Phase Error Correction for Synthetic-Aperture Phased-Array Imaging Systems

If one replaces the ordinary single receiver of a synthetic-aperture radar (SAR) with a linear array of receivers underneath the wings of an aircraft, one obtains a 3-D signal history (two spatial dimensions plus the frequency dimension) that allows the computation of a 3-D image (angle-angle-range) of a scene. Because of the limited extent of the wingspan, the cross-track resolution is limited, driving one to use high frequencies, such as 94 GHz, having a wavelength of 3.2 mm. At such short wavelengths, the motion ofthe wings during the synthetic-aperture integration time will cause large phase errors that will severely blur the image. This paper describes an approach to measuring and correcting these and other phase errors. The approach involves having three transmitters, each at a slightly different monotone frequency. Relative to the first receiver, the second is displaced along the direction of the array of receivers and the third is displaced perpendicular to that direction. The array of receivers can separate the three corresponding signals reflected from the ground from one another. We will show mathematical analysis that allows us to determine the phase errors at each receiver from these three signals. It is required either that the three transmitters experience the same phase errors (so they should be rigidly mounted together) or that the phase errors at the three transmitters are measured. No measurement of phase errors on the receivers is required.