Abstract: A signal processing method for use in echo-location systems (radar, sonar, laser radar) to obtain accurate estimates of the target reflectivity sequence in a random (non-Gaussian) noise environment, including ambient noise, reverberation, and clutter. The method derives the statistical characteristics of the random background environments and then constructs and maximizes the corresponding approximate likelihood function using iterative methods. The approximate maximum likelihood estimates are generated on the basis of an approximation of the ideal likelihood function which is maximized using computationally efficient algorithms.

Abstract: Automatic focusing of radar or sonar imaging systems, for example, synthetic aperture radar (SAR) or synthetic aperture sonar (SAS) systems, is accomplished by using high order measurements, such as the quadruple product of the range compressed signal g(x). The phase error is estimated by a recursive or non-recursive algorithm from the phase of the quadruple product. The estimated phase error is applied to the range-compressed SAR or SAS image data which is then azimuth compressed to obtain the focused image data.

Abstract: In echo-location and imaging systems, interference suppression is accomplished by shaping the transmitted signal waveform. Interference suppression permits more accurate range estimates or image formation, respectively. These specially shaped waveforms can also be used in communication systems to secure more accurate message transmission in the presence of interference. Shaping of the transmitted signal may be accomplished in two ways: (1) by convolving an elemental waveform with a multi-level pseudorandom sequence (i.e., binary, ternary, Poisson, etc.); or (2) by transforming nonlinearly the amplitude of the elemental waveform to give it high skewness. Interference suppression in the former case is achieved through matched filtering (replica correlation or pulse compression), and in the latter case it is accomplished by bispectrum-bicoherence processing.

Abstract: A method and apparatus for enhancing resolution of range estimates in all echo location systems and, specifically, such systems as Radar, Sonar, and Synthetic Aperture Radar (SAR), for example. The invention utilizes high order signal processing to "sharpen" or contract the main lobe of the processing system ambiguity function and suppress its side lobes, for a given transmission pulse bandwidth. The method and apparatus may be implemented in the frequency domain or time domain. Enhanced resolution is achieved by using a filter (MSC filter), according to this invention, in the echo location data processing system so that the received echo data is processed by the MSC filter to produce a signal that exhibits enhanced range resolution. The MSC filter output, H(.omega.), which is specific to the transmitted pulse waveform, is the ratio of a high-order manipulation of the transmitted signal with its modified spectral profile.