# Robust Space Time Adaptive Processing Methods for Synthetic Aperture Radar
## 1 Introduction  
Synthetic aperture radar (SAR) has been more and more widely used in civilian and military fields [1; 2; 3]. Along with growing crowdedness of the electromagnetic spectrum and rapid development of SAR interference technology [4; 5; 6; 7], SAR is now faced with increasingly serious unintentional or intentional interference, and the algorithm of interference cancellation in SAR has received growing attention in recent years. The common methods of interference cancellation in SAR can be divided into two categories. One is to utilize the difference between the interference and radar echo in the time domain or frequency domain so that the interference cancellation can be realized through filters in the time or frequency domain. This includes the classical frequency notch method and the time-frequency filtering method [8; 9]. Some advanced methods based on short-time Fourier transform, wavelet transform, empirical mode decomposition, eigen subspace filtering and independent component analysis (ICA) [10; 11; 12; 13; 14] have also been proposed. These methods not only cause certain loss to the echo signal itself, but also cannot effectively suppress intentional interference from jammers such as wide-band noise jamming or dense repeater jamming. Another kind of method is based on digital array processing with multi-channels, using adaptive digital beam forming (ADBF) or the space time adaptive process (STAP) algorithm for interference cancellation [15; 16; 17]. Multi-channels in the SAR can be used to form a beam in the direction of the desired ground returns and to steer a null in the direction of the interference simultaneously. With the rapid development of the multi-channel SAR system and the improvement of digital signal processing, these methods have shown more and more advantages with regards to excellent performance, wide application and high flexibility. Moving target indication (MTI) is a common radar mission involving the detection of airborne or surface moving targets. To detect moving targets in the background of clutter and interference, it is necessary to suppress clutter and interference by various methods. Single-channel techniques that exploit the Doppler spectrum or the short time Fourier transform are used in general for static clutter discrimination from moving targets. In multi-channel systems, STAP is an advanced multi-channel adaptive processing technique used for clutter mitigation and interference cancellation purposes and has been widely treated for general airborne radar [18; 19]. Furthermore, the STAP technique has also been introduced into the SAR system for both clutter suppression and high resolution imaging of moving targets [20; 21]. The typical application of STAP in airborne radar is the ground moving target indication (GMTI) mode, and the coherent processing interval (CPI) of the GMTI mode is only tens of milliseconds. It can be considered that the direction of arrival (DOA) from the jammer is constant in the CPI, which means the interference is stationary. Since the CPI in SAR is usually more than a few seconds, due to the high speed motion of the platform, the DOA of the jammer is time-varying, which means the interference is non-stationary in the CPI. There are two main kinds of methods for non-stationary interference cancellation. One is to widen the null width near the interference's DOA in the adapted beam pattern by digital array processing [22], but this may lead to a loss in the depth of the nulls. The other method for non-stationary interference cancellation is time-varying STAP [23; 24], which means using different adaptive weight vectors for each pulse. However, due to the deformation of the beam pattern from pulse to pulse, the SAR echo would be modulated in the slow time domain, resulting in the degradation of the azimuth focusing performance.