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GPS Precise Point Positioning : Some Recent Developments

GPS surveying has traditionally been carried out in the differential (relative) positioning mode. This is mainly due to the higher positioning accuracy obtained with differential positioning in comparison with GPS point positioning. A major disadvantage of GPS differential positioning, however, is its dependency on the measurements or corrections from a reference receiver or network; i.e. two or more GPS receivers are required. New developments in GPS positioning show that a user with a standalone GPS receiver can obtain positioning accuracy comparable to that of differential positioning. Such technique is known as precise point positioning (PPP). A major drawback of PPP, however, is that about 30 minutes or more is currently required to achieve centimetreto decimetrelevel accuracy. This relatively long convergence time results from remaining un-modelled GPS residual errors. A major residual error component, which affects the convergence of PPP solution, is higher-order ionospheric delay. In this paper, we rigorously model the second-order ionospheric delay, which represents the bulk of higher-order ionospheric delay, for PPP applications. Firstly, raw GPS measurements from a global cluster of international GNSS service (IGS) stations are corrected for the effect of second-order ionospheric delay. The corrected data sets are then used as input to the Bernese GPS software to estimate the precise orbit and satellite clock corrections. It is shown that the effect of second-order ionospheric delay on GPS satellite orbit ranges from 1.5 to 24.7 mm in radial, 2.7 to 18.6 mm in along-track, and 3.2 to 15.9 mm in cross-track directions, respectively. GPS satellite clock corrections, on the other hand, showed a difference of up to 0.067 ns. To examine the effect of the secondorder ionospheric delay on the PPP solution, new data sets from several IGS stations were processed using a modified version of GPSPace software, which accepts second-order ionospheric corrections. It is shown that accounting for the second-order ionospheric delay improved the PPP solution convergence time by about 15% and improved the accuracy estimation by 3 mm.