High Duty Cycle (HDC) Sonar Processing Interval and Bandwidth Effects for the TREX’13 Dataset Randall Plate, Doug Grimmett SPAWAR Systems Center Pacific San Diego, CA, USA rplate@spawar.navy.mil , grimmett@spawar.navy.mil 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. Keywords— continuous active sonar; high duty cycle sonar; continuous time LFM I. INTRODUCTION Recently, there has been emerging interest in the concept of High Duty Cycle (HDC) Sonar. Unlike Pulsed Active Sonar (PAS), which listens for echoes in between short transmission bursts, HDC sonar attempts to detect echoes amidst the continual interference from source(s) transmitting with nearly 100% duty cycle. A schematic of the two contrasting approaches is shown in Fig. 1. A potential advantage of HDC sonar is an increased number of continuous detection Fig. 1. Depiction of three cycles of PAS (top, blue) and HDC (bottom, red) transmission methods. The PAS “listens after transmit” and the HDC method “listens while transmit”. Tpri is the ping repetition interval and T is the LFM signal duration. opportunities, leading to improved target detection, localization, tracking, and classification. In addition, lower transmission source levels are possible. Of course, appropriate HDC-specific processing must be employed to enable detection in the presence of continuous transmissions. The HDC concept can be implemented within monostatic or multistatic systems, though in the case of multistatic systems the additional challenges of multisource mutual interference and effective data fusion must be overcome. Like PAS systems, HDC sonar systems may employ a variety of signal types. Single FM waveforms provide good target ranging measurements, but no Doppler information. Alternatively, CW waveforms provide good target Doppler (range-rate) measurements, but no ranging information. Systems, including multistatic systems, can gain valuable geometrically complementary detection opportunities when both signal types are used [1]. Finally, one may consider broadband waveforms which attempt to provide both good range and Doppler measurements simultaneously [2]. An HDC system which solely employs CW waveforms is unable to estimate target range, unless detections from multiple sensors (e.g., within multistatic configurations) are fused. These may be utilized for cross-fixing within a target tracker to obtain an unambiguous geographic localization [3]. An HDC system which employs the FM waveform provides unambiguous target range measurements. Furthermore, unlike Frequency Frequency Time Time Transmit & Listen Listen Transmit Tpri T T PAS HDC U.S. Government work not protected by U.S. Copyright