High Duty Cycle (HDC) sonar processing interval and bandwidth effects for the TREX'13 dataset

Abstract

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. HDC sonar presents an additional processing parameter, not available with PAS, which is the processing interval. The processing interval is a selectable subset of time within a CAS repetition cycle used for coherent processing. Hence, the choice of processing interval may be used to tune the performance of the sonar to local environmental conditions and to the operational scenario. Theoretically, increasing the processing interval increases target detectability, but in practice other factors should also be considered. In real acoustic environments, sound propagation is subject to temporal and spectral spreading effects, and these may limit the processing gains to lower levels than expected. Target Doppler can also become a more significant issue with longer processing intervals. Shorter processing intervals provide an increased number and rate of detection opportunities, which can be a significant advantage, leading to improved target holding, localization, tracking, and classification. This paper describes the various expected effects of the processing interval on performance for continuous-time LFM signals. It presents an analysis conducted on the TREX'13 sea trial dataset, and shows various results achieved as a function of processing interval. The results are explained and compared with theoretical expectations, and show the complicating effects of a real acoustic environment. In particular, we see the limitation on performance gains with increasing the processing interval due to acoustic environmental spreading effects, the target's physical extent and Doppler effects. Comparisons are shown between echoes from three different targets: mobile compact, mobile extended, and fixed. The evaluation describes performance using the quantities of Received Level, SNR, echo time-extent, and delay bias.

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

“…Once the effect of spreading is resolvable by the signal resolution, losses will occur, and as a result, further increases in time or bandwidth will not deliver the predicted gains. Previous work has shown that the background suppression follows theoretical predictions [2]. Here, we focus on the environmental effects on signal losses.…”

“…III. THE LCAS'15 SEATRIAL DATA Previous experimentation [2] has shown that one can estimate the coherence limit of the acoustic channel using the CTLFM signal. However, using a single, sub-banded CTLFM waveform, there is no way to determine if the channel's coherence limit is due to time spreading only or due to Doppler spreading only ('singly spread' channel), or spreading in both time and Doppler ('doubly spread' channel).…”

“…• If there is Doppler shift (range-rate), it must be constant Doppler without accelerations and, appropriate Doppler-matched processing is performed to avoid the smearing effect of processing large TB waveforms (this is necessary even for FM waveform which typically does not require it) [2].…”

Cooperative robotic networks for underwater surveillance: an overview

“…One may want to maintain the same bandwidth for a CAS 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 significant coherence limitations associated with the acoustic channel [188][189][190][191]. If sub-band processing is applied, it can not only provide a higher measurement rate [147,192] but can significantly mitigate the effects of limited time coherence of the channel [191] in CAS mode.…”

“…To experimentally examine the advantages and limitations of CAS (or HDCAS) with respect to the more traditional PAS mode [193], a series of sea trials have been conducted. Among others, the 2013 sea campaign Target and Reverberation Experiment (TREX‐2013) conducted in the Atlantic was partly devoted to evaluate the impact of pulse duration on echo statistics [194] and evaluate sonar performance with respect to environmental spreading effects, the target's physical extent and Doppler effects [190, 195]; the Littoral Continuous Active Sonar (LCAS) joint research and multinational project started in 2015 and, led by NATO STO CMRE, aims at the evaluation of CAS performance in littoral waters, generally characterised as a high‐reverberation environment, hence more challenging for CAS. Two sea campaigns were conducted in 2015 and 2016, respectively, in Mediterranean coastal waters in the context of LCAS project [191].…”