Imagine that a ship wishes to enter a port in conditions of reduced visibility. The navigator’s problem is how to ascertain that his position is accurate and reliable. The GPS system alone cannot ensure the reliability of his position. Where there is no local DGPS system, the combined use of GPS and EGNOS offers the navigator significantly increased accuracy and enables him to define a level of confidence corresponding to his position.
Figure: Example of a port environment
The level of confidence, known as the HPL, is a circle centred on the ship’s current position that assesses the risk to the ship to be 107 for every 150 seconds that it remains inside the circle. EGNOS has been designed to guarantee the position within a maximum radius of 40m (horizontally) 99% of the time. In the event of an anomaly in the GPS constellation, EGNOS warns the navigator within six seconds that his position can no longer be guaranteed. This system enables the navigator to enter the port in full safety without risking a collision with the harbour walls.
EGNOS helps to improve accuracy by correcting the measurements in the GPS signals and in particular by providing the integrity service.
Figure: Functional architecture of the terminal
All the messages distributed by EGNOS are used in this application, particularly the sigma (or s symbol) udre and sigma (or s symbol) give parameters.
In this application it is considered that the receiver does not calculate the radius of protection by itself, this is done by the navigation computer. In this case, the receiver must be able to provide all the raw GPS data (pseudoranges, navigation messages) and all the EGNOS messages.
The receiver only provides the computer with the raw GPS and EGNOS data. It is not designed to provide a position. The stages involved in implementing this system are as follows:
- Correct the pseudoranges for each GPS satellite using the EGNOS messages and exclude the pseudoranges for satellites declared by EGNOS as ‘Do Not Use’ and ‘Not Monitored’;
- Calculate a PVT solution using these corrected pseudoranges;
- Calculate in parallel the HPL;
- Display for the user the position, HPL and, if necessary, any integrity alarms.
More sample applications
Precision farming with EGNOS
Creating a time distribution system
Using EGNOS through the EDAS service
Using EGNOS's integrity mechanisms