Global Positioning
    System
 The System
 The Position Solution
 Hardware
 Ionospheric And
    Atmospheric Delays
 The Signals
 Dynamic Positioning
 Static Positioning
 Coordinate Systems
 Conclusions

Static Positioning

The Global Positioning System has revolutionised the geomatics industry, it is now not only possible to perform traditional surveying tasks in radically less time but there is a vastly increased scope of tasks to be performed. Recent examples in the Department of Geomatics include the use of GPS to develop a mapping system that can coordinate rail tracks at 60kmh^-1, developing computional procedures to include GPS positioning in aero-triangulation, using GPS to coordinate facility mapping systems, incorporating high precision GPS measurements into dam movement surveys, and the much more routine use of GPS to coordinate sea floor mapping. Where once we would use laser or radio distance measuring machines for determining coordinates of control points we now use two GPS units.

The highest level of accuracy obtainable with GPS requires use of two units, one as a base station and the other visiting the points of interest. It is then possible to compute the vector between these two units to a much higher degree of accuracy than we can compute absolute latitude and longitude. Practically all the systematic errors that can occur in GPS positioning can be eliminated if we measure to a set of satellites simultaneously from two receivers, a process known as differential positioning.

Differential Positioning

The typical configuration for differential positioning is shown below, two units receiving signals from the same constellation of satellites at the same time. The relative position of the two units can be determined to a very high accuracy, in many cases better than a centimetre. If one of these units was located over a point for which we had ground control coordinates it is then theoretically possible to obtain highly accurate coordinates for the other point. This is indeed the case, providing all the computations are performed in the one coordinate system.

This technique can be also applied if one of the receivers is on a moving platform, or is moved between points of interest while the base station remains fixed and continues to observe to the same satellites. This produces new procedures known as rapid-static positioning, kinematic positioning and pseudo-kinematic positioning. In general the units store the observations to the satellites and are down-loaded to PC type computers at the end of the project. The differential solution is then computed using the complete set of data from all the receivers. The latest hardware systems can transmit the corrections between the base station and the rover allowing the solution to be determined on-the-fly so that positioning accuracies of around 0.01m are available in real-time.