Frequency-Agile Radar Electronics for the Soil Moisture Active/Passive (SMAP) Mission

The radar aboard the Soil Moisture Active/Passive (SMAP) instrument is an L-band scatterometer/synthetic aperture radar (SAR) with uniquely designed capabilities for global retrieval of surface soil moisture. Several key science requirements have shaped the radar architecture, including requirements on terrestrial radio frequency interference (RFI) mitigation, calibration stability, and on silencing radiated emissions from the electronics in the presence of the sensitive radiometer aboard SMAP. The design uses an unconventional conical scan sequence to collect SAR data over a wide swath while the radar frequency-hops over 1217-1298 MHz in order to avoid RFI from ground sources. In this paper, we discuss: 1) the design trades that led to the choice of a frequencyhopping, "tunable LO" receiver scheme that significantly reduces the SMAP radar susceptibility to RFI; 2) loopback calibration to minimize bias errors in the transmit-power × receiver-gain product; 3) hardware design techniques for decreasing radiated emissions levels in the 1.41-GHz radiometer band; and 4) pre-launch performance results from flight radar environmental testing.