Temporal and Doppler coherence limits for the underwater acoustic channel during the LCAS'15 high duty cycle sonar experiment

Unlike conventional Pulsed Active Sonar (PAS) which listens for target echoes in between short-burst transmissions, High Duty Cycle (HDC) sonar attempts to detect echoes amidst the continual interference from source(s) transmitting with nearly 100% duty cycle. Continuous transmission waveforms may be of several types. Linear Frequency Modulated waveforms (LFMs) provide good range measurements but less accurate target Doppler; Continuous Wave signals (CWs) provide good Doppler measurements but less accurate target range. More sophisticated waveforms (e.g. PRNs) attempt to combine the positive effects of both of these waveforms. The potential advantage of HDC waveforms is either increased time-bandwidth product processing gain, or an increased number of detection opportunities, depending on the selection of the sonar receiver's processing time window (interval). For continuous time LFM (CTLFM) waveforms, very large time-bandwidth products can be achieved (about more than one order of magnitude larger than PAS waveforms). However, there are practical limits to the amount of coherent time-bandwidth gain that is achievable, due to the temporal and Doppler spreading effects of the underwater acoustic channel. This paper investigates the temporal and Doppler coherence limits of the underwater channel experienced during the Littoral Continuous Active Sonar 2015 experiment (LCAS'15), and compares them to theoretical expectations.