As mentioned before, the positional accuracy of GPWS currently attainable is approximately 15m. At present, this extent of accuracy can only be enhanced to 5m or less by using Differential GPS (DGPS). In this system, an accurately measured reference station calculates its position by means of GPS and then generates correction data on the basis of the calculated and the known position. This data is sent, e.g. by radio, to the users in the vicinity of the reference station. A significant issue in DGPS is the distance between the reference station and the user: the greater the distance, the greater the positional error.
There are three basic variants when transmitting correction data:

•    Transmission of the positional error
In this variant, the difference between known and computed position is transmitted to the user. An advantage of this method is the small amount of data to be transmitted. Its drawback consists of the fact that the calculated correction data is only valid for the respective constellation of satellites used by the reference station for determining its position.

•    Transmission of corrected slant ranges
Here, correction data for slant ranges towards individual satellites is determined and transmitted to the user. This variant involves the disadvantage of an increased data rate. Moreover, the reference station must be equipped with a highly accurate clock.

•    Transmission of all raw data
In this method, all raw data (propagation delay, phase, Doppler shift) measured by the reference station is transmitted to and processed by the user, which yields the highest accuracy results. However, the amount of data to be transmitted is very big.

If, like in surveying, accuracies in the millimeter range are to be attained, this is possible so far only by using DGPS. Local tropospheric effects have to be compensated by providing appropriate correction data. In this context, the satellite positioning service (SAPOS) of the German State Survey is to be mentioned. This service is based on a network of permanently operated GPS reference stations.

In vehicle navigation it is possible to use additional odometry data (velocity, acceleration) to make the positioning stable. Thereby the loss of GPS signals can be temporarily compensate (e.g. tunnel). For the time being, this requires in-built navigation systems.

Source: Skript Navigation I (Beyer/Wigger) TU Darmstadt