Calibration of Active Electronically Scanned Arrays on Aerial Vehicles Through SAR Image Formation

In-situ calibration of Active Electronically Scanned Arrays (AESA) is a challenging problem that is still considered open. In this paper we propose a novel method for calibration of such arrays on aerial vehicles using the backscatter from the terrain below. To limit the extent of the terrestrial targets, the relative motion of the vehicle with respect to the ground can be used to form a pixelated synthetic aperture radar (SAR) image of the scene. The individual pixels of this image are used as calibration targets for a pair of transmitter and receiver modules. This way for an array with N transceiver modules,
$\boldsymbol{NXN}$
SAR images can be formed, co-registered, and used to determine the phase variations across the array elements. It is interesting to note that the measured phase difference between the backscatter from two co-registered SAR images can be averaged over all pixels to provide a much better estimate of the array phase error. Array channels can be aligned within the measurement phase errors, allowing for calibration without the needs for signals-of-opportunity or strategically placed calibration targets. Monte Carlo simulation of array beamforming with random phase difference errors on each element can then be used to determine degradation of array performance metrics.