Radio Occultation Soundings in Ionosphere and Space Weather Applications: Achievements and Prospects

Abstract

It is well known that the GNSS (Global Navigation Satellite System) signals are affected by the dispersive ionosphere, when the signals pass through it on their way from the transmitter to the receiver. The impact of the ionosphere is an error source that is usually eliminated in the ground based GNSS navigation applications and in the retrieval of the characteristics of the neutral atmosphere from the RO (Radio Occultation) soundings. However, both RO soundings and ground based GNSS observations can also be used to monitor the characteristics of the ionosphere and the plasmasphere. RO soundings for ionospheric tomography were proposed for the first time already in 1988 (Austen et al. 1988). The first demonstration of the soundings were performed by the GPS/MET mission. Many other RO missions (Oersted, CHAMP, SAC-C, and GRACE) have also included ionospheric soundings into their mission objectives. IOX (Ionospheric Occultation Experiment) was a dedicated ionospheric RO instrument onboard the PICOSat satellite. The ongoing FORMOSAT-3/COSMIC mission has already provided over 800 000 ionospheric soundings during the first 14 months of the mission. The new data produced by the GNSS soundings has generated significant advances in the radio science of the Earth’s atmosphere and the ionosphere. Global coverage and continuous data production have enabled new types of services providing information about the status of the ionosphere in NRT (Near Real Time). Some emerging applications of GNSS observations include e.g., regional high resolution ionospheric tomography and monitoring and prediction of ionospheric scintillation. This paper gives an overview of the scientific achievements with the ionospheric RO soundings from the past and from the current RO missions, and discusses the prospects of the GNSS “space weather” applications in the near future.