Mismatched filters for Doppler-sensitive active sonar pulses

A technique used for designing radar poly-phase codes, is used to design mismatched filters for long-range active sonar systems, where transmit waveforms are Doppler-sensitive. The long ranges and operational scenarios involved mean that only single-ping operations are considered. The waveforms under consideration are long-duration Cox combs and sinusoidally frequency modulated (SFM) pulses, arbitrarily selected to occupy the frequency range 1 to 2 kHz. When used in conjunction with a matched filter, these waveforms offer an attractive Doppler resolution, but suffer from significant range sidelobes. The proposed mismatched filters attempt to improve their range resolution or lower their sidelobes across the zero-Doppler region of the range-Doppler domain. The designed cross-ambiguity functions achieve range sidelobes lower than -32.5 dB for the Cox comb, and -33.0 dB for the SFM pulse, as well as acceptable Doppler sidelobes at non-zero delays. This comes at a price of a signal-to-noise ratio (SNR) loss of 18.5 dB for the Cox comb, and 20.5 dB for the SFM pulse.