Seminar Ozean, Eis, Atmosphäre

S. Heise DLR, Institut für Kommunikation und Navigation, Kalkhorstweg 53, D 17235 Neustrelitz, Germany Whereas Global Positioning System (GPS) ground based observations are well proved as a powerful tool in terms of ionosphere monitoring, the installation of GPS receivers on board of Low Earth Orbiting satellites (LEO's) offers new opportunities to use GPS measurements in observing the ionosphere on global scale. After an introduction to the ionosphere, the GPS system and the CHAMP mission, GPS measurements on board the German small satellite CHAMP are reported to be used for imaging the topside ionosphere and plasmasphere. Beside the GPS radio occultation measurements the GPS TRSR2 flight receiver tracks up to 8 satellites simultaneously for precise orbit determination using a dedicated zenith looking antenna. Since these dual frequency GPS data are measured with a sampling rate of 0,1 Hz, they provide valuable information on the ionization state of the topside ionosphere and plasmasphere. To determine absolute values of the Total Electron Content (TEC) along the numerous radio links, the differential carrier and code phases are combined in a least squares sense and subsequently calibrated by considering the differential code biases of the corresponding transmitter-receiver combination. These instrumental biases are estimated in a separate way by applying a model assisted technique under nighttime ionospheric conditions that promises a low ionization level and therefore small calibration errors. In the following up to 3000 calibrated TEC measurements derived for a full CHAMP revolution are assimilated into the Parameterized Ionospheric Model (PIM) that includes also the plasmasphere. The assimilation technique and corresponding results providing a 2D-reconstruction of the ionosphere/plasmasphere electron density in the CHAMP orbit plane from the CHAMP altitude up to GPS orbit heights will be discussed. Validation results will be presented by comparing imaging data with electron density measurements from the Langmuir probe on board CHAMP and from coordinated incoherent scatter radar measurements on the ground.