Constrained mutual coupling estimation for array calibration

The presence of unknown mutual coupling between array elements is known to degrade the performance of high resolution direction-finding and space-time adaptive processing (STAP) methods. It is also known that the mutual coupling between array elements is inversely related to their separation and may be negligible for elements separated by a few wavelengths. This results in a sparse coupling matrix structure for even moderately large arrays and a resultant sparse array distortion matrix that also includes gain and phase errors. This paper develops a method for estimating the sparse distortion matrix subject to linear constraints that force zeros in known positions in the estimated distortion matrix. This results in a substantial reduction of the unknown parameters to be estimated, with attendant improvement in estimation accuracy and reduced computation. Comparative computer simulation results are presented that demonstrate this improvement and also the resulting improvement in high resolution direction-finding methods.