Improving Real Time Positioning Efficiency Using the Singapore Integrated Multiple Reference Station Network (SIMRSN)

Abstract

The Singapore Integrated Multiple Reference Station Network (SIMRSN) is designed to support concurrent high precision GPS positioning for both real-time and post-processing applications. It provides the framework to generate 'correction terms' that account for the ionospheric and tropospheric delay, which are computed in real-time from data collected by the GPS reference stations. These correction terms can be applied by users to reduce systematic biases in the double-differenced GPS phase observations. Hence the efficiency of ambiguity resolution (and precise real-time positioning in general) will be improved within the coverage of the network. In order to generate the correction terms, dual-frequency ambiguities must first be fixed to their integer values. However, even with precisely known coordinates, it is not easy to fix ambiguities between GPS reference stations in real-time, especially for newly risen satellites. A new method is proposed for this purpose. The methodology is as follows: the wide-lane ambiguities are first resolved, then the L1 ambiguities (with an effective wavelength of 10.7cm) and the relative tropospheric zenith delay are estimated using the ionosphere-free observable via a Kalman filter. When the estimated (float) L1 ambiguity meets certain criteria, this ambiguity will then be fixed. After fixing the ambiguities between the reference stations, an epoch-by-epoch, satellite-by-satellite, 2D linear correction term model for the double-differences is used. This option greatly reduces the number of parameters that need to be broadcast, and hence potentially improves the real-time performance of the system. The proposed methodology has been successfully used in the case of the SIMRSN. Test results indicate that the proposed method is robust, the double-differenced residuals at the user GPS are greatly reduced and the efficiency of single-epoch ambiguity resolution also improved.