
these scan angles, the bandwidth of the clutter is narrower than at
higher scan angles and allows large PRIs without degradation from
Doppler ambiguities. Thus, an agile beam allows interleaving of
pulses from multiple beams. In accordance with the present
teachings, sequential illumination is performed within a PRI to
multiple beam locations using the agile beam. The interleaving of
beams reduces map formation times compared to conventional
techniques using sequential arrays. For example, utilizing the
present teachings, a beam scan over a range of .+-.50 degrees may
be achieved within 1.2 seconds with a 20:1 beam sharpening ratio
using the timeline shown in FIGS. 5 and 6 herein. Beam switching
times of 10 microseconds are assumed with register controlled beams
(i.e., flicker mode).
[0053] Thus, the present invention has been described herein with
reference to a particular embodiment for a particular application.
Those having ordinary skill in the art and access to the present
teachings will recognize additional modification applications and
embodiments within the scope thereof.
[0054] It is therefore intended by the appended claims to cover any
and all such applications, modifications and embodiments within the
scope of the present invention.
[0055] Accordingly,
* * * * *