U.S. patent application number 09/948959 was filed with the patent office on 2003-03-13 for adaptive digital beamforming radar technique for creating high resolution range profile for target in motion in the presence of jamming.
Invention is credited to Yu, Kai Bor.
Application Number | 20030048214 09/948959 |
Document ID | / |
Family ID | 25488434 |
Filed Date | 2003-03-13 |
United States Patent
Application |
20030048214 |
Kind Code |
A1 |
Yu, Kai Bor |
March 13, 2003 |
ADAPTIVE DIGITAL BEAMFORMING RADAR TECHNIQUE FOR CREATING HIGH
RESOLUTION RANGE PROFILE FOR TARGET IN MOTION IN THE PRESENCE OF
JAMMING
Abstract
A wideband adaptive digital beamforming technique for
maintaining a high range resolution profile of a target in motion
in the presence of jamming utilizes a sequence of adaptively
calculated narrowband jamming cancellation weights. The adaptive
weights are calculated such that the desired frequency dependent
gain is maintained toward the target center. These adaptive weights
tend to preserve the range profile quality and low range sidelobes.
This technique also tends to eliminate signal cancellation problems
as well as adaptive weight modulation effects.
Inventors: |
Yu, Kai Bor; (Niskayuna,
NY) |
Correspondence
Address: |
DUANE MORRIS LLP
100 COLLEGE ROAD WEST, SUITE 100
PRINCETON
NJ
08540-6604
US
|
Family ID: |
25488434 |
Appl. No.: |
09/948959 |
Filed: |
September 7, 2001 |
Current U.S.
Class: |
342/16 ; 342/140;
342/162; 342/17; 342/192; 342/196; 342/90; 342/96; 342/97 |
Current CPC
Class: |
G01S 13/282 20130101;
G01S 7/36 20130101; G01S 13/32 20130101; G01S 7/411 20130101; G01S
7/2813 20130101; G01S 13/426 20130101; G01S 13/003 20130101 |
Class at
Publication: |
342/16 ; 342/17;
342/90; 342/96; 342/97; 342/162; 342/140; 342/192; 342/196 |
International
Class: |
G01S 007/36 |
Claims
What is claimed is:
1. A method for developing a high resolution range (HRR) profile
for a radar target of interest in the presence of jamming
interference, said method comprising the steps of: transmitting an
HRR waveform; receiving an echo signal resulting from said
transmitted HRR waveform, said echo signal comprising a plurality
of echo signal segments; forming a respective beam pattern for each
echo signal segment, wherein at least one null of each beam pattern
is steered toward at least one interference and a frequency
dependent gain of each beam pattern is maintained toward a center
of said target of interest; and producing an HRR target profile
from said beam patterns.
2. A method in accordance with claim 1, further comprising the
steps of: detecting said target of interest; and tracking said
target of interest.
3. A method in accordance with claim 2, wherein said echo signal is
compensated for motion of said target of interest.
4. A method in accordance with claim 1, wherein said beam patterns
are calculated adaptively.
5. A method in accordance with claim 1, wherein said HRR waveform
comprises at least one of a chirp waveform and a stepped frequency
waveform.
6. A method in accordance with claim 1, further comprising the step
of filtering said beam patterns with a plurality of narrowband
filters.
7. A method in accordance with claim 1, wherein weights used to
form said beam patterns are calculated in accordance with the
following equation: 3 W i = C i - 1 g S A ( T x S , T y S , f i ) g
S A ( T x S , T y S , f i ) H C i - 1 g S A ( T x S , T y S , f i )
g ( T x S , T y S , f i ) wherein: i is an index indicating which
echo signal segment is being processed; C.sub.i is a covariance
matrix estimate of an i.sup.th echo signal segment; W.sub.i is said
weight of said i.sup.th echo signal segment; g.sub.SA(T.sub.x,
T.sub.y, f.sub.i) is an array gain vector used as a steering
vector; H indicates a complex conjugate transpose;
g.sub..SIGMA.(T.sub.x.sup.S,T.sub.y.sup.S, f.sub.i) is a sum beam
gain at frequency f.sub.i and steering direction
(T.sub.x.sup.S,T.sub.y.sup.S); S is a superscript indicating a
steering direction toward said center of said target of interest;
T.sub.x is an azimuth directional cosine calculated in accordance
with the following equation, T.sub.x=cos(.beta.)sin(.theta.),
wherein .theta. is a steering angle, in azimuth, off boresight of
an antenna array and .beta. is a steering angle, in elevation, off
boresight of said antenna array; T.sub.y is an elevation
directional cosine calculated in accordance with the following
equation, T.sub.y=sin (.beta.).
8. A radar system creating a high range resolution (HRR) profile,
said system comprising: an HRR waveform generator for generating an
HRR waveform; an antenna array for transmitting said HRR waveform
and for receiving an echo signal resulting from said transmitted
HRR waveform, said echo signal comprising a plurality of echo
signal segments; a beamformer for forming a respective beam pattern
for each echo signal segment, wherein at least one null of each
beam pattern is steered toward at least one interference and a
frequency dependent gain of each beam pattern is maintained toward
a center of said target of interest; and a pulse compressor for
compressing said echo signal.
9. A system in accordance with claim 8, further comprising: a
detector for detecting said target of interest; and a tracker for
tracking said target of interest.
10. A system in accordance with claim 9, further comprising a
motion compensator for compensating motion of said target of
interest encoded within said echo signal.
11. A system in accordance with claim 8, wherein said beamformer is
an adaptive beamformer.
12. A system in accordance with claim 8, wherein said HRR waveform
comprises at least one of a chirp waveform and a stepped frequency
waveform.
13. A system in accordance with claim 8, further comprising a
dechirper for dechirping said echo signal.
14. A system in accordance with claim 8, wherein weights used to
calculate said beam patterns are accordance with the following
equation: 4 W i = C i - 1 g S A ( T x S , T y S , f i ) g S A ( T x
S , T y S , f i ) H C i - 1 g S A ( T x S , T y S , f i ) g ( T x S
, T y S , f i ) wherein: i is an index indicating which echo signal
segment is being processed; C.sub.i is a covariance matrix estimate
of an i.sup.th echo signal segment; W.sub.i is said weight of said
i.sup.th echo signal segment; g.sub.SA(T.sub.x, T.sub.y, f.sub.i)
is an array gain vector used as a steering vector; H indicates a
complex conjugate transpose; g.sub..SIGMA.(T.sub.x.sup.S,
T.sub.y.sup.S, f.sub.i) is a sum beam gain at frequency f.sub.i and
steering direction (T.sub.x.sup.S, T.sub.y.sup.S); S is a
superscript indicating a steering direction toward said center of
said target of interest; T.sub.x is an azimuth directional cosine
calculated in accordance with the following equation,
T.sub.x=cos(.beta.)sin(.theta.), wherein .theta. is a steering
angle, in azimuth, off boresight of an antenna array and .beta. is
a steering angle, in elevation, off boresight of said antenna
array; and T.sub.y is an elevation directional cosine calculated in
accordance with the following equation, T.sub.y=sin (.beta.),
wherein .beta. is a steering angle, in elevation, off boresight of
said antenna array.
15. A computer readable medium having embodied thereon a computer
program for causing a computer to create a high resolution range
(HRR) profile for a radar target of interest in the presence of
jamming interference, the computer readable program comprising:
means for causing said computer to transmit an HRR waveform,
wherein said HRR waveform comprises at least one of a chirp
waveform and a stepped frequency waveform; means for causing said
computer to receive an echo signal resulting from said transmitted
HRR waveform, said echo signal comprising a plurality of echo
signal segments; means for causing said computer to form a
respective beam pattern for each echo signal segment, wherein at
least one null of each beam pattern is steered toward at least one
interference and a frequency dependent gain of each beam pattern is
maintained toward a center of said target of interest; and means
for causing said computer to create an HRR profile from said beam
patterns.
16. A computer readable medium in accordance with claim 15, further
comprising: means for causing said computer to detect said target
of interest; means for causing said computer to track said target
of interest; and means for causing said computer to dechirp said
echo signal.
17. A computer readable medium in accordance with claim 16, wherein
said computer program further comprises means for causing said
computer to compensate said echo signal for motion of said target
of interest.
18. A computer readable medium in accordance with claim 15, wherein
said computer program further comprises means for causing said
computer to form said beam patterns adaptively.
19. A computer readable medium in accordance with claim 15, wherein
weights used to calculate said beam patterns are accordance with
the following equation: 5 W i = C i - 1 g S A ( T x S , T y S , f i
) g S A ( T x S , T y S , f i ) H C i - 1 g S A ( T x S , T y S , f
i ) g ( T x S , T y S , f i ) wherein: i is an index indicating
which echo signal segment is being processed; C.sub.i is a
covariance matrix estimate of an i.sup.th echo signal segment;
W.sub.i is said weight of said i.sup.th echo signal segment;
g.sub.SA(T.sub.x, T.sub.y, f.sub.i) is an array gain vector used as
a steering vector; H indicates a complex conjugate transpose;
g.sub..SIGMA.(T.sub.x.sup.S, T.sub.y.sup.S, f.sub.i) is a sum beam
gain at frequency f.sub.i and steering direction (T.sub.x.sup.S,
T.sub.y.sup.S); S is a superscript indicating a steering direction
toward said center of said target of interest; T.sub.x is an
azimuth directional cosine calculated in accordance with the
following equation, T.sub.x=cos(.beta.)sin(.theta.), wherein
.theta. is a steering angle, in azimuth, off boresight of an
antenna array and .beta. is a steering angle, in elevation, off
boresight of said antenna array; and T.sub.y is an elevation
directional cosine calculated in accordance with the following
equation, T.sub.y=sin (.beta.), wherein .beta. is a steering angle,
in elevation, off boresight of said antenna array.
20. A radar system for developing a high resolution range (HRR)
scene profile comprising: a transmitter for generating and
transmitting an HRR waveform; a receiver for receiving an incoming
waveform including returns from a radar scene; a processor coupled
to the receiver for transforming the received incoming waveform to
produce and HRR scene profile; said processor, said processor
operative to form beam patterns for portions of said incoming
waveform and for steering at least one mull of each beam pattern
toward an interference for producing said profile.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to radar processing
and more specifically, to a radar process and system for creating
and maintaining the quality of a high resolution range profile for
a target in the presence of jamming.
BACKGROUND
[0002] Modern radar systems having high resolution capability are
useful in many situations, such as target detection, target
discrimination, target recognition, and terrain imaging. Such radar
systems are frequency agile and operate at rapidly varying
frequencies. These radar systems are vulnerable to diverse threats
such as intentional jamming, spoofing, and radar frequency
interference (RFI). Also, in air and missile defense applications,
the target platform may also comprise countermeasures such as
jamming and chaff.
[0003] Of particular interest are systems having high resolution in
range. The performance of high range resolution (HRR) systems is
degraded in the presence of jamming interference. Typically,
jamming is in the form of a high power transmission designed to
impair a radar system's performance. Jamming may comprise a signal
modulated with noise or other disruptive information. The object of
typical jammers is to impair the performance of a radar system's
receiving electronics and/or obscure display of potential targets
of interest. The source of jamming interference may be mobile or
may be relatively stationary (e.g., land based systems). HRR
processing is vulnerable to interference due to jamming because it
requires a relatively wide operational bandwidth, thus increasing
the chances that a jammer at a particular frequency will be in the
operational bandwidth. HRR processing is also vulnerable to jamming
interference because of the relatively long coherent integration
time associated with HRR processing. This increases the likelihood
that a jammer will transmit while the HRR echoes are being
received. Therefore, to avoid performance degradation due to
jamming interference, it is desirable to eliminate jamming
interference from the received signal (e.g., via cancellation,
attenuation).
[0004] Jamming interference is typically cancelled by adaptively
forming beam patterns, wherein nulls of the beam patterns are
steered in the direction of the source(s) of jamming interference.
Many existing adaptive techniques require a training period in
which a signal is not present (such as during a passive listening
period), or a period in which the signal value is low compared to
jamming interference (such as in a search radar system) in order to
distinguish signal energy from jammer energy. However, during HRR
processing, signal content is available in all frequency samples.
Thus, conventional adaptive techniques may cancel desired signal
content in addition to canceling jamming interference. Also,
conventional adaptive techniques tend to modulate the signal of
interest, causing degradation in sidelobe performance, as a result
of changing adaptive weight values. Thus, conventional adaptive
techniques may degrade the image quality of an HRR profile.
[0005] Wideband jamming cancellation, in conjunction with stretch
processing, was introduced in a document entitled "Nulling Over
Extremely Wide Bandwidths When Using Stretch Processing",
proceedings of Adaptive Sensor Array Processing (ASAP), March 1999.
The technique introduced in that document processed a wideband
signal as a sequence of narrowband signals. However, this technique
does not address the signal cancellation or the adaptive weight
modulation problems described above. Thus a need exists for an HRR
process that can create an HRR profile and maintain the quality of
the profile in the presence of countermeasures.
SUMMARY OF THE INVENTION
[0006] A system and method for creating a high resolution range
(HRR) profile for a radar target of interest in the presence of
jamming interference include transmitting an HRR waveform and
receiving an echo signal resulting from the transmitted HRR
waveform. Beam patterns are formed for each echo signal segment of
the echo signal such that at least one null of each beam pattern is
steered toward at least one interference and a frequency dependent
gain of each beam pattern is maintained toward the center of the
target of interest. The HRR profile is created from the beam
patterns.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The foregoing and other objects, aspects and advantages will
be better understood from the following detailed description of a
preferred embodiment of the invention with reference to the
drawings, in which:
[0008] FIG. 1A is a graph of an exemplary relationship between time
and frequency of a chirp waveform, in accordance with an embodiment
of the present invention;
[0009] FIG. 1B is a diagram of an envelope of an exemplary stepped
frequency waveform 36 in accordance with an embodiment of the
present invention;
[0010] FIG. 2 is a functional block diagram of an exemplary HRR
processing system in accordance with an embodiment of the present
invention;
[0011] FIG. 3 is a flow diagram of an exemplary process for
creating an HRR profile in accordance with an embodiment of the
present invention; and
[0012] FIG. 4 is a block diagram of a radar system comprising an
antenna array 60 and computer processor 62 in accordance with an
exemplary embodiment of the invention.
DETAILED DESCRIPTION
[0013] It is an object of many radar systems to obtain long
detection range and fine range resolution. High range resolution
(HRR) processing is advantageous, inter alia, to distinguish
targets that are relatively close together, to create detail target
images, to aid in target recognition, and to form detailed ground
images. One means for accomplishing this objective is to transmit
an HRR waveform, which provides the desired average power (thus
providing desired detection range) and decoding the received echoes
resulting from the HRR waveform(s) via pulse compression, and
performing a weighted inverse fast Fourier Transform (FFT).
[0014] An HRR waveform may comprise a continuous LFM (linear
frequency modulated) chirp waveform or a stepped frequency waveform
(pulses of energy). FIG. 1A is a graph of an exemplary relationship
between time and frequency of a chirp waveform, in accordance with
an embodiment of the present invention. A chirp waveform is a
waveform wherein the frequency of the waveform is either increased
or decreased at a constant rate with respect to time. Curve 34
indicates the frequency of an exemplary chirp waveform as a
function of time. Curve 34 is monotonically increasing, although in
another embodiment of the invention, curve 34 is monotonically
decreasing. Curve 34 is depicted as a straight line, indicating a
linear relationship between the frequency and time of the generated
waveform. Although a linear HRR waveform is described herein, a
nonlinear HRR waveform, such a parabolic HRR waveform, is also
envisioned.
[0015] FIG. 1B is a diagram of an envelope of an exemplary stepped
frequency waveform 36 in accordance with an embodiment of the
present invention. Waveform 36 comprises a plurality of waveform
segments 35 each having a different frequency, f.sub.1 through
f.sub.N-1. Frequencies f.sub.0 through f.sub.N-1 may increase or
decrease with respect to time. The duration, in time, of each
waveform segment is denoted as T.sub.1 through T.sub.N,
respectively. In an exemplary embodiment of the invention,
durations T.sub.1 through T.sub.N are equal. The separation between
waveform segments 35 is .DELTA.f. In an exemplary embodiment of the
invention, .DELTA.f is the same between all segments 35 of waveform
36.
[0016] Pulse compression comprises a delay line or filter (or
similar means), that introduces a time delay into a signal. The
time delay is inversely proportional to the frequency of the
signal. Thus, the introduced time delay decreases with frequency at
the same rate as the frequency of the echoes increases. For
example, referring to FIG. 1B, if f.sub.0 is the highest frequency
and f.sub.N-1 is the lowest frequency, f.sub.1 will take less time
to pass through the pulse compressor than f.sub.N-1. Also, echoes
resulting from each segment 35 of a transmitted HRR waveform will
be delayed in the same manner. The result is that all frequencies
are aligned at the output of the pulse compressor to the same time.
Thus, the information contained in each echo resulting from each
transmitted waveform segment 35 are superimposed upon one another.
Thus, echoes from closely spaced targets are merged in the received
echo resulting from the HRR waveform, but are separate at the
output of the pulse compressor.
[0017] FIG. 2 is a functional block diagram of an exemplary HRR
processing system 200 in accordance with an embodiment of the
present invention. In an exemplary embodiment of the invention
antenna 12 is an antenna array. Transmitter 14 receives a modulated
center frequency from waveform generator/oscillator 20. Waveform
generator/oscillator 20 is a frequency agile oscillator, thus being
capable of changing frequencies rapidly. Transmitter 14 is a radar
transmitter of any appropriate type well known in the art. Waveform
generator 20 generates HRR waveforms used to create a HRR profile
in accordance with the present invention. In an exemplary
embodiment of the invention, the waveforms generated by waveform
generator 20 comprise chirp waveforms and/or stepped frequency
waveforms. Waveform generator 20 may also generate the waveform(s)
used for detecting radar targets of interest. Waveforms may include
LFM and stepped frequency waveforms.
[0018] Antenna 12 transmits radar signals 13 and receives reflected
radar signals 15 (echoes), and provides signals 17 corresponding to
these echoes to dechirper 16. Dechirper 16 dechirps the received
echo signal. Dechirping comprises multiplying the chirped signal by
a signal provided by the waveform generator/oscillator 20 having
the same slope as the chirped signal. Dechirping produces a
baseband signal, which is provided to digital beamformer 18.
Adaptive digital beamformer 18 (ADBF) is a frequency dependent
beamformer. Beam patterns are formed for each received echo from
each segment of the transmitted HRR waveform in beamformer 18. In
an exemplary embodiment of the invention, beams are adaptively
formed in adaptive digital beamformer 18. Beamformer 18 provides
beamformed signals 19 to range bin selector 22. In an exemplary
embodiment of the invention, a target tracker operates in
conjunction with HRR processing system 200 (tracker not shown in
FIG. 2). The tracker updates information pertaining to the location
of a target of interest (e.g., updates estimated angle of arrival
and estimation range). Tracker information 25 is provided to range
bin selector 22 to determine the target center, and the resultant
signal 23 is provided to pulse compressor 24. The resultant signal
is pulse compressed by pulse compressor 24 using the same type of
waveform that was used to generate the HRR waveform (i.e., LFM or
stepped frequency waveform). Pulse compression comprises a matched
filtering process, wherein the provided signal is convolved with a
replica of the HRR waveform.
[0019] The compressed signal 31 is motion compensated and then
provided to an inverse FFT processor 32 to create an HRR profile.
Quadratic phase motion compensation comprises multiplying the phase
component of the compressed signal 31 by its complex conjugate to
remove higher order (quadratic) terms, which are related to
velocity. If this term is not removed, the HRR profile image may
comprise modulation distortion. The quadratic phase motion
compensated signal is
[0020] In operation, a target of interest is detected and tracked
using conventional narrowband waveforms. These narrowband waveforms
may include CW and/or FM (linear and nonlinear) waveforms. Target
tracking comprises updating and maintaining positional information
pertaining to a target of interest (e.g., estimated arrival angel,
estimated range). In an exemplary embodiment of the invention, once
target tracking is commenced, HRR processing is commenced. HRR
processing, in accordance with an exemplary embodiment of the
invention, comprises transmitting an HRR waveform, receiving echo
signals resulting from the transmitted HRR waveform, adaptively
beamforming segments of the received echo signals to produce
beamformed data, performing pulse compression on the beamformed
data, performing quadratic phase motion compensation on the
compressed data, providing the compensated compressed signal to an
inverse FFT processor for producing an HRR profile (image).
[0021] FIG. 3 is a flow diagram of an exemplary process for
creating an HRR profile in accordance with an embodiment of the
present invention. A target of interest is detected in step 40 and
tracked in step 42. This detection and tracking may comprise any
means known in the art. HRR processing in accordance with the
present invention may be accomplished independent of the detection
and tracking of the target of interest. However, the quality of the
HRR profile may be improved if motion compensation is performed in
accordance with information provided by the tracker. The HRR
waveform is transmitted in step 44. The HRR waveform may comprise
an FM chirp waveform and/or a stepped frequency waveform. Echoes
resulting from the interaction of the transmitted HRR waveform and
objects including the target(s) of interest, ground clutter,
targets not of interest, and other objects, are received in step
46. The chirped received echo signal is dechirped in step 47 using
an LFM dechirping waveform having the same slope as the transmitted
chirped signal.
[0022] Beamforming is performed in step 48. In an exemplary
embodiment of the invention, beamforming is performed adaptively.
Each echo signal corresponding to each segment 35 of the
transmitted HRR waveform is beamformed. That is, a beam pattern is
formed for each received echo signal corresponding to each segment
35, and weights are calculated for each beam pattern. If the
transmitted HRR waveform is a chirp waveform, the received echo is
separated into segments and beamforming is performed for each
segment.
[0023] HRR processing is vulnerable to interference due to jamming
because it requires a relatively wide operational bandwidth. This
increases the chances that a jammer at a particular frequency will
be in the operational bandwidth. HRR processing is also vulnerable
to jamming interference because of the relatively long coherent
integration time associated with HRR processing, thus increasing
the likelihood that a jammer will transmit while the HRR echoes are
being received. Therefore, it is generally desirable to eliminate
jamming interference from the received signal (e.g., via
cancellation or attenuating) in order to avoid performance
degradation. Jamming interference may be canceled adaptively or
manually.
[0024] In an exemplary embodiment of the invention, jamming
interference is cancelled adaptively by forming beam patterns,
wherein nulls of the beam patterns are steered in the direction of
the source(s) of jamming interference. Many existing adaptive
techniques require a training period where no signal is present
(such as during a passive listening period), or a period where the
signal value is low compared to the jamming interference signal
(such as in a search radar system) in order to distinguish signal
energy from jammer energy. However, during HRR processing, signal
content is available in substantially all frequency samples. Thus,
conventional adaptive techniques may cancel desired signal in
addition to canceling jamming interference. Further, conventional
adaptive techniques tend to modulate the signal of interest,
causing degradation in sidelobe performance as a result of changing
adaptive weight values. Thus, conventional adaptive techniques may
degrade the image quality of the HRR profile. An HRR process in
accordance with the present invention tends to overcome these
problems by tracking the target center, with respect to range, for
each pulse and constrain the adaptive processing such that the
frequency dependent gain is maintained toward the target center for
each adaptive processing block.
[0025] Still referring to FIG. 3, beams are formed with respect to
the target center in accordance with information provided by the
tracker. Weights are adaptively calculated for each received echo
signal corresponding to each waveform segment 35 (see FIG. 1B) of
the transmitted HRR waveform, and the weights are calculated to
steer at least one null toward an interference, and within the
constraints that frequency dependent gain is maintained toward the
target center.
[0026] A mathematical description of the adaptive weights in
accordance with the present invention follows. In this description,
the underlying target model consists of K scattering centers
located at positions represented by R.sub.k, k=1, 2, . . . K. The
frequency measurement for each range bin of the received echo is in
accordance with the following equation. 1 r i = k = 1 K g S A ( T x
k , T y k , f i ) - j2 f i ( 2 R k c ) + J i + n i , i = 0 , 1 , ,
N - 1 , ( 1 )
[0027] where, r.sub.i is the vector of the array (or sub-array)
measurement for the selected range bin of the i.sup.th pulse,
g.sub.SA is the array (or sub-array) gain vector at frequency
f.sub.i and steering direction (T.sub.x, T.sub.y), J.sub.i and
n.sub.i are the jamming interference and noise component,
respectively, c is the speed of light in a in the transmission
medium, R.sub.k is the position of the k.sup.th scattering center,
and T.sub.x is the azimuth directional cosine and T.sub.y is the
elevation directional cosine calculated in accordance with the
following equations.
T.sub.x=cos(.beta.)sin(.theta.) (2)
T.sub.y=sin(.beta.), (3)
[0028] where .theta. is the steering angle, in azimuth, off
boresight of the antenna array, and .beta. is the steering angle,
in elevation, off boresight of the antenna array.
[0029] Because there is no passive listening period for wideband
imaging (HRR profile), there is a potential for signal
cancellation. This is due to the adaptive beamforming algorithm
attempting to calculate weights, which steers a null towards the
signal. This signal cancellation can be avoided if a constraint is
used such that the frequency dependent gain is steered toward the
target reference center. This constraint will maintain the mainlobe
of the beam pattern steered substantially toward the target center,
and allow nulls to be formed in the direction of jamming
interference. Weights formed within this constrain, in accordance
with an exemplary embodiment of the invention are calculated in
accordance with the following equation. 2 W i = C i - 1 g S A ( T x
S , T y S , f i ) g S A ( T x S , T y S , f i ) H C i - 1 g S A ( T
x S , T y S , f i ) g ( T x S , T y S , f i ) , ( 4 )
[0030] where, the index i indicates the number of processing blocks
into which the received echoes are separated for processing,
C.sub.i is the covariance matrix estimate of the i.sup.th
processing block, W.sub.i is the adaptive weight of the i.sup.th
block, g.sub.SA(T.sub.x, T.sub.y, f.sub.i) is the array (or
sub-array) gain vector used as the steering vector, H indicates the
complex conjugate transpose, g.sub..SIGMA.(T.sub.x.sup.S,
T.sub.y.sup.S, f.sub.i) is the sum beam gain (tapered beam pattern
steered toward the target) at frequency f.sub.i and steering
direction (T.sub.x.sup.S, T.sub.y.sup.S), and the superscript S
indicates the steering direction toward the target center, which is
constant for each processing block.
[0031] A sum beam is typically the weighted sum of the sub-array
measurements. Sum beam antenna patterns usually peak at the desired
signal direction and have low sidelobes. Accordingly, a desired sum
beam gain is achieved and maintained towards the target reference
for all pulses while jamming is cancelled. In this manner, the
image or HRR profile quality is maintained with low range sidelobes
and can be used for target discrimination or recognition
applications.
[0032] Referring again to FIG. 3, pulse compression is then
performed on the beamformed data at step 50. The selection of the
bin representing the target center is determined in accordance with
target range information provided by the tracker. This is followed
by pulse compression and quadratic phase motion compensation. This
information aids in compensating for target motion. Target motion
compensation improves the quality of the HRR profile (image) by
reducing quadratic phase error.
[0033] Tapered, inverse FFT processing is performed on the
quadratic phase motion compensated data at step 52. Weighted
tapering is optional, however weighted tapering processing may
enhance the quality of the HRR profile. Processing a wideband
signal using narrowband techniques (e.g., dechirping with a linear
frequency modulated waveform followed by a bandpass filter) is also
referred to as stretch processing. Stretch processing is described
in a document entitled "Nulling Over Extremely Wide Bandwidths When
Using Stretch Processing", proceedings of Adaptive Sensor Array
Processing (ASAP), March 1999, which is hereby incorporated by
reference in its entirety. Stretch processing is a process, which
enables processing of wideband waveforms (e.g., HRR waveforms) with
narrowband processing techniques. Stretch processing comprises
converting pulse delay time, in range, to frequency. Thus the
received energy from any one range has a constant frequency, and
the received energy from different ranges may be separated by well
know narrowband processing techniques, such as narrowband filtering
with a plurality of narrowband filters via a Fast Fourier Transform
(FFT). Thus, targets, which are relatively closely spaced in range,
are distinguishable in the HRR profile. The pulse compressed,
weighted, tapered, inverse FFT signals are formed into images in
step 54.
[0034] The present invention may be embodied in the form of
computer-implemented processes and apparatus for practicing those
processes. The present invention may also be embodied in the form
of computer program code embodied in tangible media, such as floppy
diskettes, read only memories (ROMs), CD-ROMs, hard drives, high
density disk, or any other computer-readable storage medium,
wherein, when the computer program code is loaded into and executed
by computer processor 32, the computer processor 32 becomes an
apparatus for practicing the invention. The present invention may
also be embodied in the form of computer program code, for example,
whether stored in a storage medium, loaded into and/or executed by
computer processor 32, or transmitted over some transmission
medium, such as over electrical wiring or cabling, through fiber
optics, or via electromagnetic radiation, wherein, when the
computer program code is loaded into and executed by computer
processor 32, the computer processor 32 becomes an apparatus for
practicing the invention. When implemented on a general-purpose
processor, the computer program code segments configure the
processor to create specific logic circuits.
[0035] The present invention may be embodied in the form of
computer-implemented processes and apparatus for practicing those
processes. FIG. 4 is a block diagram of a radar system comprising
an antenna array 60 and computer processor 62 in accordance with an
exemplary embodiment of the invention. HRR waveforms are created by
processor 62 and provided to antenna array 60. HRR waveforms are
transmitted by antenna array 60. Reflected energy resulting for the
transmission of the HRR waveforms is received by antenna array 60
and is provided to computer processor 62. Computer processor 62
performs processes for forming beam patterns, performing pulse
compression, performing stretch processing, and generating images
in accordance with the present invention, as herein described.
Processing may also be performed by special purpose hardware.
[0036] The present invention may be embodied in the form of
computer-implemented processes and apparatus for practicing those
processes. The present invention may also be embodied in the form
of computer program code embodied in tangible media, such as floppy
diskettes, read only memories (ROMs), CD-ROMs, hard drives, high
density disk, or any other computer-readable storage medium,
wherein, when the computer program code is loaded into and executed
by computer processor 62, the computer processor 62 becomes an
apparatus for practicing the invention. The present invention may
also be embodied in the form of computer program code, for example,
whether stored in a storage medium, loaded into and/or executed by
computer processor 62, or transmitted over some transmission
medium, such as over electrical wiring or cabling, through fiber
optics, or via electromagnetic radiation, wherein, when the
computer program code is loaded into and executed by computer
processor 62, the computer processor 62 becomes an apparatus for
practicing the invention. When implemented on a general-purpose
processor, the computer program code segments configure the
processor to create specific logic circuits.
[0037] Although illustrated and described herein with reference to
certain specific embodiments, the present invention is nevertheless
not intended to be limited to the details shown. Rather, various
modifications may be made in the details within the scope and range
of equivalents of the claims and without departing from the spirit
of the invention.
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