The GPS/MET experiment was the first active atmospheric microwave occultation experiment using the existing GPS Li and L2 frequencies to measure the atmospheric refractive index. One major limitation to this tecimique is that the presence of water vapor in amounts typically found in the lower troposphere (below 5-7 km) causes an ambiguity between the contributions of dry air and moisture to the refractive index. Additionally the profiles of other gases, such as ozone, cannot be measured using the Li and L2 frequencies. A new satellite remote sensing technique to independently monitor atmospheric water vapor and ozone is under development. It will include small satellites with both transmitter and receiver capabilities on each. The frequencies will be located around the 22 and 183 GHz water vapor and the i95 GHz ozone absorption lines. The receivers will also have the capability to observe the Li and L2 GPS frequencies. Simulation studies show that this new active occultation technique has the potential to provide accurate profiles of water vapor and ozone, as well as refractivity, temperature and pressure.
The global positioning system (GPS) is based on a constellation of 24 transmitter satellites orbiting the earth at approximately 21,000 km altitude. The original goal of the GPS was to provide global and all-weather precision positioning and navigation for the military. Since this original concept was developed, several civilian applications have been conceived that are making use of these satellites. GPS/MET is one such application. GPS/MET is sponsored by NSF, FAA, NOAA, and NASA. The goal of GPS/MET is to demonstrate the feasibility of recovering atmospheric temperature profiles from occulting radio signals from one of the 24 GPS transmitters. On April 3, 1995, a small radio receiver was launched into a 750 km low- earth orbit and 70 degree inclination. As this receiver orbits, occultations occur when the radio link between any one of the 24 GPS transmitters and the low-earth orbiting (LEO) receiver progressively descends or ascends through the earth's atmosphere. With the current constellation of GPS transmitters, approximately 500 such occultations occur in each 24-hour period per LEO receiver. Several hundred occultations have been analyzed to date, where some type of confirmational data has been available (i.e., radiosonde, satellite, numerical analysis gridded data). In this paper, we present a brief outline of the method followed by a few typical temperature soundings that have been obtained.
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