Sounding the Ionosphere by GPS Measurements on CHAMP

Abstract

Ground based GPS measurements are used successfully since several years to monitor the vertical Total Electron Content (TEC) in an operational manner. The installation of GPS receivers on board of LEO (Low Earth Orbiting) satellites such as CHAMP offers new opportunities of ionospheric remote sensing. Beside the GPS radio occultation measurements the German small satellite CHAMP tracks up to 8 GPS satellites simultaneously for precise orbit determination using a dedicated zenith looking antenna. These 0.1 Hz sampled navigation measurements are permanently performed and provide valuable information on the ionization state of the upper ionosphere and plasmasphere between CHAMP and GPS altitude on global scale. After preprocessing and calibration, link related TEC measurements are derived from the GPS navigation observations on board CHAMP. The three dimensional electron density distribution is reconstructed by assimilating the TEC measurements of a full CHAMP revolution into the ionospheric/plasmaspheric model PIM. For this purpose the discretization of PIM on a global voxel structure is necessary. The assimilation method is based on an iterative algorithm which adapts the initial model assumption to the TEC measurements by applying multiplicative modifications. In this talk reconstruction results are presented for selected assimilation examples by means of two dimensional slices along the respective CHAMP orbit plane. The assimilation technique will be described briefly.We present validation results using electron density measurements from the Langmuir Probe on board CHAMP, from incoherent scatter radars and from ionosondes. Potentials and limitations of the reconstruction technique will be addressed.