The atmospheric limb sounding technique making use of radio signals transmitted by the Global Position System (GPS) has emerged as a promising approach for global atmospheric measurements. As demonstrated by the proof-of-concept GPS Meteorology (GPS/MET) experiment and more recently by the CHAMP and SAC-C missions, the GPS radio occultation (RO) sounding data are of high accuracy and high vertical resolution. On 15 April 2006, the joint U.S.-Taiwan COSMIC/FORMOSAT-3 mission, a constellation of six microsatellites, was launched from the Vandenberg Air Force Base. These satellites are being deployed to their final orbits during the coming year. During the early phase of the deployment, the satellites are closely located. This offers a unique opportunity to examine the precision of the GPS RO measurements. The COSMIC data are available in near real-time for global weather analysis and prediction and post-processed results will be made available for climate monitoring. As of the time of this writing (1 August 2006), COSMIC is producing approximately 500 GPS RO soundings per. This number will be increased with improved receiver firmware and as the satellites are further separated through the deployment process.

Radio occultation measures phase and amplitude of the microwave signals emitted from GPS. These signals are inverted to obtain profiles of signal bending, atmospheric refractivity, pressure temperature and water vapor. The main objective of the COSMIC/FORMOSAT-3 mission is to demonstrate the value of these radio occultation products for weather forecasting, climate monitoring, space weather prediction and ionospheric research. This presentation will provide an overview of the COSMIC/FORMOSAT-3 program. We will present results on the estimate of the precision of GPS RO soundings in the early phase of the mission, as well as the impact of COSMIC/FORMOSAT-3 data on hurricane prediction. For further information on the COSMIC/FORMOSAT-3, please refer to http://www.cosmic.ucar.edu/.