Correction of inaccuracies in estimation of the narrow-band ambiguity surface
William Harry Marsh · The Journal of the Acoustical Society of America · 1977
A narrow-band ambiguity surface (NAS) is the coherence or correlation between two signals as a function of the Doppler difference and time delay difference between them. A peak in the NAS indicates the presence of a coherent relationship with specific Doppler and time delay differences between the two signals. An estimate of the NAS can be obtained by using time sequences of Discrete Fourier Transforms of the two signals and using a narrow-band approximation for Doppler compensation. This sort of compensation is required when processing propagation data, taken from a moving source. Inaccuracies may arise from the narrowband approximation when large numbers of degrees of freedom are used in the estimate. Such inaccuracies occur not only in coherence estimation, but in wide-band correlation estimation using FFT and inverse FFT for the correlation process if a narrow-band approximation is used for Doppler compensation. The nature of these inaccuracies is described and related to such factors as signal bandwidth, Doppler range, and integration time. Procedures are then outlined for minimizing approximation effects on the coherence estimate.