Ambiguities in space-based SAR using a circular reflector antenna

Reflector antennas are quite widely used in radar devices, including space-borne synthetic aperture radars (SARs). One of the problems of such SARs is to ensure the unambiguous measurement of the Doppler frequency of received signals. The shape of the Doppler spectrum of the echo signal is determined by the shape of the antenna pattern in the azimuthal plane. Thus, in general, the narrower the pattern (the larger the antenna size), the lower the level of the interference signal received at the side peaks of the ambiguity function. However, a significant increase in the antenna size is limited by the difficulties of its placement on the SAR launcher, which is especially important for small spacecraft. On the other hand, the required beam width in the elevation plane must satisfy two conditions: ensuring swath and ambiguity detuning. The time ambiguity interval is equal to half the distance of radio wave propagation during the pulse repetition interval. A technical contradiction arises between reducing the level of azimuthal and elevation ambiguity of SAR. The purpose of the article is estimation of azimuth and range ambiguities in a space-based SAR using a circular reflector antenna. In this regard, the material in the article is relevant. We consider a normal looking view which is performed using X-band space-borne SAR. The reasons for the emergence of ambiguities are considered. The necessary mathematical calculations are presented and calculations are made of the projection of circular antenna pattern and the achievable levels of azimuth and range ambiguity for space-based SARs. The obtained values were compared with similar data for a rectangular antenna. An analysis of the influence of the viewing angle and pulse repetition frequency on the ambiguity level was carried out. The results obtained make it possible to estimate the range and azimuth ambiguity level in space-borne SAR and present requirements for the need and level of additional suppression.