A Polarimetric Active Transponder With Extremely Large RCS for Absolute Radiometric Calibration of SMAP Radar

This paper presents a new single-antenna polarimetric active radar calibrator (PARC) intended for the polarimetric and absolute radiometric calibration of the NASA's soil moisture active passive (SMAP) radar. The PARC receives and retransmits the SMAP signal through a dual-polarized horn antenna with 17-dB gain at the center frequency of 1.26 GHz with 100 MHz of bandwidth. The transmit and receive polarizations are perpendicular and are isolated from each other using a precision orthomode transducer (OMT) specially designed for this application. The antenna is rotated 45° in the plane perpendicular to the direction of incidence, so that the scattering matrix of the PARC with respect to SMAP polarization coordinates has equal entries that enable radiometric calibration of all four channels simultaneously. As SMAP radar resolution is coarse (1 km), a point target with a very large radar cross section (RCS) is required to provide a high signal-to-clutter ratio. The proposed PARC can provide RCS values as high as 80 dBsm to achieve a 30-dB signal-to-clutter ratio. The PARC is controlled by a microcontroller to autonomously start minutes before the SMAP radar is expected to scan the area, stabilize the amplifier gain, record the magnitude of the pulses transmitted from the radar, and transmit these data to a base station. The design procedures of the OMT and the antenna, as well as control and RF circuits, are discussed, a number of leakage cancelation techniques are introduced to increase the isolation between the ports, the RCS of the fabricated PARC is fully characterized, and the image of the PARC as seen by SMAP is presented.