The radio occultation (RO) technique, which makes use of radio signals transmitted by the Global Position System (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere in all weather and over both land and ocean. As demonstrated by the proof-of-concept GPS/Meteorology (GPS/MET) experiment and more recently by the CHAMP (CHAllenging Minisatellite Payload) and SAC-C (Satellite de Aplicaciones Cientificas-C) missions, GPS RO data are shown to be of high precision, accuracy and vertical resolution. On 15 April 2006, the joint Taiwan-U.S. COSMIC/FORMOSAT-3 (Constellation Observing System for Meteorology, Ionosphere, and Climate and Formosa Satellite mission #3; hereafter COSMIC) mission, a constellation of six microsatellites, was launched from Vandenberg Air Force Base in California. Since 28 July 2006, COSMIC has been providing a large number of RO soundings to support the research and operational communities. The number of soundings is expected to increase to about 2,500 per day by mid 2007 as the satellites are further separated and reach their final orbits. As of the end of July 2007, a total of 571,504 atmospheric soundings have been processed.

Recent results have shown that the GPS RO data from COSMIC are of better quality than those from the previous missions and penetrate much farther down into the troposphere; from 70 to 90 percent of the soundings reach to within 1 km of the surface on a global basis. COSMIC data are making a positive impact on operational global weather forecast models. Following several months of testing, European Centre for Medium-range Weather Forecasts (ECMWF) began using the COSMIC data operationally on 12 December 2006. The National Centers for Environmental Prediction (NCEP) turned on operational assimilation of COSMIC data on 1 May 2007, and the United Kingdom Meteorolorical Office on 15 May 2007. As of 1 August 2007, there are 561 registered users of the data representing major U.S. and international Universities, the leading operational weather centers, research laboratories, and several private companies.

COSMIC data are shown to be useful in validating weather prediction and ionospheric models. With the ability to penetrate deep into the lower troposphere using an advanced open loop tracking technique, the COSMIC RO instruments have shown the capability to observe the structure of the tropical atmospheric boundary layer, providing valuable information on low level atmospheric water vapor. The information on water vapor had a positive impact on modeling the genesis of tropical cyclones, and the prediction of heavy rainfall events. The value of RO for climate monitoring and research is demonstrated by comparing observations from COSMIC and other RO missions and showing that the precision of RO observations is mission-independent. We also show that COSMIC observations are capable of inter-calibrating microwave measurements from the Advanced Microwave Sounding Unit (AMSU) on different satellites. Finally, unique and useful observations of the ionosphere are being obtained using the RO receiver and two other instruments on the COSMIC satellites, the Tiny Ionosphere Photometer (TIP) and the Tri-Band Beacon.