ABSTRACT
Real-time high precision GPS surveying and navigation applications have been constrained to 'short range' due to the presence of distance-dependent errors in the between receiver single-differenced observables. Over the past few years, the use of a GPS reference station network, to extend the inter-receiver distances (user-to-reference station), has attracted great interest. This network-based approach can be extended to include GPS/GLONASS receivers. In order to model the distance-dependent errors such as the ionospheric and tropospheric biases, the ambiguities in the GPS/GLONASS reference station network should first be fixed to their correct integer values. However, even with precisely known station coordinates, it is still a challenge to fix the ambiguities in reference station networks, especially when a new satellite rises above the horizon. In this paper two procedures for ambiguity resolution, suitable for real-time implementations, in GPS/GLONASS reference station networks are suggested.
The first procedure is single-epoch ambiguity resolution after an ambiguity is initialized. As the distance-dependent errors (atmosphere errors and orbit errors) exhibit a high degree of temporal correlation for short time spans, the double-differenced residuals can be represented as a linear function of time for short periods of up to a few minutes. On an epoch-by-epoch and satellite-by-satellite basis these systematic errors (or biases) can be estimated using previous measurements with fixed ambiguities, and precisely predicted for use in ambiguity resolution during the following measurement epochs. The second procedure is suitable for a newly risen satellite, or after a long data gap. Atmospheric biases also exhibit strong spatial correlations between satellite pairs. The atmospheric delay information derived from other satellites, with fixed ambiguities, can be used in predicting the atmospheric bias for a newly risen satellite, and for those satellites that have unknown ambiguities associated with them. A test data set from a GPS/GLONASS reference station network was used to evaluate the performance of these procedures. The experimental results show that the proposed procedures can reliably and efficiently resolve the integer ambiguities of reference station networks, in real-time, on a single-epoch basis.