A GLOBAL MAPPING TECHNIQUE FOR GPS-DERIVED IONOSPHERIC TOTAL ELECTRON-CONTENT MEASUREMENTS

A worldwide network of receivers tracking the transmissions of Global Positioning System (GPS) satellites represents a new source of ionosph eric data that is globally distributed and continuously available. We describe a technique for retrieving the global distribution of vertica l total electron content (TEC) from GPS-based measurements. The approa ch is based on interpolating TEC within triangular tiles that tessella te the ionosphere modeled as a thin spherical shell. The high spatial resolution of pixel-based methods, where widely separated regions can be retrieved independently of each other, is combined with the efficie nt retrieval of gradients characteristic of polynomial fitting. TEC pr edictions from climatological models are incorporated as simulated dat a to bridge significant gaps between measurements. Time sequences of g lobal TEC maps are formed by incrementally updating the most recent re trieval with the newest data as it becomes available. This Kalman filt ering approach smooths the maps in time, and provides time-resolved co variance information, useful for mapping the formal error of each glob al TEC retrieval. Preliminary comparisons with independent vertical TE C data, available from the TOPEX dual-frequency altimeter, suggest tha t the maps can accurately reproduce spatial and temporal ionospheric v ariations over latitudes ranging from equatorial to about +/-65 degree s.