Digital Beamforming on Receive Array Calibration: Application to a Persistent X-Band Surface Surveillance Radar

Unbalances in gain and phase of receiving chains as well as mutual coupling effects among radiating elements degrade the performance in direction of arrival estimations (DOA) in digital array radars (DAR). These effects can be described by an unknown distortion matrix by which the received signals are in fact multiplied. The calibration of the array consists then on finding an approximation to this matrix, so that is possible to compensate the received signals prior to their processing. This article presents the active calibration procedure used to improve the targets DOA estimations of a surface surveillance radar. The radar is a high range resolution system that performs the digital beamforming only on receive. To calibrate the system, an auxiliary rotating radiating source plus an electro-optical delay line are used. The waveform used in the calibration is the same that the system use in operation and the signal processing chain used by the system to detect targets is included In the calibration algorithm so it can be viewed as a whole-system calibration. Matched filtering and integration allow to obtain a high enough signal to noise ratio in the calibration process. Results are presented for both the calibration algorithm, using computer simulations, and for the calibration data set, to finally show the Improvements in DOA estimation using the monopulse technique after calibration.