Effect of Thai Ionospheric Maps (THIM) model on the performance of network based RTK GPS in Thailand

Abstract

In 2008, the DOL-NRTK (Department of Lands-Network-based Real Time Kinematic) GPS system was established to support cadastral surveying applications in Thailand using the Virtual Reference Station (VRS) concept. Currently the DOL-NRTK network consists of 11 reference stations located in the Central Plain region of Thailand with spatial coverage ranging from 30 to 125 km with an average spacing around 60 km. A previous study confirmed that ionospheric errors are the main error sources that affect the NRTK performance. Also the ambiguity fixing success rate was significantly improved when the final Global Ionospheric Maps (GIM) from the Center for Orbit Determination in Europe (CODE) were applied. However, the final GIM is the global model and may not perfectly fit with smaller regional test areas in Thailand. Therefore, an investigation on the use of a regional ionospheric model was required. In this paper, the Thai Ionospheric Map (THIM) generated by the Bernese software is described. An investigation on the performance of NRTK with the aid of the THIM and the final GIM in various different reference station spacing, i.e. 10–20, 30–50, 50–60 and 60–80 km are compared. This investigation is performed using a large number of GPS observations (31 consecutive days) and all available Continuous GPS (CGPS) stations in the Central Plain region of Thailand. Test results indicate that the NRTK positioning performance with the aid of the THIM yields the best solutions when compared to the use of the final GIM and no model, especially in the case of the middle reference station spacings (i.e. 30–50 and 50–60 km spacing). Therefore, it can be concluded that the THIM can effectively mitigate the ionospheric bias in the NRTK mode in Thailand.