The role of performance modelling in active phased array SAR

Phased array antennas play an important role in many radar applications and their use has increased during recent years in space-based remote sensing applications. Their success is mainly due to high agility in reconfiguring pattern, quick steering capability along both elevation and azimuth axis, easy packaging on spacecraft, low sensitivity to T/R module failures, high achievable directivities. In the frame of SAR systems implementation and calibration activities, a SAR simulator is required to support engineering activities devoted to scenario definition, performance assessment, estimation of error effects on signal Tx/Rx/Cal chain. Such a tool shall also implement an electromagnetic antenna model which is required to predict antenna performance, in terms of beam shape, directivity and sidelobe levels, with high accuracy and reliability, by keeping into account characterisation data provided at various levels (both pre-flight and in-flight), mutual coupling, VSWRs, insertion losses, amplitude and phase errors. This paper discusses preliminary results achieved by mean of a SAR simulator which implements a non-electromagnetic antenna model developed by Alcatel Alenia Space Italia for SAR instrument calibration and phased array antenna pattern prediction, which is based on array factor computation by mean of fast Fourier transform applied on the excitation matrix, where nominal excitation values are corrected on the base of near field pre-flight and in-flight measures. In particular, the predicted beam is achieved by matching together both information on antenna configuration (operative frequency, bandwidth, element spacing, number of T/R modules, TDLs, failures), data coming from near field measures (pre optimisation and post optimisation holograms) and in-flight telemetries. This paper presents the former results obtained by loading pre- flight measured data, and shows the high accuracy achieved in directivity computation, beam shape prediction and pointing angle - estimation, by comparing such results with those achieved by mean of an electromagnetic validated model. The tool, developed in Matlab, operates a correction of nominal antenna excitations by applying over them a pre-distortion array obtained by opportunely sampling the near field holograms acquired during pre-flight tests and correcting such information to compensate for probe to AUT distance. The effect of correction is evident both along azimuth and elevation cuts, even if the elevation pattern shows some discrepancies over far sidelobe regions. In any case, the general good matching toward validated analyses demonstrates the effectiveness of such approach which offers high accuracy even with low computational complexity. Furthermore, the model is designed to account for in-flight deviations from nominal behaviour, like those due to module failures or component degradation. The model loads calibration data and status telemetries and updates its database in order to predict the whole sets of beams in the most accurate way. As a project and analysis tool, the model also implements the possibility to cycle over independent variables to perform statistical analyses. In such way, the effects of frequency variations, graceful degradation, pointing deviations, amplitude/phase and random errors may be analysed and predicted in terms of expected values and variances.