A long‐range, high‐resolution Doppler sonar for fishery stock assessment

Abstract

A new approach is developed for horizontal‐looking active sonar that uses continuous bi‐/multistatic transmission of very long period m‐sequences. Compared to conventional monostatic ping rather than listen sonar, this approach improves the Doppler resolution by a factor of 20 and increases FOM by perhaps 13 dB by increasing the temporal coherent processing gain. The improved time/Doppler resolution helps to discriminate fish school from bottom reverberation, and improves the resolution of taxonomic parameters of fish assemblages. The improved FOM allows coverage of large areas of shallow oceans with source levels low enough to avoid environmental concerns for marine mammals. The principal obstacle to continuous transmission sonar operation is the masking of the target return by the time and frequency leakage of the direct path arrivals. Unique time‐correlation property of m‐sequences elimination time leakage and a process calls coordinate zeroing (CZ) eliminates all the Doppler leakage. A linear temporal Doppler search (LTDS) algorithm is used to shift the detection problem to a high‐resolution pulse arrival‐time/Doppler plane with a noise‐limited background. Ultrafast Hadamard transforms lighten the computational burden. The combination of these ideas is presented including the theory, numerical experiments that demonstrate the concept, and verification with real ocean signals.