RFI at L-BAND: AQUARIUS and SMOS
Le Vine, David1; de Matthaeis, Paolo2; Kerr, Yann3; Richaume, Philippe3; Ruf, Chris4
1NASA/Goddard Space Flight Center, UNITED STATES; 2GESTAR, UNITED STATES; 3CESBIO, FRANCE; 4University of Michigan, UNITED STATES
Passive microwave measurements from space in the spectral window at L-band are important for remote sensing of soil moisture and ocean salinity. Two instruments are currently in orbit making these measurements, the Aquarius radiometers aboard the Aquarius/SAC-D observatory (NASA) and the MIRAS interferometric radiometer aboard SMOS (ESA). Although the spectral window at 1.413 GHz where these instruments operate is restricted for passive use only, man-made radio frequency interference (RFI) is an issue; and especially over land RFI is severe enough in several parts of the world to threaten the retrieval of surface parameters.
Although the sources of the RFI are common to both sensors, the appearance and character of its effect depend on the sensor itself (e.g. polarization, antenna pattern, bandwidth, etc). The MIRAS and Aquarius radiometers represent quite different technology: MIRAS is an interferometric instrument that synthesizes the radiometer antenna beam in software whereas Aquarius employs a conventional antenna imaging in a pushbroom fashion. MIRAS has better spatial resolution and images the same pixel from several look angles which has facilitated locating sources of RFI (and having them shut down ). Aquarius has high radiometric sensitivity and short-time sampling which enhances detection of low level RFI.
Provisions for detecting and mitigating RFI are among the unique features of the Aquarius radiometers. Aquarius employs rapid sampling and a ''glitch detection'' algorithm . The ''glitch detector'' examines each short-time sample to detect outliers which are presumed to be associated with RFI. The idea is to sample much faster than the Nyquist rate for imaging with the goal that samples corrupted with RFI can be identified and removed without having to discard the entire pixel. This approach appears to be working well and Aquarius is producing maps of salinity of high scientific quality over the open ocean .
The distribution of RFI as reported by Aquarius is consistent with the observations of SMOS. Both sensors detect persistent and significant sources of RFI over land in Eastern Asia (China) and Eastern and Central Europe and associated with radars in the DEW line in North America. In addition to high power radar, RFI is detected by Aquarius in the vicinity of the population centers of the world including large cities in South America and Africa.
Examples of the global distribution of RFI as detected by the two sensors will be presented. Combining resources from the two missions to characterize RFI at L-band will help understand how to best to mitigate and control the interference and help future missions such as SMAP prepare.
1. R. Oliva, E. Daganzo, Y.K. Kerr, S. Mecklenburg, S. Nieto, P. Richaume, et al, ''SMOS radio frequency interference scenario: status and actions taken to improve the RFI environment in the 1400-1427-MHz passive band.(soil moisture and ocean salinity)(radio frequency interferences), IEEE Trans. Geosci & Remote Sensing, ISSN 0196-2892, Vol 50 (#5), pp 1427 - 1439, May, 2012
2. C. Ruf, and S. Misra, ''Detection of rado-frequency interference for the Aquarius radiometer'', IEEE Trans. Geosci Remote Sens., Vol 46 (#10), 3123-3128, 2008
3. D.M. Le Vine et al., ''Aquarius: The Instrument and Initial Results'', pp 1-3, MicroRad 2012, 12th Specialist Meeting on Microwave Radiometry and Remote Sensing of the Environment, March, 2012: DOI10.1109?MicroRad.2012.6185226.