Dynamic or Kinematic GPS

Dymanic or Kinematic GPS is becoming the default method of navigation for small craft as the complex computations previously necessary are all performed on-board the receiver. They work independently of the weather, 24 hours per day ( but not under trees or water), perform spheroidal calculations and automatically convert between coordinate datums.

Static GPS

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 downloaded 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.