Patent ID: 8127138
Filing Date: 2012-02-28
Classification: G01S

Abstract:
1. A method for embedding and detecting a digital watermark in the time-frequency plane of an active sonar signal comprising the steps of: taking a short time Fourier transform of the active sonar signal according to the following equation adding a digital watermark signal to the short time Fourier transform of the active sonar signal wherein the digital watermark signal is generated from a pseudo random sequence by a secret key, α(n,ω recovering a time domain representation of the active sonar signal after the digital watermark has been added to the sonar signal by inverting the short time Fourier transform of the active sonar signal after the digital watermark has been added according to the following equation deriving a digital watermark sequence in a time domain from the inverted short time Fourier transform of the active sonar signal after the digital watermark has been added according to the following equation combining the derived digital watermark sequence in the time domain with an original sonar signal resulting in a digitally watermarked sonar signal according to the following equation transmitting the digitally watermarked sonar signal through a second active sonar signal that periodically pings for a target; receiving the digitally watermarked sonar signal through a sonar system; processing the received digitally watermarked sonar signal through a plurality of Doppler decoding filters; computing the short time Fourier transform of the output of the plurality of Doppler decoding filters and generating a time frequency power plot; selecting a region where the watermark is expected to be located in said time frequency power plot; subtracting from said time frequency power plot a signal component which also has an appropriate channel response; detecting the digital watermark in the digitally watermarked sonar signal through informed correlation statistics that include knowledge of the digital watermark, the secret key, a functional form of a sonar and an estimate of the digital watermark after said digital watermark has propagated through a sound channel with assumed channel clutter characteristics according to the following equation selecting a Doppler match of correlated watermark region; and plotting Λ to determine a peak, thereby confirming an existence of the digital watermark.