The Soil Moisture Active Passive (SMAP) mission will have the first L-band radar/radiometer sensor suite dedicated to global measurements of soil moisture. For the radar sensor, the requirements for achieving high backscatter measurement accuracy from low-Earth orbit present a unique design challenge in the presence of terrestrial radio frequency interference (RFI). The SMAP radar shares the same 1,215 to 1,300 MHz spectrum used by high-power ground-based transmitters like air-route and defense surveillance radars, which can generate strong interference in a conventional fixed-frequency spaceborne radar. The noisy ground environment motivated the development of a frequency-hopping (self-tuning) feature in the radar design. As the SMAP spacecraft orbits across various regions of the Earth, the radar continually adjusts its RF operating frequency to quieter areas of the spectrum for improved fidelity in soil-moisture science data observations.
A “tunable LO” (local oscillator) receiver architecture has been developed with narrowband analog/IF filtering to significantly reduce the SMAP radar’s susceptibility to RFI. A key advantage of the tunable LO scheme is the excellent selectivity of the IF filter stage, which blocks out-of-band noise before the A/D (analog-to-digital) conversion stage. A collateral benefit of the tunable LO approach is that the smaller receiver system bandwidth relaxes requirements on A/D sampling speed. Lower-speed A/D converters typically operate at lower power consumption and over a wider dynamic range.
During performance testing of the flight radar, it was verified that the 0.4 dB RFI error budget was satisfied with 0.2-dB margin. The SMAP radar is designed with RFI mitigation capabilities that include:
- dynamically tunable operating frequency, to allow the instrument to frequency-hop around noisier parts of the spectrum, and over particular regions of the Earth;
- sharp RF/digital receiver filtering with a high degree of selectivity for rejecting out-of-band interference (80+ dB rejection); and
- radar telemetry for flagging range lines contaminated by RFI, so that this bad data can be removed in science data post-processing on the ground.
The radar design uses a hybrid digital-analog approach to update radar operating frequencies dynamically in orbit. On the transmit leg, direct-digital synthesis is used to generate the pair of tunable frequencies for V- and H-polarized, 1-MHz chirp pulses; the fixed-LO upconverter and high-power amplifier are wideband (>80 MHz) to accommodate the range of chirped RF frequencies over 1,217 to 1,298 MHz, adjustable in 1.25-MHz steps. Each receive leg uses a single-stage heterodyne downconverter with tunable LO and FPGA-based back-end digital processor.
This work was done by Mark A. Fischman, Harry S. Figueroa, Kayla Nguyen, Andrew C. Berkun, Charles T-C Le, Mimi Paller, and Gerald J. Walsh of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49415