The Detection and Mitigation of RFI with the Aquarius L-Band Scatterometer

The Aquarius sea-surface salinity mission includes both an L-band radiometer and scatterometer. The scatterometer senses sea-surface roughness to provide a radiometric correction to the radiometer's measurement of ocean surface brightness temperature, thus allowing the estimation of salinity from the corrected brightness temperature (1). Both RF sensors are subject to radio-frequency interference (RFI) in their respective L-band wavelengths (1400-1427 MHz for the radiometer, 1258-1262 MHz for the scatterometer). Both instruments require highly sensitive measurements of received power, such that the existing RFI environment at L-band poses significant challenges. The mitigation of the RFI environment for the radiometer has been documented elsewhere (2,3). This paper describes the results of a study evaluating the severity of terrestrial RFI sources on the operation of the Aquarius scatterometer radar, and the design of a scheme to both detect and remove problematic RFI signals in the ocean backscatter measurements. We will also examine implications of the growing world of global navigation satellite systems, generating space-based L-band RFI, for the Aquarius scatterometer, scheduled to launch in 2010 with a 3-year lifetime. 2. EXPECTED RFI ENVIRONMENT In 2004, the Aquarius and Hydros (4) projects commissioned, with the NASA Earth Science Spectrum Management Office, an RFI environmental study to understand, quantify, and mitigate RFI generated by ground radars on the two orbiting instrument systems. Realistic beam patterns, orbit geometry, and instrument characteristics were used for modeling the Aquarius Instrument, both radiometer and scatterometer subsystems. A large sampling of non-classified North American radars that transmit at L-band were included in the analysis as RFI sources. This yielded about 180 RFI sources, mostly air traffic control and air defense radars, with a wide variety of peak powers, pulse rates, scan rates, beam widths, and frequency- hopping characteristics. Although all these sources are located on land, they can be seen by the Aquarius Instrument as soon as they pop over the horizon as observed from the satellite (approximately 3000 km distant). Although no main-beam to main-beam viewing occurs, the main beam of the RFI sources pass through the side-lobes of the three Aquarius antenna beams (oriented in a fixed, pushbroom configuration forming a ~370 km-wide swath on the ocean surface). The Aquarius main beams view the side-lobes of the ground radars, but this always occurs over land, thus not affecting the ocean salinity measurements.