Beside the GPS radio occultation measurements of the troposphere and ionosphere, the CHAMP satellite tracks presently up to 8 GPS satellites simultaneously for navigation. The received dual frequency GPS signals provide valuable information on the ionization state of the topside ionosphere and plasmasphere. This paper briefly describes a new developed model assisted technique to improve the three dimensional electron density estimation above the CHAMP orbit from these GPS measurements. Application of this assimilation algorithm provides a 2-D electron density slice of the ionosphere/plasmasphere system within the CHAMP orbit plane from the CHAMP altitude up to GPS orbit heights. We estimate the accuracy of the GPS TEC calibration and perform a first validation by comparing the retrieved electron density of the ionosphere/plasmasphere slice at CHAMP altitude to simultaneous electron density measurements of the Langmuir Probe onboard CHAMP.