4D Imaging of the Ionosphere using GPS data and an Application to Ionospheric Calibration of Radar Altimeters

We show how it is possible to combine GPS/MET oc-cultation data with ground data collected from 100+ IGS stations to perform stochastic tomography of the ionospheric electron content with a 3D global grid of voxels extending up to 2000 km above the mean surface of the Earth, and thus produce temporal series of 3D images of the ionospheric electron content. A correlation functional approach that enforces smoothness of the images is used, and a Kalman lter is used to assimilate the data and propagate the solutions in the time direction. Even with Anti-Spooong on, we have checked that the combination of ground and occulta-tion data and the use of smoothing techniques is robust enough for vertical resolution in a multi-layer model analysis. This has been demonstrated with simulations and the method has been tested by contrasting the GPS estimates with independent sources of data. We discuss as well some of the work we have performed in the area of GPS data ingestion in the Parametrized Iono-spheric Model (PIM). As an application, we compare TEC measurements from the NASA Radar Altimeter and DORIS instrument on board TOPEX/POSEIDON with GPS TEC estimates, and evaluate diierent GPS data analysis strategies. We verify that global tomo-graphic GPS analysis using a voxel grid is well suited for ionospheric calibration of altimeters. We show that a 1-day t of 20-second-averaged NRA ionospheric correction data versus GPS tomographic TEC data has a bias of 3.4 TECU and a root mean square deviation of 3.2 TECU.