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.
Application of Improved Space-Time Adaptive Processing in Sonar
In order to use the theory of space-time adaptive processing (STAP) in Sonar, an improved STAP for sonar was proposed. Firstly, the principle of STAP was analyzed according to the echo model of radar. Secondly, comparing the echo model of sonar with radar, the improved STAP was presented and the space-time coupling steering vector was used in space time processing. Finally, the improved STAP was applied in space time processing and objective detection. Simulation results illustrate that the improved STAP is beneficial to underwater objective detection, and applying Space-time coupling steering vector can improve the accuracy of the direction estimation of target.
Theory of unequally-spaced arrays
Although recently unequally-spaced arrays have been shown to be useful, the theory has not been fully developed, except for the use of matrices, computers, or the perturbation method. This paper presents a new approach to the unequally-spaced array problem. It is based on the use of Poisson's sum formula and the introduction of a new function, the "source position function." By appropriate transformation, the original radiation pattern is converted into a series of integrals, each of which is equivalent to the radiation from a continuous source distribution whose amplitude and phase distribution clearly exhibit the effects of the unequal spacings. By this method, it is possible to design unequally-spaced arrays which produce a desired radiation pattern. This method is effective in treating arrays of a large number of elements, and unequally-spaced arrays on a curved surface. Three examples are shown to illustrate the effectiveness of the method. The problem of sidelobe reduction for the array of uniform amplitude, which was attacked by Harrington, is treated by our method. A numerical example is shown for 25-db sidelobe level. Also, the problem of secondary beam suppression is attacked with the use of the Anger function. The interesting problem of azimuthal frequency scanning by means of an unequally-spaced circular array is also shown, using the method of stationary phase.