In recent years, the global positioning system (GPS) has been exploited via radio occultation techniques to obtain profiles of refractivity, temperature, pressure and water vapor in the neutral atmosphere and electron density in the ionosphere. The GPS MET experiment, which placed a GPS receiver in a low-Earth orbit, provided a wealth of data which was used to test this concept and the accuracy of the retrievals. Several investigations have already demonstrated that the retrieval accuracies obtained with GPS MET is already comparable, if not better. than the more traditional atmospheric sensing techniques (e.g.. radiosondes). Even though the concept of atmospheric profiling via radio occultation is quite a simple one. care Must be taken to separate the numerous factors that can affect the occulted signal. These include the motion of the satellites, clock drifts. relativistic effects, the separation of the ionosphere and the neutral atmosphere, and the contribution of the tipper atmosphere where sensitivity of the GPS signal is weak. In addition, care must be taken to use proper boundary conditions. use proper smoothing intervals and interpolation schemes to avoid retrieving artificial atmospheric structures, and most importantly detect and correct phase measurement errors introduced by sharp refractivity gradients in the atmosphere. This work describes in some detail the several steps involved in processing Such data. In particular. it describes a system that was developed at the Jet Propulsion Laboratory and used to process the GPS MET data. Several examples of retrieved refractivity, temperature and water vapor profiles are shown and compared to analyses from the European Center for Medium-range Weather Forecast (ECMWF). Statistical comparisons of GPS MET and ECMWF temperatures for data collected during June 21-July 4. 1995. indicate that differences are of order 1-2 K at northern latitudes where the ECMWF analyses are most accurate. (C) 2002 Elsevier Science Ltd. All rights reserved.
