Mitigation of Terrestrial Radar Interference in L-Band Spaceborne Microwave Radiometers

Terrestrial radars operating in the 1215-1400 MHz radio-location and navigation spectrum allocation are important for air traffic safety, homeland security, and national defense. For low-frequency observations of soil moisture and ocean salinity, Earth-observing microwave radiometers are allocated EarthExploration Satellite Service (EESS) spectrum for operating at 1400-1427 MHz. The proximity of powerful long-range radars to the passive allocation makes observing a challenge. Three aspects of mitigation to RFI are discussed in this paper: survivability, operability, and excisability (SOE). Modeling and simulations of NASA's Hydros and Aquarius radiometers were performed to examine the impacts of radar interference. The results are applied to the three aspects of mitigation SOE and the affects on the radiometer requirements are discussed. The physics of microwave thermal emission dictate that low frequencies be used for radiometers to measure ocean-surface salinity and soil moisture (through any reasonable amount of vegetation). The Earth Exploration Satellite Service (EESS) enjoys an exclusive passive allocation at 1400-1427 MHz, a band in which transmission is prohibited [I]. Two L-band microwave radiometers will be launched into orbit before the decade's end. (It was three until NASA canceled it's Hydros mission [2] to measure global soil moisture and freeze-thaw state see http:/ihydros.~sfc.nasa.gov). NASA's Aquarius radiometer will measure ocean surface salinity on a global scale [3] and ESA's SMOS mission will measure soil moisture [4]. If radiometer receivers where perfectly selective to their allocated band and neighboring transmitters had perfect control of outof-band (OOB) emissions, radio-frequency interference (RFI) would not be an issue. Experience shows, however, this is not the case [5]-[8]. Since no L-band radiometers have flown in space since Skylab, airborne and ground-based experience augmented by analysis must be used to predict the potential impact of the spectrum environment. By experience, the most problematic interference is due to terrestrial radars TR's and a previous analysis for SMOS also predicts this to be the case [91. In this paper, the impacts of interference due to TR's operating below 1400 M H ~ on the engineering requirements of Aquarius and Hydros are analyzed. The RFI analyses herein are from a study of TR's commissioned by the NASA Earth Science Spectrum Management Office for the Aquarius and Hydros missions [lo]. Given the impacts, three aspects to mitigation are proposed: survivability, operability, and excisability; or SOE. Survivability deals with avoiding damage from RFI. This means proper filtering and limiting. A radiometer achieves operability when it can measure, without error, the antenna or brightness temperature in the presence of interference. Selective receivers operating in quite spectrum are required for operability. When filtering and frequency selection is not enough to avoid interference, then the RFI might be excisable. A number of techniques have been proposed and demonstrated recently [Ill-[14]. These fall into three basic categories: temporal, spectral, and statistical some techniques are a combination of two or more. We draw a distinction between operability and excisability when signal analysis beyond conventional radiometric techniques is required. The Aquarius and Hydros approaches to SOE are