Space-time processing for suppression of bottom reverberation
The major problem for sonars operating in shallow water is the detection of low Doppler targets in the presence of severe bottom reverberation. For a modern sonar with an azimuthally omni-directional transmitter; the Doppler of bottom reverberation is proportional to /spl nu/cos /spl theta/ where /spl nu/ is the speed of the sonar platform and /spl theta/ is measured from the bow or heading direction. For a towed array receiver, reverberation of various Doppler arriving on sidelobes limits the effectiveness of advanced waveforms. This smearing of sidelobe reverberation over a Doppler range of /spl plusmn//spl nu/ makes it difficult to detect targets with speeds less than that of the sonar platform. Standard adaptive beamforming algorithms are ineffective due to the conflict between desire for narrow frequency bins for good Doppler confinement and wide frequency bins for rapid convergence. We present space-time processing algorithms that overcome these difficulties and permit the detection of low Doppler targets from moving platforms.
Sparse sample matrix inversion with diagonal loading for reverberation suppression
The main problem of active sonar detection is that the reverberation has spatial and temporal two-dimensional coupling characteristic which is similar to clutters in airborne radar. This paper presents a sparse sample matrix inversion with diagonal loading (SSMI-DL) for reverberation suppression in active sonar. The proposed method transfers Pulse Doppler radar STAP processing techniques into active sonar application, which utilizes the inverse reverberation covariance matrix to obtain adaptive weights and avoids singular reverberation covariance matrix with diagonal loading due to limited available received data. Simulation results are provided to verify that the SSMI-DL is more effective for improving slowly moving target detection for CW pulse than the conventional methods.
On grating plateaux of nonuniformly spaced arrays
This paper investigates the optimization and synthesis of nonuniformly spaced arrays with respect to the flattening of their grating plateaux. The analysis begins with the proof that the exponential spacing gives flat space-factor grating plateaux, using Poisson's sum and the stationary phase method. The theory then is generalized to include arrays of nonisotropic elements. It is found that results of the above analysis can be reduced to very simple forms. Through the use of Parseval's theorem, the theory of the space-factor gain of nonuniformly spaced arrays is also developed.