In Situ Monitoring of Channel Imbalances of Air/Space-Borne Antenna Arrays Through Multistatic Radar Imaging

Maintaining calibration of phased array antennas in situ is a challenging problem that is still considered open. A critical step in the array calibration process is characterizing the channel imbalances between elements, so that they can be equalized. In this article, we propose a novel method for monitoring channel imbalances at elements in such arrays mounted on flying platforms using the backscatter from distributed targets within the field of view. The motion of the platform can be used to measure the backscatter of the scene in multiple positions. Synthetic aperture radar (SAR) images can be formed from these measurements using a common transmitter/receiver module and a paired receiver/transmitter module. The individual corresponding pixels of the coregistered images can be used to determine the amplitude and phase variations (channel imbalances) across the array elements. The measured phase difference between the backscatter from two coregistered images can be averaged over all pixels to provide a much better estimate of the array channel imbalances in the presence of noise. Changes in channel imbalances can be monitored in situ without the need for signals-of-opportunity or strategically placed calibration targets. This could be used in the development of more elaborate array calibration algorithms, or to maintain a factory calibration during flight. The proposed method is validated under controlled conditions by forming images of a random rough surface with a two-antenna system. Average channel imbalances in amplitude and phase over frequency are recovered within a small fraction of a dB and a fraction of a degree, respectively.