The GPS radio occultation technique (Kursinski et al., 1997) uses satellite-to-satellite limb soundings between low-Earth orbiters and GNSS transmitters to measure accumulated refraction of the signal while it propagates through the atmosphere. Based on Doppler shift and known occultation geometry the so-called bending angle can be calculated and assigned to perigee points where the refraction is critical. The idea has its origin in extraterrestrial space missions to explore atmospheres of Mars (Fjeldbo and Eshleman, 1968) and Venus (Fjeldbo et al., 1971) in Mariner program. The proof-of-concept GPS/MET mission launched in 1995 was the first experiment on Earth’s atmosphere (Kursinski et al., 1996) that made the radio occultation technique an important tool for weather forecasting since the number of receiver platforms as well as new generation transmitters is constantly increasing. The most successful mission FORMOSAT-3/COSMIC which brought constellation of six low-Earth orbiters in 2006 provides both, ionospheric total electron density profiles for space weather applications and neutral atmosphere soundings that are operationally assimilated into numerical weather prediction models in the form of bending angle or refractivity profiles (Cucurull et al., 2007; Poli et al., 2010). Since COSMIC satellites already exceed their lifetime, the follow-on mission is planned to be launched on equatorial (expected in 2018) and polar orbits (under evaluation by NOAA’s Observing System Simulation Experiments) which together with growing EUMETSAT constellation of MetOp satellites will provide continuous retrievals.
Fig. Radio occultation event (source: cosmic.ucar.edu)
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