Global GNSS Radio Occultation Mission for Meteorology, Ionosphere & Climate

Abstract

The term “occultation” is widely used in astronomy when an object in the foreground optically occults objects in the background, and it refers to a geometry involving the emitter, the planet and its atmosphere if any, and the receiver changes with time.1 Radio occultation (RO) is a remote sensing sounding technique in which a radio frequency signal emitted from a spacecraft passes through an intervening planetary atmosphere before arriving at the receiver, and is used to study the planetary atmosphere properties in the interplanetary mission (Fjeldbo & Eshleman, 1965). The atmospheric RO observations represent a planetary scale geometric optics experiment in which the atmosphere acts as a big optical lens and refracts the paths and propagation velocity of electromagnetic wave signals passing through it (Kursinski et al., 2000). The first RO experiment started with the Mars flyby by Mariner- IV in 1964 (Kliore et al., 1965). When Mariner-IV satellite passed behind and emerged from the other site of Mars, the extra carrier phase delay and amplitude variation of the microwave signals were observed. These observed data provided very first valuable atmospheric and ionospheric density information by using the inversion techniques (Melbourne et al., 2005). Since then a series of planetary experimental missions were planned to study the atmospheres and ionospheres of the planets and their moons (Yunck et al., 2000). The limb sounding of the earth’s atmosphere and ionosphere using the RO technique can be performed with any two cooperating satellites before the United States’ Global Positioning System (GPS), the first Global Navigation Satellite Systems (GNSS), becoming operational (Lusignan et al., 1969). A few early RO experiments from a satellite-to-satellite tracking link had been conducted. These included the occulted radio link between ATS-6 (Applications Technology Satellite-6) and GEOS-3 (Geodetic and Earth Orbiting Satellite-3) and between the Mir station and a geostationary satellite (Liu et al., 1978; Yakovlev et al., 1996).