Sub-band processing of continuous active sonar signals in shallow water

Continuous active sonar (CAS) is a special case of active sonar where the ping duration approaches the ping repetition interval. Sub-band processing is a method of breaking up the long-duration CAS transmission to achieve faster processing update rates than pulsed active sonar (PAS). This paper compares experimental signal-to-reverberation ratio (SRR) measured for CAS and PAS echoes. The processing bandwidth was varied, and the processing integration time was proportional to the bandwidth because linear frequency modulated (LFM) pulses were employed. The data were collected during back-to-back CAS and PAS runs during the TREX13 sea trial, which was conducted in shallow, 20-m water near the shore of Florida, USA. Sub-band processing was applied to the CAS data as well as to the PAS data to allow comparison, although for PAS the increased updates are not at equal intervals over the ping cycle as in CAS. Increasing the number of sub-bands increases the update rate, but the corresponding decrease in bandwidth is expected to reduce SRR. Examining the trade off between update rate and SRR is an important first step in understanding how CAS performs in the littoral environment.