Radio Occultation Techniques for Probing the Ionosphere

Abstract

The availability of L-band radio signals permanently transmitted by a fleet of satellites belonging to Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS) has opened a new dimension for ionosphere sounding. Whereas ground-based measurements of propagation effects, such as travel time delays and phase changes, have been well established since the mid-nineties, space based GNSS measurements onboard Low Earth Orbiting (LEO) satellites are rather new. Immediately after the proof-of-concept GPS/MET experiment on Microlab-1, flown within the years 1995 -1997, has demonstrated the huge potential of the limb sounding technique on LEO satellites for atmosphere/ionosphere sounding, the development of improved inversion techniques, assimilation methods and powerful processing systems made a big progress in recent years. The radio occultation technique provides a rather simple and inexpensive tool for a global profiling of the entire vertical electron density structure from satellite orbit heights down to the bottom of the ionosphere, not achieved so far by any other technique. The reception of multi-satellite navigation signals, affected on their travel through the ionosphere, provides integral key information on the ionospheric state. Modern inversion and data assimilation methods allow a reliable reconstruction of the electron density structure if the amount of data is sufficient. Extensive information provided by current and future satellite missions with GNSS receiver onboard enables permanent monitoring of the Earth’s co-rotating plasma environment in near-real time. The obtained global data sets contribute to a comprehensive understanding of solar-terrestrial relationships, and are valuable for developing and improving global ionospheric models and provide operational space weather information. Consequently, accuracy and reliability of space based Communication/Navigation radio systems will take benefit from this knowledge.