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10.7 GHzRFI in AMSR-E SSTs (descending passes) near Europe increasing yearly: 
18.65 GHzWe have identified and characterized a source of RFI contamination in descending AMSR-E 18.7 GHz vertically and horizontally polarized channels, beginning September 2007. The location, timing, and frequency of the interference are highly consistent with HDTV broadcasting activities from DirecTV satellite 10 in geosynchronous orbit at 102.8 West Longitude. AMSR-E ocean products derived from descending 18.7 GHz observations are potentially impacted along all U.S. coastal waters from September 2007 forward. DirecTV-10 launched July 7, 2007, and began broadcasting operationally after about 2 months of testing (http://en.wikipedia.org/wiki/DirecTV-10). The first significant DirecTV interference with AMSR-E we have detected occurred on September 15, 2007. The locations of interference are consistent with geosynchronous satellite signals reflecting off the ocean surface into AMSR-E's field of view. DirecTV-10 is reportedly broadcasting at 18.648 GHz (http://www.lyngsat.com/dtv10.html), well within AMSR-E's bandwidth at 18.7 GHz. Having similar broadcasting capabilities, DirecTV-11 (http://en.wikipedia.org/wiki/DirecTV-11) launched March 19, 2008 and began broadcasting on July 31, 2008, from geosynchronous orbit at 99.2 West Longitude. DirecTV-11 may also be impacting AMSR-E at 18.7 GHz. If so, this will likely increase the amplitude of the RFI more than the spatial extent, as the 2 broadcasting satellites are only 3.6 degrees apart in longitude. The geometrically consistent nature of the RFI lends confidence that a high quality flag can be developed to accurately exclude impacted observations. The dynamics are very similar to RFI from Hotbird and Astra satellites previously identified and flagged in AMSR-E descending passes near Europe. Experience with those sources of RFI should be applicable to these new sources. The DirecTV RFI problem was first detected in WindSat data by NRL. The 18.7 GHz channels on WindSat have significantly wider bandwidth than AMSR-E, thus the RFI contamination is more substantial and obvious. WindSat has been impacted by the DirecTV nationwide beams, which broadcast at frequencies below the sensitivity of AMSR-E at 18.7 GHz. The DirecTV spot beams, however, are using frequencies that impact AMSR-E. Initially, it was hoped that the spot beams would limit the spatial extent of impact on AMSR-E. Further investigation revealed that DirecTV spot beams cover nearly the entire U.S. coastline. Spot beams are used to beam different local programming to different local markets, thereby allowing reuse of the same set of frequencies. As the U.S. coast is well populated, the constellation of spot beams provides coverage similar to the nationwide beams. Spot beams are also reported to serve Alaska and Hawaii, but the broadcast angle is unlikely to ever reflect into AMSR-E’s field of view.
AMSR-E RFI Flag: Geostationary Satellite Glint AngleRFI intensity is affected by glint angle, geographic location, and surface roughness. Glint Angle: The magnitude of RFI from these sources is highly correlated with how directly the broadcast signals reflect off of Earth's surface into AMSR-E's field of view. We find the Geostationary Satellite Glint Angle by computing the scalar angle difference between the reflection vector (boresight, or observation vector reflected off the Earth observation point) and the vector from the Earth observation point to the geostationary orbit position. A glint angle of 0° is a direct reflection; glint angles above 20° are unlikely to have significant RFI. Geographic Location: The broadcasting satellites direct their power very specifically to serve markets. Outside of these geographic areas, low glint angles do not correlate with intense RFI. Ascending passes are not affected at all. On descending passes, ocean RFI is most intense near coastal areas. Surface Roughness: RFI is also correlated with surface roughness of the ocean. Low wind, smooth waters yield intense RFI confined to lower glint angles. Rough, windy waters result in RFI of more moderate intensity, but over a larger area including higher glint angles. Unfortunately, the RFI makes wind retrievals unreliable, complicating any attempt to utilize this correlation. We have not studied the impact over land.
Last updated: January 08, 2010 |
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10.7 GHz
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