Performance of network-based RTK GPS in low-latitude region: A case study in Thailand

Thailand has established a Network-based Real Time Kinematic (NRTK) GPS system using the Virtual Reference Station (VRS) concept since 2008. Currently, the Thai NRTK consists of 11 reference stations located in the central part of Thailand with averaged reference station spacing at 60 km. A previous test suggested some problems with the low rate of ambiguity-fixing and the large number of position jumps even when the ambiguity-fixed solutions could be obtained. In this paper, the position performance of NRTK is tested with a large number of GPS observations (31 consecutive days) and different reference receiver spacing, 10-20, 30-50, 50-60 and 60-80 km, with the use of all available Continuous GPS (CGPS) stations in the central part of Thailand. Test results indicate that the NRTK positioning performance is degraded when the reference station spacing is increased. It is also found that the ionospheric bias is the main error source that affects the performance of NRTK in Thailand. Even with the 10-20km reference station spacing, reliable ambiguity-fixed solutions could hardly be obtained during the period of high ionospheric variation. Thus, it is recommended that the reference station spacing should be kept less than 30km. In order to achieve the reliable NRTK solutions especially in low-latitude region like Thailand, the ionospheric bias should be properly handled.