Abstract
Radio Frequency Interference (RFI) is a growing problem in Synthetic Aperture Radar (SAR) systems as scientific motivations push the radars to lower frequencies and as more wireless services share the frequency spectrum. New, advanced SAR instruments, such as NASA’s EcoSAR, DBSAR and DLR’s Tandem-L mission, employ a multichannel architecture capable of Digital Beamforming (DBF). Radars with DBF are capable of notching the antenna pattern in specific directions, which can be utilized to suppress RFI on board or in post-processing. A well-researched beamformer for this purpose is the Minimum Variance Distortionless Response (MVDR) Beamformer. However, the number of interferers that can be removed through notching is limited by the number of receive channels. It is therefore essential to adaptively change the antenna pattern notching throughout the image in time and frequency for the best results with a given number of receive channels. In this paper, we present several methods to achieve this notching by making use of the spatial SAR signal distribution in range time, range frequency, azimuth time and azimuth Doppler that is inherent to the SAR imaging geometry. Because this distribution is time-variable and yet predictable, it can be used to improve the angle of arrival estimation of the RFI and the adaptive notching. The presented methods can be applied to a Digital Beamforming (DBF) SAR signal with multiple channels in elevation and/or in azimuth. Simulations show that the proposed methods increase the ability to notch out-of-swath interference from multiple directions and lessen the impact on in-swath interference. The improvement of the interferometric coherence for a single-pass interferogram acquired by NASA’s EcoSAR system (capable of DBF in elevation) is demonstrated. The removal of periodic RFI artifacts is achieved.
Highlights
Radio Frequency Interference (RFI) is a problem that is continuously growing due to the demand for wireless services and the finite nature of the frequency spectrum [1]
EcoSAR is an airborne Digital Beamforming (DBF) Synthetic Aperture Radar (SAR) instrument developed at the NASA Goddard Space Flight Center [21,51,52,53]
EcoSAR is only equipped with eight receive channels per antenna, 7 notches can be placed towards interference
Summary
Radio Frequency Interference (RFI) is a problem that is continuously growing due to the demand for wireless services and the finite nature of the frequency spectrum [1]. The methods presented in this work are unique as the basis of the RFI suppression is to utilize the spatial distribution of the SAR signal that is inherent to its imaging geometry This distribution is variable in time and yet predictable because the arrival time of the radar return is determined by the distance. The angles of arrival of the interferers can be estimated for all time instances when the radar position is not identical to the instantaneous radar return position This information can be utilized as input for an optimal and variable antenna pattern notching throughout the image (Note that this is only possible with DBF and not with conventional phased arrays that need to use the same antenna pattern for the entire receive window).
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