The atmospheric limb sounding technique based on the Global Positioning System (GPS) has been shown to be accurate and of very high vertical resolution. The fact that the GPS radio occultation (RO) technique is not affected by clouds or precipitation, has no instrument drift, and requires no calibration makes it ideally suited for climate monitoring and global weather prediction. The Constellation Observing System for Meteorology, Ionosphere, and Climate (ROCSAT-3/COSMIC) mission will be launched in late 2005, and will provide ~2,500 GPS radio occultation soundings per day to support operational weather prediction, climate analysis, and ionospheric research. Other radio occultation missions are expected to coincide with COSMIC promising additional data. In this paper, we provide status overview of the COSMIC mission, describe its science goals, and review selected GPS RO studies that are relevant to weather prediction and climate analysis.
The Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) is a joint Taiwan-U.S. space mission, with a plan to launch a constellation of six micro-satellites in late 2005. Each satellite will carry three instruments: a Global Positioning System (GPS) Radio Occultation (RO) receiver, a Tiny Ionospheric Photometer (TIP), and a Tri-Band Beacon (TBB). The COSMIC constellation will provide up to 3,000 RO soundings that are distributed relatively uniformly around the Earth. The raw measurements made by the GPS RO receivers are the phase and amplitude of the GPS radio signals (L-band with wavelengths L<sub>1</sub> ~19.0 cm and L<sub>2</sub> ~ 24.4 cm), which can be used to derive the vertical profiles of temperature, moisture and electron density. The TIP and TBB instruments will provide additional ionospheric measurements. The COSMIC data from these three instruments are expected to make a significant impact on global weather prediction, climate and ionosphere monitoring and research. This paper presents (1) an overview of the COSMIC system; (2) CDAAC results from two recent GPS RO missions, CHAMP and SAC-C; and (3) the potential impact of COSMIC data on numerical weather prediction as indicated by recent observing system simulation experiments (OSSEs).