Abstract: Most GPS (gobal position system) users who require higher precision now operate differentially with respect to a known reference station to eliminate the effects of SA (selective availability) and significantly reduce common station errors. As the distance between a roving receiver and its reference increases, the commonality of errors is reduced and applying range corrections from a single reference spiotation may not provide optimal results for point positioning. Daily, the GSD (geodetic survey division), NRCan(natural resources Canada) generates precise GNSS (global navigation satellite system) satellite orbits and clocks in a standard format which are contributed to the IGS (international GPS service). Higher frequency satellite clocks are subsequently computed from the CACS (Canadian active control system). New developments in GNSS positioning show that a user with a single GNSS receiver can obtain positioning accuracy comparable to that of differential positioning (i.e., centimeter to decimeter accuracy). In this study, the GPS data for a control point CFG113B from two measurement epochs were submitted to CSRS (Canadian spatial reference system) for post processing. Sources of errors and the model used in the solution are discussed. The earth and space based models for attaining the derived accuracies is highlighted. The results of the adjustment using PPP (precise point positioning) model were compared with existing GPS derived coordinates of the same station. The results revealed a shift of 7 m in horizontal direction for the two stations. An analysis of the accuracy of the adjusted unknown shows a standard deviation of 17 mm in horizontal position and 32 mm in elevation. This shows that PPP can give mm level results with a longer observation period and using precise orbit and clock corrections.
　　Key words: GNSS (global navigation satellite system), PPP (precise point positioning) model, international GPS station, ITRF(international terrestrial reference frame).
　　1. Introduction
　　Precise point positioning is a technique of GNSS(global navigation satellite system) measurement in which absolute position of points is determined by eliminating systematic errors through precise modelling of many of the error sources such as ionospheric delay, tropopheric as well as receiver antenna. In this method, the coordinates of a site are determined with respect to the orbits of the satellites.
　　The highest accuracy is achieved using IGS(international GPS service) final products where the accuracy of the orbit class improves with time [1]. Highly accurate results are only possible in post processing modes [2]. As a result of tropospheric delay models parameters, measurement time should not be less than 30 minutes per station in order to achieve PPP accuracy.
　　2. On-line Processing of Ikpoba Dam Geodetic Control Data
　　Presented herein is the implementation of precise point positioning in which an observation control data for CFG113B carried out in Benin city, Nigeria were submitted to Canadian spatial reference system online global GPS processing service CSRS-PPP via internet for post processing. The CSRS-PPP (Canadian spatial reference system-precise point positioning) service provides post processed position estimates over the internet of GPS data files submitted by the user.
　　Precise position estimates are refered to the CSRS standard (NAD 83) as well as ITRF. Single station position estimates are computed for users operating in static or Kinematic modes using precise GNSS orbits and clock corrections.
　　The online PPP service is designed to minimize user interaction while positioning the best possible solution for the given observation availability. Currently, users need only to specify the mode of processing static or Kinematic and the reference frame for position output.
　　The data submitted were for a control point CFG113B measurement which served control point for dam deformation monitoring. The purpose was to determine the absolute coordinates of the control point which can then be used for estimating absolute movement of the dam structure between epoch.
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