Temporal and spatial variability of the bias between TOPEX- and GPS-derived total electron content

Abstract

Total Electron Content (TEC) predictions made with the GPS-based La Plata Ionospheric Model (LPIM) and the International Reference Ionosphere (IRI95) model were compared to estimates from the dual-frequency altimeter onboard the TOPEX/Poseidon (T/P) satellite. LPIM and IRI95 are evaluated for the location and time of available T/P data, from January 1997 to December 1998. To investigate temporal and spatial variations of the TEC bias between T/P and each model, the region covered by T/P observations was divided into 10 latitude bands. For both models and for all latitudes, the bias was mainly positive (i.e. T/P values were larger); the LPIM bias was lower and less variable than the IRI95 bias. To perform a detailed analysis of temporal and spatial variability of the T/P-LPIM TEC bias, the Earth’s surface was divided into spherical triangles of 9° sides, and a temporal-varying regression model was fitted for every triangle. The highest TEC bias was found over the equatorial anomalies and it was attributed to errors of LPIM. A significant TEC bias was found at +40° latitude and it was attributed to errors in the T/P Sea State Bias (SSB) correction. To separate systematic errors in the T/P TEC from those caused by LPIM, altimeter range biases estimated by other authors are analysed in connection with the TEC bias. This analysis suggested that LPIM underestimates the TEC, particularly during summer, in the Southern Hemisphere; while T/P C-band SSB calibration is worse during winter, in the Southern Hemisphere