Repeat-Sequence Coding for Improved Precision of Doppler Sonar and Sodar

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Abstract
Repeat-sequence coding is a robust method for improving the precision of velocity estimates from incoherent Doppler sounders. The method involves transmitting a number of repeats of a broadband “subcode.” The Doppler shift is estimated from the complex autocovariance of the return, evaluated at a time lag, equal to the subcode duration. The repeat-sequence code is an extension of the simple pulse-train concept developed in the early days of radar. By transmitting codes, rather than discrete pulses, the average transmitted power is increased. A model is developed here to predict performance enhancement for specified codes. The model is based on the sample error of the covariance estimates. It explicitly accounts for the lag used. Root-mean-square precision is enhanced roughly in proportion to the square root of the time-bandwidth product of the subcode. Coded pulse technology has been implemented on a variety of Doppler sonar systems at Scripps Institution of Oceanography and used in both open-ocean (volume-scattering) and shallow-water (surface-scattering) applications. Field measurements of sonar precision roughly agree with predictions of the model, although with some increase in error.

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