Signal-to-noise ratio gain in multi-channel SAR with digital beamforming

Abstract

Multiple digital channels allow to improve the performance of synthetic aperture radar (SAR). One example is the ratio of swath width to azimuth resolution, whose upper limit is increased compared to conventional single channel SAR, as is well known from numerous publications. Related to this, is the question how the signal-to-noise power ratio of a digital beam-forming (DBF) system compares to that of a conventional SAR. Available literature does not provide a clear answer, further, straight forward application of the known radar equation may lead to erroneous results. The problem can be segregated into the elevation and azimuth dimension. In elevation, SCan-On-REceive (SCORE, also known as SweepSAR) allows using a high gain, time-varying receive antenna pattern, which improves the signal power. In azimuth, multiple channels are employed, raising the question of whether the power level increases when combining the digital data streams during processing. This paper uses a descriptive approach to derive an expression for signal-to-noise ratio in multi-channel SAR systems. An equivalent closed expression for the antenna elevation pattern is derived, which does not depend on the time variable. Multi-azimuth channel operation is described through an equivalent single channel system. These are combine into a simplified SNR equation, taking the oversampling gain into account.