Measurements of signal coherence for high and low duty cycle sonars in a shallow water channel

Military sonars must detect, localize, classify, and track submarine threats from distances safely outside their circle of attack. However, conventional Pulsed Active Sonars (PAS) have duty cycles on the order of one percent which means that 99% of the time, the track is out of date. In contrast, High Duty Cycle (HDC) sonars have duty cycles approaching 100% which enable near-continuous updates to the track. If one can overcome technical challenges such as the high dynamic range required by the receiver, then HDC should significantly improve tracking performance in the (approximately) free-field environment that one encounters in the deep ocean. Improvements in tracking performance in shallow water, however, are not assured since both targets and clutter will be tracked continuously and HDC may increase false tracks to an unacceptably high level — essentially continuously tracking the clutter. Furthermore, one would typically want to maintain the same bandwidth for an HDC system as for the PAS system it might replace. This will provide a significant increase in the time-bandwidth product, but may not produce the increase in gain anticipated if there are coherence limitations of the acoustic channel that result from environmental variability. To compare performance of HDC with conventional PAS in the littorals, a set of experiments was conducted as part of the Target and Reverberation Experiment (TREX) in spring 2013. This was one of the first scientifically controlled experiments conducted in the littorals to compare environmental effects on these two active sonar techniques. In most active sonar experiments, the source, receiver, and target are moving so that one cannot cleanly isolate the impact of platform motion from that of environmental variability. In the experiment discussed in this paper, however, R/V SHARP was fixed in a four-point mooring, and a free-flooding ring transmitter and a horizontal line array receiver were deployed from the deck and mounted a few meters above the seabed. The target, an air-filled hose, was deployed approximately 3 km away and anchored to the seabed with minimal surface expression, thereby drastically reducing its motion. This fixed-fixed geometry provided an opportunity to examine the effect of the acoustic channel on signal coherence. An initial examination of the echo levels from an air-filled passive target shows that there is very little difference in the coherence loss of HDC and PAS pulses. However, the standard deviation of the HDC echo levels is about 0.1 dB lower than PAS under similar environmental conditions; this may be because the longer HDC pulses provide some averaging of the effect of surface roughness on the coherent gain.