Validating low earth orbiting satellite altimeter observed ionosphere TEC using global ionosphere map (GIM)

The dual frequency radar altimeter of TOPEX/Poseidon (T/P) provides an extensive database of vertical electron distributions over the ocean. TOPEX ionosphere data were compared with global ionosphere maps (GIMs) to validate single frequency altimeter ionosphere corrections using GIM. T/P dual frequency altimeter measures nadir electron content from satellite altitude to the ocean surface. To compare T/P TEC with GIM we eliminate the super-satellite electron density profiles using the International Reference Ionosphere (IRI) model. Global Ionosphere Maps (GIMs) are GPS-data driven products that provide the integrated total electron densities between the ground and GPS satellite altitude of about 20,000 km. Regional GPS receiver networks provide additional high spatial TEC data as input for an alternate representation of GIM. Regionally enhanced TEC representation using GIM were also compared with T/P TEC. Specifically, we compared GIM developed by the Center for Orbit Determination in Europe (CODE) with TEC from T/P. Global comparison indicates that GIM lack sufficient resolution and accuracy compared to T/P TEC. The limitation of GIM is attributed to the sparse station locations in large areas of Asia and the oceans. We also compared GIM results with TEC from GEONET which is a regional GPS network on the Japanese Islands. GEONET provides high temporal and spatial resolution TEC data over the Japan islands. Single frequency altimeter ionosphere corrections using GIM should be used with care. Ionosphere corrections are best derived from regional high density GPS network TEC results.