Frequency shifting coded transponders are routinely used by almost any aircraft. This transponder when receiving a radar pulse from air traffic control (ATC) radar, transmits a frequency shifted pulse on which information is coded (12 bits of status/identification and the barometric altitude). The transmitted pulse is received by a secondary radar antenna and the coded information is displayed beside the radar plot on the ATC display screen. Both radar cross section (RCS) enhancement and status/identification capability would be interesting in air-to-ground synthetic aperture radar (SAR) imaging. Direct transposition of the 50 year-proven ground-to-air technique is jeopardized by three difficulties: First, the transponder echoes are dispersed in a significant clutter. Second, the transponders are potentially more clustered on the ground than aircraft are in the sky. Last, because azimuth resolution, instead of being obtain from a wide rotating antenna, is in SAR derived from coherent summing during long "integration time" thus tightening the frequency shift quality requirement. Here, diverse solutions are proposed for air-to-ground coded SAR transponders especially with under-foliage applications in mind.
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