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The COSMIC radio occultation mission represents a revolution in atmospheric sounding from space, with precise, accurate, and all-weather global observations useful for weather, climate, and space weather research and operations.
The global positioning system (GPS) radio-occultation (RO) limb-sounding technique for sounding Earth's atmosphere was demonstrated by the proof-of-concept GPS Meteorology (GPS/MET) experiment in 1995-97 (Ware et al. 1996). Following GPS/MET, additional missions, that is, the Challenging Minisatellite Payload (CHAMP; Wickert et al. 2001) and the Satellite de Aplicaciones Cientificas-C (SAC-C; Hajj et al. 2004), have confirmed the potential of RO sounding of the ionosphere, stratosphere, and troposphere.
At 0140 UTC 15 April 2006, six microsatellites were launched into a circular, 72° inclination orbit at an altitude of 512 km from Vandenberg Air Force Base, California (Cheng et al. 2006). The mission is a collaborative project of the National Space Organization (NSPO) in Taiwan and the University Corporation for Atmospheric Research (UCAR) in the United States. The mission is called the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) in the United States and the Formosa Satellite Mission 3 (FORMOSAT-3) in Taiwan.' All satellites began delivering useful data within days after the launch (Anthes 2006). This paper summarizes the mission and the early scientific results, with emphasis on the radio-occultation part of the mission.
The primary payload of each COSMIC satellite is a GPS radio-occultation receiver developed by the National Aeronautics and Space Administration's (NASA's) Jet Propulsion Laboratory (JPL). By measuring the phase delay of radio waves from GPS satellites as they are occulted by the Earth's atmosphere (Fig. 1), accurate and precise vertical profiles of the bending angles of radio wave trajectories are obtained in the ionosphere, stratosphere, and troposphere. From the bending angles, profiles of atmospheric refractivity are obtained. The procedures used to obtain stratospheric and tropospheric bending angle and refractivity profiles from the raw phase and amplitude data for the COSMIC mission are described by Kuo et al. (2004).
The radio-occultation method for obtaining atmospheric soundings is summarized by Kursinski et al. (1997) and in a special issue of Terrestrial, Atmospheric and Oceanic Sciences (2000, Vol. 1, hereafter TAO). TAO also describes the RO method; the application of RO to weather, climate, and ionospheric research; and the COSMIC mission. The refractivity...