In-Situ Calibration of Active Electronically Scanned Antenna Arrays Through SAR Imaging

In situ array antenna calibrations are methods devised to calibrate fielded arrays that cannot be accessed or must be calibrated frequently due to rapidly changing environmental conditions, such as those mounted on air/space-craft. There is no existing method that maintains optimal operation in real time and works for diverse array configurations without relying on extraneous fielded equipment and without requiring modifications to the array hardware. As a solution to this problem for arrays mounted on flying platforms, we propose to use backscatter measurements of the terrain within the array's field of view, along with the relative motion of the array with respect to the ground to form synthetic aperture radar (SAR) images to use as calibration targets. Because the ground below is always visible, calibration can be accomplished at any time. A common transmitter/receiver element (or an ensemble of them) can be used to form an image with each receiver/transmitter element. Corresponding pixels between co-registered images can be used as calibration targets to determine amplitude and phase variations across the array. The measured channel imbalances can be averaged over the many pixel pairs (where each pair is used as a calibration target) to reduce the effects of system noise and fading on the measurements. The proposed method is validated with laboratory measurements where we demonstrate the ability to measure receiver channel imbalances between two antennas using SAR images of a random rough surface distributed target. Amplitude and phase differences in the receivers are recovered within a fraction of a dB and fraction of a degree, respectively.