On the Design of Direct Sequence Spread-Spectrum Signaling for Range Estimation

Precise range measurement by time-of-flight sonar is important for underwater positioning, oceanography (tomography) and marine geology (geodesy). This paper reports on the design of spread-spectrum codes for range measurement in a variety of important underwater environments. Direct sequence spread-spectrum (DSSS) signal processing has many advantages over continuous wave techniques for time-of-flight range estimation: improved precision, extended effective range, robustness to ambient or jamming noise, increased update rate, and simultaneous multi-user capability. Design of appropriate DSSS codes requires matching the code parameters to the acoustic operating environment to maximize system performance. We consider three canonical ocean environments: a laboratory test-tank, the littoral zone, and the deep water channel. The characteristics of these acoustic channel models directly influence the code design to maximize range estimate performance. We parameterize the design choices for DSSS codes by code type, code length, carrier frequency, and chip rate. The paper concludes with experimental results for spread- spectrum range estimation in a shallow water dynamic environment and on an operation ROV in deep water.