Ultra-wideband, high RCS, active calibration targets for foliage/ground penetrating SARs

This paper describes the design, development, test, and operation of compact, active electronic targets having high and constant values of radar cross section (RCS of 20 to 40 dBsm). The active targets were specifically designed to be used for reference calibration of airborne synthetic aperture radars (SARs) used in foliage and ground penetrating (FOPEN, GPEN) applications. The use of active (vs. passive) calibration targets in this application was driven by the need to achieve physically compact reference targets with high, constant RCS over the ultra-wide bandwidths and for relatively long radar wavelengths typical of FOPEN/GPEN radars. Two target types were developed. One type operates from 30 MHz to 90 MHz and was designed to respond to a stepped frequency radar waveform. The other target type operates from 100 MHz to 500 MHz and was designed to respond to an impulse radar waveform. For each target type, two different models were built: one for above ground operation and the other for burial below ground. A description of the active targets is provided along with an example of the design methodology which included electromagnetic modeling of the target antennas above and below the ground. Lessons learned during the target development process are discussed. A description of field tests using the active targets is presented along with a comparison of modeled and measured target RCS and an example of the active target imaged by an airborne SAR.