Assessment of corrected time-step method for nominal ionospheric gradient calculation: A comparative analysis with spatial approaches

The effect of ionospheric delay on the ground-based augmentation system under normal conditions can be mitigated by determining the value of the nominal ionospheric gradient (sigma(vig)). The nominal ionospheric gradient is generally obtained from Continuously Operating Reference Stations data by using the spatial single-difference method (mixed-pair, station-pair, or satellite-pair) or the temporal single-difference method (time-step). The time-step method uses only a single receiver, but it still contains ionospheric temporal variations. We introduce a corrected time-step method using a fixed-ionospheric pierce point from the geostationary equatorial orbit satellite and test it through simulations based on the global ionospheric model. We also investigate the effect of satellite paths on the corrected time-step method in the region of the equator, which tends to be in a more north-south direction and to have less coverage for the east-west ionospheric gradient. This study also addresses the limitations of temporal variation correction coverage and recommends using only the correction from self-observations. All processes are developed under simulations because observational data are still difficult to obtain. Our findings demonstrate that the corrected time-step method yields sigma(vig) values consistent with other approaches.