Remotely sensing the earth's atmosphere using the global positioning system (GPS) - The GPS/MET data analysis

The Global Positioning System/Meteorology (GPS/MET) project is an active satellite-to-satellite remote sensing experiment using the radio occultation technique. Due to the atmospheric index of refraction and gradient of the index of refraction, GPS signals propagate through the earth's atmosphere along a slightly curved path and with slightly retarded speeds. When these signals arrive at a receiver aboard a low earth orbit satellite, the receiver records an excess phase delay compared with the phase delay of a straight line propagation in a vacuum. Using the Abel integral equations, the phase delay rates with time can be converted into the atmospheric index of refraction profile; then, using the hydrostatic equation: the pressure and temperature profiles may be derived. This paper describes the principles of the GPS/MET occultation experiment and the detailed data analysis procedure. Data smoothing technique and error analysis are also discussed. Some GPS/MET intermediate and final retrieval results, such as ray bending angle and temperature and pressure profiles, are presented for illustration. Although random noise in the GPS/MET measurements leads to an uncertainty of similar to 0.1-0.3 K in the retrieved temperatures, comparisons of the GPS/MET retrieval results with radiosonde measurements, other remote sensing observations, and numerical analyses show that without the multipath corrections the GPS/MET temperature profiles accurate to within 1 degrees-2 degrees C can be obtained from similar to 5-7 to similar to 40 km above the ground. Despite the fact that a few outstanding problems in the GPS/MET data retrievals remain to be addressed, overall the GPS/MET occultation method has been demonstrated to be capable of producing accurate, all-weather, round-the-clock. global refractive index, density, pressure, and temperature profiles of the troposphere and stratosphere.