M-sequences and bi-static active sonar

Abstract

The application of m-sequences to bi-static active sonar is evaluated with data from acoustic propagation experiments in the Florida Straits. Over the past 20 years, several numerical methods for processing M-sequences have been developed by Birdsall, Metzger and others. These methods come together for a possible sonar application by the following approach: (1) continuous transmission and reception of long m-sequences, (2) synchronous sampling to form a CON (complete ortho-normal) data set, (3) direct blast removal by HCCO (hyperspace cancellation by coordinate zeroing), and (4) a full range waveform Doppler search. Ultra-fast Hadamard transforms speed up the direct waveform pulse m-sequence pulse compression and the inverse pulse waveform transform and thereby allow timely execution of the intensive computational burden. The result is a numerically demonstrable approach that produces a gain of 36 dB over a simple pulse and 16 dB over other active signals. In addition, the direct arrivals and their Doppler leakage are eliminated, thus shifting the detection problem from a reverberation limited to a noise limited Doppler process. Here, m-sequence data from propagation experiments are re-processed to test the fundamental concepts. Data results are shown to be in close agreement with ideal numerical simulations.