Estimation of Range Ambiguities for Correlation Based Calibration

Abstract

There is an evolutionary trend for Synthetic Aperture Radar (SAR) missions to employ multiple channels in azimuth and elevation. Paired with Digital Beam-Forming (DBF) techniques, it allows to achieve a high resolution and a wide swath. In elevation, a narrow receive beam is formed via Digital Beam-Forming (DBF) that is steered in real-time towards the position of the signal on the ground - known as Scan-On-Receive (SCORE). Further, this leads to an improved Signal-to-Noise Ratio (SNR) and suppressed range ambiguities. Due to the combination of the channels for the DBF the beamformed patter is affected by interchannel mismatches. These mismatches result from error sources of the individual channels (drifts, coupling, gain compression, noise, etc.). If the pattern is not optimally formed, the overall performance can suffer, e.g. a degradation of the resolution, decreased SNR or higher azimuth and range ambiguity levels. To mitigate the mismatches a calibration of the channels is mandatory. The real-time calibration of a multi-channel system employing DBF in elevation can be performed with a correlation-based calibration. The raw data of the radar clutter signal is correlated between the channels to estimate the channel mismatches. A refined model is necessary to account for effects like spaceborne geometry, transmit signal properties, ambiguities and other effects. Deterministic variations of the ambiguities, e.g. from the antenna pattern, range and position, can be accounted for by the model. However, inhomogeneities in the scene can lead to a change of the power and of the correlation phase of the ambiguities. This leads to a significant increase of the calibration error. Here, a data-based DBF SAR calibration method as well as a mathematical model (focusing on a monostatic planar antenna array SAR) are introduced. A least square minimization method is proposed to estimate the power of the ambiguities by utilizing the correlation information between the channels in combination with the expected correlation phase due to the ambiguity position. Further, the effects of inhomogeneities in azimuth and elevation on the correlation phase are analyzed as well as the influence of the changed correlation phase on the proposed ambiguity power estimation method. Simulation results showing the performance of the proposed method are presented.