Clutter reduction using Doppler sonar in a harbor environment

Abstract

A high frequency experiment was conducted in the Woods Hole Harbor in Massachusetts to evaluate the effectiveness of Doppler sonar for discriminating targets from reverberation. Using a pulsed linear frequency modulated signal, one finds that the matched filtered outputs are filled with high-level discrete backscattered returns, referred to as clutter, which are often confused with the target echo. The high level non-target returns have an amplitude distribution that is heavy-tailed. Using a Doppler-sensitive binary-phase-shift-keying signal coded with an m-sequence, the target echo and clutter can be separated by Doppler and delay, and tracked using the Doppler spectrogram (Dopplergram). The Doppler filtered time series show a background reverberation with a Rayleigh-like amplitude distribution, with an improved signal-to-(peak) reverberation ratio compared with that without Doppler filtering. The reduced reverberation level with Doppler processing decreases the probability of false alarm (Pfa) for a given threshold level. Conversely, for a given Pfa, the higher signal-to-(peak) reverberation ratio implies a higher probability of detection. Transmission loss measurement was conducted to estimate some of the system parameters, e.g., the source level and target strength relative to the noise level.

Experiments conducted in the marine test field at Sizihwan coastal area in Kaohsiung, Taiwan

“…The test site in the Kaohsiung harbor has a relatively flat bottom of ∼ 10 m. We also found that the transmission loss (TL) does not vary significantly between different tracks, and can be reasonably modeled as a function of range. Comparing this result with the TL data collected in Woods Hole Harbor, Cape Code, Massachusetts [4], it has been shown that non-flat bathymetry has significant impact on the TL and need to be carefully measured along the propagation path for each target look direction for system analysis. Relevant results have been published in Ref.…”

An Iterative Deconvolution-Time Reversal Method with Noise Reduction, a High Resolution and Sidelobe Suppression for Active Sonar in Shallow Water Environments

“…The auto-ambiguity function of the transmitted signal s ( t ) with finite energy is defined as [ 19 ]: where superscript * is conjugation, and is the time compressed or dilated version of the transmitted signal s ( t ), where α is the compression/dilation ration, referred as the Doppler ratio. One sees that the Doppler and range resolution is determined predominantly by the ambiguity function of the transmitted signal.…”

“…For the traditional methods, the width of the peak of the ambiguity surface reveals the resolution in delay and Doppler-the accuracy to which a target's range and radial speed can be estimated. In active sonar, the pulsed continuous wave and linear/hyperbolic frequency modulation signal are often used in sequence [19]. When there is only one target, one can determine that target range and speed using linear/hyperbolic frequency modulation signal and pulsed continuous wave separately.…”

“…This is unrealistic for the previous works due to the duration of transmitted signal is very long in order to guarantee the resolution and detection performance. The active sonar needs the transmitted signal with long duration to combat the noise [1,19,20]. However, this is a non-trivial task for the ID-TR method since it can provide a high resolution and super gain using iterative deconvolution and time reversal processing.…”

“…It cannot exploit the difference in Doppler between a moving target and the bottom clutter to separate the target echo from the clutter. Analysis based on experimental data demonstrated that a moving target can be "unambiguously" identified in the high Doppler space due to the absence of a high-Doppler background [19,20].…”