Using multipath signals to estimate the range of high speed underwater targets

Sound from a torpedo will be received by an underwater hydrophone along multiple paths. Due to different arrival angles, the signals from the multiple paths will be Doppler shifted relative to each other. By separating the multi-path signals an estimate of the range can be found using trigonometric relationships. A high spectral resolution estimator called the fast orthogonal search (FOS) is used to estimate tones from the torpedo whose frequency spacing is much less than the fast Fourier transform resolution limit. An adaptive candidate FOS algorithm (AFOS) was developed to detect two tones with closely frequencies and short record lengths. The torpedo detection algorithm used record lengths of 10 s and estimated the range of the approaching torpedo to within 4.2 m for initial angles of approach ranging from 0° to 20 °. and ranges less than 3000 m.