Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Reexamination Certificate
2002-04-23
2004-02-24
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
C342S160000
Reexamination Certificate
active
06697010
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a radar system and method for detecting moving targets. More specifically, the present invention relates to a synthetic aperture radar (SAR) system for identifying moving targets from a set of data returns received at a single receive phase center.
BACKGROUND OF THE INVENTION
Aircraft-borne radar systems have been designed in the past as ground mobile target indicators (GMTI). An airborne GMTI radar system radiates a plurality of electromagnetic pulses to illuminate a specific area on the ground. These pulses reflect off of moving targets as well as stationary clutter and targets. The reflected pulses are received by the radar system. The received pulses are processed by the radar system to reject stationary clutter and targets, and detect moving targets.
A conventional GMTI radar system utilizes multiple antennas to accomplish moving target detection. The number of antennas varies from 3 to 8. These antennas could be separate dish antennas, or different parts of a phased array antenna system. In the transmit mode, one of these antennas radiates from a transmit phase center. In alternative transmit modes, all or a selected group of the antennas radiate coherently together. In other words, they act as a single larger antenna with a single transmit phase center. The transmit phase center is the point from which the outward spreading electromagnetic pulses would seem to have originated, as seen by an observer from a distance. When the radar system radiates, there is a single transmit phase center, the location of which is controlled by electrical means and is not necessarily at the spatial center of the transmit antennas.
In the receive mode, the 3-8 antennas receive the reflected pulses as separate antennas. In other words, the antennas deliver separate outputs corresponding to different receive antenna phase centers. The separation of moving targets from stationary clutter is a key objective of a GMTI radar system. A variety of implementations have been demonstrated to meet this objective, and to meet a variety of other system requirements. Some of these implementations are referred to as Synthetic Aperture Radar (SAR) processing, Doppler Radar processing, Displaced Phase Center Antenna (DPCA) processing, and Space Time Adaptive Processing (STAP). A discussion of DPCA and SAR based moving target detection is given by Mehrdad Soumekh, “Synthetic Aperture Radar Signal Processing with MATLAB Algorithms,” John Wiley & Sons, Inc., New York, 1999, page 561 to 585.
Conventional implementations of a GMTI radar system require multiple separate receiving antennas. Described as follows are two conventional methodologies for performing GMTI. The first methodology for performing GMTI relies on achieving clutter rejection based on spatial correlation between received pulses. The second methodology for performing GMTI relies on achieving clutter rejection based on both spatial and temporal correlation of consecutive received pulses.
The first methodology for performing GMTI will be described with respect to a conventional GMTI radar system that is designed to synthesize a very large array antenna, using an aircraft that flies along a straight line. This is a basic configuration of synthetic aperture radar (SAR). The SAR radiates from different locations along the flight path of the aircraft. These locations are denoted by {s
1
, s
2
, s
3
, s
4
, . . . , s
n
, . . . }, which denotes a set of closely spaced points along a straight line. The motion of the aircraft causes the transmit phase center of the GMTI radar system to pass through these points. When the transmit phase center of the radar system is aligned with one of these points, the radar radiates a pulse. Since the aircraft flies at a constant velocity, the radar radiates at regular intervals.
The pulse repetition frequency (PRF) of the radar and the velocity of the aircraft are chosen so that the reflected pulses would be arriving at the radar in a proper manner, described as follows.
FIG. 1A
depicts a GMTI radar system
100
that has 5 antennas with phase centers denoted by c
0
, c
1
, c
2
, c
3
, and c
4
. The radar system selects the phase center denoted by c
3
as the transmit phase center and radiates a pulse from all 5 antennas. At the time of radar transmission the antenna phase centers are aligned with the locations {s
98
, s
99
, s
100
, s
101
, s
102
}. Thus, the radar effectively radiates a pulse from the location s
101
. A short interval later, the reflected pulse arrives.
FIG. 1B
illustrates a short time later when the reflected pulse is received back by the radar. By the time the aircraft radar system receives the reflected pulse, the aircraft has moved forward, and the phase centers of the antennas, {c
0
, c
1
, c
2
, c
3
, c
4
}, are aligned with {s
99
, s
100
, s
101
, s
102
, s
103
}. The radar system uses these antennas to receive the reflected pulse. Each of the five antennas detects radiation, generating five receive “outputs” or “channels”. The output from the receiving antenna at c
2
is denoted the mono-static SAR signal The terminology “mono-static” refers to the notion that the location for the transmission of the out-going pulse (or subsequent out-going pulses) and the reception of the reflected pulse (or subsequent reflected pulses) are the same. The outputs from other receive channels are denoted bi-static SAR signals. The terminology “bi-static” refers to the arrangement where the locations for transmit and receive are different. For example, the signal received by antenna C
2
in
FIG. 1B
is a mono-static signal because it is received allocation S
101
and the pulse was effectively radiated from location S
101
(shown in FIG.
1
A). The signals received by antennas C
0
, C
1
, C
3
, and C
4
in
FIG. 1B
are bi-static signals.
The mono-static SAR signal is used to construct an image of the target area based on SAR processing. The resultant image is denoted a mono-static SAR image. The bi-static outputs are used to construct bi-static SAR images. If the clutter and targets in the area illuminated by the radar system are stationary, then it can be shown that any one of the bi-static SAR images could be used to produce an estimate of the mono-static SAR image, or vice versa.
Since multiple bi-static images are available, they can be combined to produce a good estimate of the mono-static image. The stationary clutter or targets that appear in the mono-static SAR image can be suppressed or substantially eliminated by subtracting the mono-static SAR image by an estimated version of the mono-static SAR image. The result of subtracting two images is another image, where the image features due to stationary clutter and targets are substantially diminished but the image features due to moving targets are enhanced.
For clarity of discussion, the above example ignored a practical matter. When the receive antennas {c
0
, c
1
, c
2
, c
3
, c
4
}, are aligned with {s
99
, s
100
, s
101
, s
102
, s
103
} as in FIG.
1
B and are configured to receive, the above example implies that the GMTI radar system is configured to radiate a pulse at the same time, so that subsequently, another reflected pulse would arrive at a later time. In practice, a GMTI radar system would not radiate and receive at the same time, rather radar pulses should be radiated between intervals of reception.
In summary, a first conventional GMTI radar system and method is comprised of several receive antennas. Several SAR images are produced using the signals received by spatially separated antennas. At one of the intermediate data processing steps, one SAR image is produced using the data from a specific receive channel (the mono-static signal), and an estimated version of the SAR image is produced using the data from the other receive channels. Clutter rejection is achieved by taking the difference of the two images. This approach to clutter rejection is based on the correlation that exists between the outputs of spatially separated
Alsomiri Isam
Lockheed Martin Corporation
Tarcza Thomas H.
Townsend and Townsend / and Crew LLP
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