The influence of typical subarray grouping with true time delay (TTD) adjustments on the performance of the antenna radiation patterns of SAR-radars and moving target indicators (MTI) is illustrated. Particularly, the relationship between the size of subarrays and the level of grating lobes is examined with regard to various beam pointing angles and bandwidth requirements for a tapered amplitude distribution of the antenna. As a general design base, to ensure a relative grating lobe level not exceeding -20 dB for a typical subarray size of 8 (with half wavelength spacing), the permitted instantaneous bandwidth is approximately 7 %. This is valid for beam pointing angles in the range of +-45 deg of angle. If wider beam scanning is required, either the usable bandwidth or the size of the subarrays must be reduced. Another essential point for the design of a subarray arrangement with TTD adjustments is the choice of appropriate quantisation steps for the TTD implementation. For a reasonable design, the minimum increments of the line length adjustment can be in the range of approximately two wavelengths. In this case, the sidelobe level of the radiation pattern for wideband SAR operation can be kept below -23 dB. It is noteworthy that with TTD increments of two wavelengths for an MTI mode operation with a typical bandwidth of 20 MHz (in X-Band) usual boresight accurcay requirements can also be met.
Active phased array antennas for airborne SAR/MTI applications
GRS, German Radar Symposium, 2000 ; 315-320
2000
6 Seiten, 7 Bilder, 2 Quellen
Conference paper
English
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