Modelling of Dispersive and Non-dispersive Effects on Network-Based Real-Time Kinematic Positioning Samsung Lim 1 , Tajul A. Musa 2 and Chris Rizos 1 1 School of Surveying and Spatial Information Systems,The University of New South Wales, NSW, Australia 2 Department of Geomatics Engineering, Universiti Teknologi Malaysia, Malaysia E-mail: s.lim@unsw.edu.au ; Tel: +61-2-9385 4505; Fax: +61-2-9313 7493 ABSTRACT The concept of network-based real-time kinematic positioning has been extensively developed in order to better model the distance-dependent errors of GPS carrier-phase measurements. These errors can be separated into a frequency-dependent or dispersive component (e.g. the ionospheric delay) and a non-dispersive component (e.g. the tropospheric delay and orbit biases). In fact, dispersive and non-dispersive errors have different dynamic effects on the GPS network corrections. The separation of the two is useful for modelling the network corrections and can provide network users with more options for their data processing strategy. A simple running average is proposed in this paper to provide a stable network correction for the non- dispersive term. It is found that the non-dispersive correction can be used to obtain better ionosphere-free measurements, and therefore helpful in resolving the long-range integer ambiguity of the GPS carrier-phase measurements. Once the integer ambiguities have been resolved, dispersive and non-dispersive corrections can be applied to the fixed carrier-phase measurements for positioning step so as to improve the accuracy of the estimated coordinates. Instantaneous positioning, i.e. single-epoch positioning, has been tested for two regional networks: SYDNET, Sydney, and SIMRSN, Singapore. The test results have shown that the proposed strategy performs well in generating the network corrections, in fixing ambiguities and in computing a user’s position. 1. INTRODUCTION Real-time kinematic (RTK) ambiguity resolution, a key step for precise GPS positioning, is complicated due to many error sources in the carrier-phase measurements. These errors can be grouped into station- and distance-dependent errors. Station-dependent errors such as receiver-based errors, multipath and measurement noise notably degrade the ambiguity resolution. As a lot of research on reducing these errors is currently being undertaken, RTK ambiguity resolution is now seriously affected by the presence of the distance-dependent errors: ionospheric delay, tropospheric delay and orbit biases. Due to the distance-dependent errors, reliable RTK ambiguity resolution is limited to relatively short inter-receiver distances, typically of the order of 10km or so. However, there exists always a strong demand to extend the baseline length, without sacrificing RTK performance. The use of multiple GPS reference stations, or a GPS network, makes it possible. GPS networks have been deployed for many years, providing opportunities to mitigate distance-dependent errors in different ways. A good example is the network of the International GNSS Service (IGS), and its products (cf. http://igscb.jpl.nasa.gov ). To date the coverage of IGS is not dense enough to be sensitive to small-scale errors, and therefore does not meet the requirement of regional or local GPS users. Although the IGS products are