Ionosphere TEC and TEC gradients estimation using a regional GNSS network

This paper presents an algorithm to estimate regional ionosphere total electron content (TEC) and TEC gradients. In this approach, the ionosphere TEC at an arbitrary ionosphere piercing point (IPP) in a local area is represented by contributions from the vertical TEC (VTEC) and spatial gradients of TEC along both latitudinal and longitudinal directions. By differencing the TEC between each pair of observable IPPs, linear equations of local TEC gradients can be established if only first order TEC spatial derivatives in the TEC representation are retained. The TEC gradients obtained from these linear equations are then combined with the VTEC of observable IPPs to generate regional TEC maps. The algorithm is tested using various combinations of a network of seven GNSS stations located in northern Europe between N46° and N53° latitude and E10° and E23° longitude with a 0.5° latitude and 1° longitude grid size over a 24-hour period. The TEC values obtained from the algorithm are compared with that from the IGS global ionosphere maps (GIM). The results show that the average difference between TECs generated from the proposed algorithm and the GIM is less than 2 TEC units, while the standard deviation of differences is about 1.5-3 TEC units. The preliminary results demonstrate that the proposed TEC gradient-based method has the potential to produce an accurate regional TEC map for precise positioning and for ionospheric monitoring using measurements from relatively sparsely located GNSS receivers. The algorithm can be implemented for real-time application and can be easily modified to incorporate other multi-GNSS measurements, such as GLONASS, Galileo, and Beidou.