Capture and Spurious Target Generation Due to Hard Limiting in Large Time-Bandwidth Product Radars

摘要

Hard limiting before pulse compression or correlation processing is a common approach to the CFAR (constant false alarm rate) problem, and it offers a good and simple solution in a single-target or scarce-target environment. With the advent of radars with a large time-bandwidth product the possibility arises that expanded radar returns due to multiple targets of interest may overlap very largely or entirely but still may be sufficiently separated to be resolved after receiver processing. In this case the compressed pulses cannot attain full amplitude at the processor output even if the signal-to-noise ratio at the input is very high; this phenomenon is known as capture and small signal suppression. The purpose of this report is to exhibit that, in addition to compressed target responses of reduced magnitudes, false targets may be generated with apparent amplitudes of the same order or exceeding those of legitimate targets. Spurious target generation in the case of chirp radar has been known for some time. The theory has been extended to maximum-length linear shift-register codes which are used as modulation functions of pulse-compression and phase-coded CW radars. It is found that a single pair of radar returns coded in this manner is subject to capture only and not to false target generation. Surprisingly, however, the addition of a third expanded signal produces a spurious response. This generation of a false target should be taken in account when the dynamic range of future phase-coded radars using linear shift-register code is specified, in partular if the radar is designed for automatic track and raid-size determination. (Author)