Analysis of effective signal design for active sensing of undersea objects/bottoms in tropical shallow waters

Abstract

Precise understanding of the sea bottom characteristics and accurate identification of objects on the seabed are critical in many underwater applications. The severe multi-path along with the time varying and random reflections from the surface and the bottom of shallow waters, present an interesting and complex signal processing problem. A thin line hydrophone array, deployed from small Autonomous Underwater Vehicles (AUV) provides significant operational advantage in terms of accessibility in shallow coastal areas and does open up substantial application possibilities. The shallow tropical waters in general present high ambient noise levels due to high density snapping shrimp beds and shipping. This often requires that for getting a reasonably good Signal to Noise Ratio (SNR) the acoustic transmissions have to be powerful to combat the noise level. However, this is not a good option for many battery powered platforms like an AUV as it heavily limits their operational endurance. An alternative is to design specific sonar signals which have high correlation properties and can provide good detection performance under severe noise conditions. Although the sonar waveform design is not new, such application specific signal processing attempt is not reported in the literature. The work will present a two-stage, Non-Linear Frequency Modulated (NLFM) signal design for active undersea sensing using a Digital Thin Line Array (DTLA). The signal design has been undertaken for the frequency band 3–10 kHz, with variations in center frequency and bandwidths. The different chirp rates in a two-stage LFM signal and non-LFM were evaluated for their effectiveness in suppressing the side lobes. The bottom reflected signal has been correlated with the direct path signal, to allow minimal difference between the received signal and the receive filter characteristics, due to the underwater channel. The well-known shallow water propagation loss model proposed by Rogers (1974) was used to simulate the underwater channel conditions in shallow water conditions. The performance metric was based on the ability to minimize the integrated side lobe level metric (ISL) of the normalized correlation function. Bottom types comprising sand and silt have been used in the simulated channel model to compare the performance.