There is a trend for the establishment of regional-scale GPS permanent receiver networks, for a variety of applications including support of high accuracy, carrier phase-based positioning for surveying and precise navigation. When implemented in real-time, GPS users located within the region enclosed by multiple GPS reference stations can precisely position by using, for example, the 'correction terms' generated and transmitted by the reference station network. For such a configuration one of the major challenges is that the integer ambiguities have to be resolved during the real-time processing of the reference network data in order to ensure the generation of the carrier phase corrections, even when the reference receivers are many tens of kilometers apart. Due to the presence of distance dependent errors in the double-differenced data (principally the ionospheric and tropospheric delays) reliable instantaneous (single epoch) ambiguity resolution is difficult in the case of medium-scale reference networks (defined here as where the reference stations are typically in the range 50-100km apart). In practice the ambiguities between the reference stations could be correctly fixed during some sort of 'initialization' procedure. However, the main challenge is to keep on fixing ambiguities instantaneously (or with minimum delay) when a satellite experiences cycle-slips, a long data gap, or when a new satellite rises above the horizon. In this paper, a three-step strategy is proposed for over-coming these challenges which is suitable for implementation in real-time. In the first step the high correlation of the atmospheric delay between adjacent epochs is used to assist the cycle-slip recovery and ambiguity resolution. Then these atmosphere models are used in a predictive manner in the double-differenced observations, on an epoch-by-epoch and satellite-by-satellite basis. Finally, these atmosphere models are applied in a real-time kinematic data processing algorithm to fix the ambiguities during situations when there is a long data gap, or when a new satellite rises between the reference stations. Test data from GPS reference stations spaced 80km apart were used to evaluate the algorithm. The results indicate that the proposed algorithm can provide reliable integer ambiguities, very quickly, in the case of medium-scale reference networks.
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