Initial Results on Ionosphere/Plasmasphere Sounding based on GPS Data Obtained On Board CHAMP

For precise orbit determination the GPS receiver on board CHAMP tracks presently up to 8 GPS satellites. Since these navigation data are measured with a sampling rate of 0.1 Hz, the dual frequency GPS signals provide valuable information on the ionization state of the topside ionosphere and plasmasphere. Combining differential carrier and code phases, absolute values of the total electron content (TEC) along the numerous radio links are estimated after calibrating the instrumental biases of each CHAMP receiver GPS transmitter combination by applying a model assisted technique. The calibrated TEC data derived for a full CHAMP revolution are assimilated into the Parameterized Ionospheric Model (PIM) that includes also the plasmasphere. The assimilation technique is briefly described. Preliminary results that provide a 2-D electron density slice of the ionosphere/plasmasphere within the CHAMP orbit plane from the CHAMP altitude up to GPS orbit heights will be discussed. First validation checks will be presented using electron density in situ measurements by the Langmuir probe on board CHAMP and electron density profiles from digisonde and incoherent scatter radar measurements for selected coincidence cases.