Stefan Schlüter

Differential Code Bias of GPS Receivers in Low Earth Orbit: An Assessment for CHAMP and SAC-C

The knowledge of the transmitter and receiver differential code biases (DCB) plays a key role for the calibration of GPS based measurements of total electron content (TEC). To estimate the DCB of the CHAMP receiver concerning the zenith looking antenna a model assisted technique has been developed which takes advantage of the known GPS biases and comparatively low TEC above Low Earth Orbiter (LEO) altitudes in polar regions and during nighttime intervals. For assistance the Parameterized Ionospheric Model (PIM) is used. Applying this method we derived a reliable bias solution for the CHAMP receiver varying within a spread RMS below 1 TECU for the years 2001 and 2002. The SAC-C receiver DCB (zenith antenna) could be derived comparatively stable. In this paper we describe the model assisted calibration technique and present differential code bias estimations for CHAMP and SAC-C. The model assisted approach has been used to estimate the CHAMP occultation DCB as well. This bias could also be assessed by simultaneous GPS observations of the zenith and occultation antenna for selected intervals in 2001. DCB estimations from both approaches are shown.

Three-Dimensional Monitoring of the Polar Ionosphere with Ground- and Space-Based GPS

Integrated electron density measurements along GPS signal ray paths provide a useful data source for 3-dimensional ionospheric reconstruction. Since a substantial data amount is required for the application of the inverse problem, ground-based inversions are restricted to regions with high ground receiver density. The additional incorporation of space-based GPS data from LEO satellites allows the monitoring in areas less densely covered by ground receivers. We present a case study of 3-dimensional reconstruction for Nov. 2001 in the North polar region. Input data are calibrated TEC derived from IGS and CHAMP measurements. The results show increased electron densities near the geomagnetic pole. The comparison of the reconstruction results with measurements of the Langmuir Probe onboard CHAMP shows a good agreement along the CHAMP orbit. The large-scale structure of the ionospheric observations are closely related to the present geomagnetic conditions.

Monitoring the 3 Dimensional Ionospheric Electron Distribution based on GPS Measurements

In the last decade GPS has become a valuable tool for determining various ionospheric and neutral atmospheric parameters. The presently existing global and regional networks of GPS ground stations (e.g. regional surveying offices, International GPS service IGS) open the opportunity to establish monitoring systems to estimate ionospheric and tropospheric/stratospheric key-parameters (e.g. electron content, temperature, water vapour) on a regular base.