Six tiny FORMOSAT-3 satellites that were sped into space on April 15, 2006 are designed for systematic mass scale radio occultation (RO) studies of the Earth atmosphere and ionosphere at different altitudes by use of the GPS signals. Termed as the Formosa Satellite-3/ Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC) mission, the new constellation’s primary science goal is to obtain in near real time the vertical profiles of temperature, pressure, refractivity, and water vapor in the neutral atmosphere, and the electron density in the ionosphere with global coverage. The measurements during five years of mission life will provide about 2,500 soundings per day, thus generating extensive information to support operational global weather prediction, climate change monitoring, ionospheric phenomena, and space weather research. The theory of RO measurements has been described previously (Gurvich and Krasilnikova, 1988; Yunck, 1988; Yakovlev, 2002; Hajj et al., 2002). During last four years, essential modernization in the RO technique has been introduced (e.g., Liou et al., 2002, 2006; Pavelyev et al., 2004 and references therein).