Initial GPS radio occultation measurements of the ionosphere onboard champ

The radio limb sounding or occultation technique is capable of deriving accurate vertical profiles of atmospheric refractivity. This has been shown by very promising results of the first active GPS limb sounding GPS/MET experiment on Microlab 1. In this paper we report early results of ionospheric radio occulatation (IRO) measurements carried out onboard the German satellite mission CHAMP (CHAllenging Minisatellite Payload) that contributes also to essential improvements of gravity and magnetic field models of the Earth. First radio occultation measurements of the ionosphere were carried out by CHAMP on 11 April 2001. Fortunately, ionospheric radio occultation measurements were switched on during the High Rate GPS/GLONASS measuring campaign (HIRAC) initiated by the International GPS Service (IGS) and supported by COST 271 activities. So a comprehensive analysis of coordinated measurements will be possible within COST 271 cooperation in the near future. The achieved accuracy of the retrieved electron density profiles is estimated by comparing them with independent ionosonde data.