12.21.2004 12:00 AM
Interference to Earth Sensing Satellites Concerns Meteorologists
If you closely monitored coverage of the hurricanes and storms this summer, you might have caught references to satellite observations other than the various wavelength infrared and visible wavelength images displayed during TV weather forecasts. These Earth-sensing satellites use microwave frequencies to, among other things, "see through" clouds to measure sea surface temperature-critical for hurricane forecasting, monitoring surface snow pack and measuring soil moisture. As you know from reading RF Report, there is more demand for all microwave frequencies. Meteorologists are concerned that powerful commercial interests will sway the International Telecommunications Union (ITU) allocate frequencies critical for earth sensing applications to their commercial ventures.

A BBC News article by Alex Kirby, Forecasters face losing key tools examines the threat. It noted that important frequencies around 6.8 GHz and 10.7 GHz have already been lost for use over land. A key concern is the 23.6 to 24 GHz band, which the BBC noted is sensitive to water vapor but not to liquid water. The article quoted Dr. Stephen English, manager of the satellite radiance assimilation group at the U.K. Met Office. "We only need a few narrow-frequency bands for Earth remote-sensing, but most of these are unique, so there is no alternative." Regarding the properties of the 23.6 to 24 GHz band, Dr. English explained, "There is no other frequency where this occurs. But car 'radars' will now be allowed to broadcast in this frequency band."

Already, NASA's Aqua satellite shows "hot spots" due to RF interference over land. Dr. English explained the implications. "The 'hot spots' are easy to spot, but more worrying is the fact that smaller variations may be RFI, or they may be due to rain. The truth is we can't tell. Therefore the channel is rendered useless not only in the 'hot spots' but everywhere, because we can no longer uniquely interpret the variations in terms of rainfall. Of course, over the ocean man-made signals are limited, so we still regard this channel as useful over the ocean, but it's no longer useful over land."

If you are interested in the images from remote sensing satellites like the NASA Aqua, visit the Naval Research Laboratories NexSat Next-Generation Weather Satellite Demonstration Project and check out the images over your location. Note that some of the images on this page are from visual and infrared satellites and would not be affected by the interference mentioned by Dr. English.


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