Underwater telemetry method using doppler compensation

Abstract

A method for Doppler compensation in a phase coherent underwater communications system includes the steps of receiving a communications signal from a plurality of underwater communications channels, where the signal consists of a sequence of raw data; then applying a Doppler estimation to the data; demodulating the raw data; low pass filtering the demodulated raw data; applying resampling and downsampling to the data at a fractional sampling rate, for n samples per symbol, and generating a synchronized, Doppler-phase-corrected output signal; then applying the synchronized, Doppler-phase-corrected fractionally sampled signal to a phase locked loop and equalizer; then comparing an estimated symbol with a decision symbol and updating a plurality of tap coefficients; and h) outputting decision symbols. The Doppler estimation is performed either by applying a concurrent sinusoidal signal in a different band than the band of the communications signal, and then applying a Fourier transform or adaptive spectral estimation over a duration larger that a packet length to thereby obtain the Doppler-shift frequency f ; or, by applying a beginning probe signal; applying an ending probe signal; applying a probe replica signal; cross-correlating the probe replica signal with the beginning probe signal and the ending probe signal using a matched filter processor; determining a difference of arrival time between the beginning and ending probe signals using the peaks of the cross-correlation outputs; taking a ratio of the difference of arrival time to a time difference of the beginning and ending probe signals to obtain a dilation/compression ratio due to the Doppler shift; and then obtaining the Doppler shift from the dilation/compression ratio.