Mapping topside ionospheric vertical electron content from multiple LEO satellites at different orbital altitudes

In this paper, we present an approach to generating a global topside ionospheric map (GTIM) using dual-frequency global positioning system (GPS) data from multiple low Earth orbit (LEO) satellites at different orbital altitudes. NeQuick2 is employed to normalize LEO data to the same observation range, and 13 LEO satellites from 2015/01/01 to 2015/09/27 are used to generate GTIM-500 (with an ionospheric range from 500 km to 20,200 km) and GTIM-800 (with an ionospheric range from 800 km to 20,200 km). First, we use the coinciding pierce point technique to study the error induced by altitude normalization. The results show that the relative bias error is approximately 1%. Then, the performance and accuracy of the GTIMs as well as the differential code bias (DCB) of GPS receivers onboard LEO and GPS satellites are compared and analyzed. The statistical results of the differences between the official LEO-DCB products and the LEO-DCBs estimated by our different solutions show a RMS improvement of 23% and 41% for GTIM-500 and GTIM-800, respectively. The improvement in RMS of GPS-DCBs for the proposed method is approximately 20%. Finally, the accuracy of GTIM is evaluated by the dSTEC assessment method. The results show that the RMS of GTIM-500 is 0.50 TECU (total electron content unit) for both methods. In terms of GTIM-800 estimated by the proposed method, the RMS has an improvement of 24%.