The topside global broadcast ionospheric delay correction model for future LEO navigation augmentation

In this paper, we propose a solution of designing a topside broadcast ionospheric model to enable the future low earth orbit (LEO) navigation augmentation (LEO-NA) services. Considering the lack of global station observations to develop the LEO-NA ionosphere model, we utilize abundant global navigation satellite system (GNSS) data from LEO satellites to determine the topside global broadcast ionospheric delay. This delay can be combined with existing GNSS broadcast ionospheric delay correction models to determine LEO-NA ionospheric delay. First, the performance of the different-order spherical harmonic (SH) model is evaluated in generating a global topside ionospheric map. The results indicate that by increasing the order from 1 to 2, the internal and external accuracy of the model improves significantly. However, increasing the order from 2 to 8 leads to a decrease in accuracy of 0.10 and 0.11 TECU (total electron content unit) for the internal and external root mean square error. Taking into account compatibility with the Beidou global ionospheric delay correction model, limited data capacity in the navigation message, ionospheric model accuracy, and computational efficiency, we select the second-order SH model as the topside ionosphere broadcast model and outline the strategy for calculating broadcast coefficients. Finally, the accuracy of the topside global broadcast ionospheric delay correction model is evaluated during periods of high and low solar activity. The mean values of root mean square in 2009 and 2014 are 1.49 and 1.88 TECU, respectively. The model in 2009 and 2014 can correct for 67.30% and 72.49% of the ionospheric delay, respectively.