Bistatic STAP for Airborne Radar
Stephen Kogon and Michael Zatman
MIT Lincoln Laboratory
244 Wood St.
Lexington, MA 02420
Phone: (781) 981-3275
Abstract Bistatic radars, ones in which the transmitter and receiver are not co-located, have been used since the first deployments of radar systems. However, the complicated nature of bistatic clutter has so far discouraged the use of bistatic radar in an airborne system for the purposes of moving target indication (MTI) with space-time adaptive processing (STAP) for clutter mitigation. In particular, the separate motion of the transmitter and receiver creates two sources of Doppler for bistatic clutter. The resulting iso-Doppler relationship can be both complicated and non-stationary in range and, therefore, difficult to compensate for as is typically done for monostatic STAP. Yet, for certain applications, operational considerations make a bistatic MTI radar very attractive. In this paper, we present various considerations for STAP in an airborne bistatic radar. We first look at the iso-Doppler and iso-bistatic range relationships in a few illustrative examples that demonstrate the complicated natureof bistatic clutter and show the strong dependence on the transmitter/receiver geometry. STAP clutter mitigation is then considered where the range dependence issue is addressed for the training of the STAP weights. Both Doppler warping, used in monostatic systems to correct for non-stationarity in range, and a derivative-based weight updating scheme are investigated. Then, we propose the use of a continuous wave (CW) signal for bistatic radar. CW signals are possible because of the separate locations of the transmitter and receiver and have the very attractive characteristic of being unambiguous in both range and Doppler. In constrast, traditional pulsed waveforms often have performance limitations due to Doppler and range ambiguities. The STAP processing of a CW waveform differs from traditional pulse-Doppler STAP used for pulsed waveforms. We outline the processing steps, including matched filtering, Doppler processing, and STAP, used for MTI in a CW bistatic radar system. The other advantages and limitations of the CW signal are discussed in the context of STAP for clutter cancellation.
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