Phase unwrapping methods for solving the ambiguity in current velocity estimation based on combined signal design

Acoustic Doppler Current Profiler (ADCP) due to the Doppler principle is a powerful instrument for the current velocity measurement, where the complex autocorrelation function is a commonly used method to estimate the Doppler frequency shift, but its inherent phase wrapping usually leads to ambiguities in the velocity estimation and then limits the measurement range. To solve this problem or weaken its influence, the cause of the phase wrapping in the complex autocorrelation function is analyzed in this paper, and two phase unwrapping methods are proposed based on the combined signal design, where the sub-pulses of the emitted signal have different time delays or different signal frequencies. Numerical simulations show that the measurement ranges can be extended significantly with the setting parameters of the emitted signals and the estimation accuracies are acceptable when SNR is above 10 dB. Comparisons are also implemented between the broadband method with the coded-pulse signal and the proposed phase unwrapping method based on the time delay difference, and the practical usage of the latter is demonstrated by running experiments in a tank.