Spaceborne GPS remote sensing for atmospheric research

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.