Effects of ionospheric correction on GPS time transfer throughout Asia

It is known that the stability of time transfer with GPS common view in the low latitudes of Asia is worse than in Europe and the United States. The ionosphere fluctuates greatly within ±20° of the equator, which seems to have a large effect on time transfer stability. We compared the results of ionospheric corrections obtained using the Global Ionosphere Map (GIM) with those obtained from actual measurements from the ionosphere. The frequency stabilities of common-view results in the areas close to the equator were worse than those in higher latitudes when using the GIM correction, while the results compensated with the measured ionospheric delay were comparable with those in high latitude areas. We compared the GIM simulated variations with the actual variations in the ionosphere around the equator and found that the accuracies in the lower latitudes were not as good as those in the higher latitudes. This seems to degrade the time transfer stability throughout the lower latitudes of Asia.