Signal multichannel processing in a space-based SAR in presence of azimuth ambiguity
Abstract
Power increase of ambiguity interference is to be faced when developing SAR (synthetic aperture radar) for a small-size SV (space vehicle). This is due to the fact that the aim to minimize the weight-size parameters of the space facility results in the linear size contraction of SAR antenna, both in the angular plane and azimuth bearing. Accordingly, antenna beam-width (ABD) appears to be bigger than usual with consequent false signals in ABD vicinity and that reduces RI (radar image) quality. This phenomenon is commonly referred to as azimuth ambiguity when examining interference towards moving SV and range ambiguity if RI is analyzed in crosswise direction. The article proposes a way to minimize azimuth ambiguity impact on the radar image quality. The phase structure of quantized signal was analyzed around the nucleus of unambiguity m-area to identify discrepancies between the ambiguity signals and the signals of the major survey area. As it follows from the examination, if we pick up two quantizations of one received signal, one of which is delayed relative to the other by half of sounding period then their phase values in the basic signal area
(m = 0) will not differ. All odd-order unambiguity m-areas will have — angular disparity phases and that enables to select odd-order false signals and eliminate them. Generally, processing of the received echo signal comprises -receiving channels, each of which has ADC (analog-to-digital converter). ADC tuning is such that the signal recorded in l-channel, is obtained by quantizing the received signal with time delay relative to 0-channel signal. The article describes an algorithm for eliminating ambiguities using such signals and shows what the number of Nc-channels should be if the maximum order of the regarded ambiguity equals to m. There was performed mathematical simulation of the algorithm for suppressing azimuth ambiguity in en-route mode in the case of one or two point targets in the same range gate observed at various angles and plus white noise. The simulation results indicated that the proposed algorithm is operational ensuring ambiguity suppression including high-order one from multiple targets and in noise environment.
