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System and method for detection and discrimination of targets in the presence of interferenceSystem and method for detection and discrimination of targets in the presence of interference description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20090027257, System and method for detection and discrimination of targets in the presence of interference. Brief Patent Description - Full Patent Description - Patent Application Claims
CLAIM OF PRIORITY AND INCORPORATION BY REFERENCE
This application claims priority of provisional application Ser. No. 60/687,661, which is incorporated herein by reference. U.S. Pat. No. 6,636,174, U.S. application Ser. No. 09/875,116 (Pub. No. 2002/0030623), application Ser. No. 10/691,245 (Pub. No. 2004/0085241), and application Ser. No. 11/180,811 (Pub. No. 2006/0082491) are also incorporated herein by reference for all purposes. FIELD OF INVENTIONThis invention relates generally to Space-Time Adaptive Processing (STAP) technology and more particularly to computationally effective discrimination, detection and tracking of targets in the presence of interference, such as environmental clutter and intentional jamming. BACKGROUND OF INVENTIONA typical prior art STAP system comprises a phased-array antenna with N transmit elements and N receive elements. The receiver antenna gain pattern can be steered in a desired direction through a beam forming process. The STAP system operates using a pulse train and coherent pulse integration. The Coherent Processing Interval (CPI) defines the duration of the pulse train. During each CPI the transmitter sends out M pulses (or signals). The time between the beginning of a pulse and the beginning of the next pulse is called a Pulse Repetition Interval (PRI). The pulses reflect from objects at different distances from the STAP system. The antenna elements then receive the reflections of the pulses. The distance to an object (or the range) may be determined by the amount of time that passes between the sending of a pulse and receiving of its reflection, referred to as time delay. The STAP system collects the reflections for each antenna element (1 through N), for each pulse (1 through M), and for each range. The data received from these reflected signals can be conceptually assembled into a three-dimensional matrix which is sometimes called “the STAP cube.” There is a trade-off associated with selecting an optimal PRI value for the prior art STAP systems. On one hand, longer PRI minimizes range ambiguity. In particular, it is desirable to receive reflections of one pulse from all targets before sending the next pulse. If PRI is relatively short, then a reflection received after a transmitted pulse would create an ambiguity as to whether it is a reflection of this pulse or the previously transmitted pulse. Selecting a longer PRI would mitigate the effects of this ambiguity. On the other hand, for coherent pulse integrating systems, longer PRI increases Doppler shift ambiguity. The inverse of PRI is called Pulse Repetition Frequency (PRF). PRF determines the maximum unambiguous Doppler shift for a target. Targets for which the absolute value of Doppler shifts is greater than one half PRF results in aliasing in the Doppler shift domain and appear to be at some Doppler shift with an absolute value that is less of than or equal to one half PRF. Prior art STAP systems, which require a high PRF to attain a desired maximum unambiguous Doppler shift value are, however, limited as to the maximum allowable transmit pulse duration. As the PRF is increased, less time is available to transmit the pulse and wait for the return. For high operating frequencies, fast target velocities, and large unambiguous distances, this can result in very short pulse durations. Transmitting short pulses leads to the need to transmit high peak powers so that sufficient total energy is transmitted to the target. Prior art STAP systems use a matched filter to detect the reflected signal. The matched filter performs well in detection of the reflected signal obscured by noise as long as the reflected signal matches the transmitted signal temporally, that is, as long as the reflected signal has not been Doppler shifted with respect to the transmitted signal. To the extent that the reflected signal has been Doppler shifted relative to the transmitted signal, the detection sensitivity of the matched filter degrades. If the Doppler shift is large enough, the detection sensitivity of the matched filter will be insufficient and an additional matched filter will be required, matching to the Doppler shifted version of the transmitted signal. The need of multiple matched filters in the prior art STAP systems is costly as it requires multiple subsequent, computationally intensive STAP detection system components. There are two general classes of signals that can be characterized for detection through a matched filter: Doppler fragile and Doppler tolerant signals. Introducing a Doppler shift in Doppler fragile signals results in quickly degrading detection sensitivity through a matched filter. Doppler fragile signals include pseudorandom number (PN) coded signals, frequency stepped COSTAS signals, and in general most long arbitrarily modulated signals. Introducing a Doppler shift in Doppler tolerant signals results in continued sufficient detection sensitivity through a matched filter for most Doppler shifts of interest. Doppler tolerant signals include signals of very short duration and linear FM chirped signals. In terms of system performance, Doppler fragile signals can be characterized as providing high Doppler shift resolution, low probability of intercept in adversarial conditions, good performance in the presence of multiple coexisting and co-operating systems, and difficult to counter with electronic jamming. Doppler tolerant signals often are characterized as easier to generate, transmit and receive, poor Doppler shift resolution, harder to conceal from undesired receivers, and easier to counter with electronic jamming. Because of the undesirability of multiple matched filters in the prior art STAP systems, they are typically designed to utilize Doppler tolerant signals. The inability of the prior art STAP systems to effectively process Doppler fragile signals limits the types of signals that can be used by such systems to only a few. This makes it easy for an enemy to detect the transmitted signals and use Electronic Countermeasures (ECM) to jam them. After receiving all signals in the CPI, which comprise the STAP cube, and constructing the STAP cube, prior art STAP systems coherently process all of the received signals across all of the antenna elements at all time delay values. This coherent processing is the equivalent of a two dimensional Fourier transformation at each range value and it effectively transforms the three dimensions of the STAP cube to angle, Doppler shift (or velocity), and range time delay (or range) and a reflected signal amplitude for each three dimensional coordinate within the STAP cube. For any given angle in the STAP cube, the time delay—Doppler shift plane is equivalent to a cross-ambiguity function of the transmitted and received signals for that look angle, over the unambiguous range and Doppler extent of the given PRI. The cross-ambiguity function of a transmitted and received signal is defined according to the following equation: Ars=∫r(t+τ/2)s*(t+τ/2)exp[j2πvt]dt, where:
s(t) is the transmitted signal,
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