In certain constrained environments, GPS and EGNOS signals may be difficult to acquire. For example, when a vehicle is driving along a road hemmed in by rows of tall buildings, the vehicle’s onboard receiver may have difficulty picking up the satellite signals. Positioning in these environments, which are known as ‘urban canyons’, is poor. The use of EDAS compensates for this EGNOS reception problem. This kind of environment also generates a lot of multipath errors, whose effects can only be dealt with using receiver-level techniques. Receiver Autonomous Integrity Monitoring (RAIM) is used to identify and exclude erroneous measurements, and multipath rejection algorithms.
The system consists of a GPS receiver that communicates with equipment running the EGNOS/EDAS correction software (GPS/EDAS correction module as shown in the diagram below). The equipment consists of an interface that is compatible with the GPS receiver, an Internet connection and an interface for sending the corrected positions to the application.
Figure: Architecture of the connection to EDAS
Typically, this architecture can be provided by a smartphone with a Bluetooth connection to communicate with the receiver, and an Internet connection over GPRS. The correction module can be integrated directly in the smartphone.
For this application, the EGNOS corrections broadcast via EDAS are used, as well as the ephemeris data and parameters of the Klobuchar ionospheric correction model also broadcast by EDAS.
For this application, it is important for the GPS receiver not to use EGNOS in native mode as the corrections are applied outside the receiver. The receiver must be able to send the time and position (longitude, latitude, altitude), as well as the list of satellites used to calculate the PVT solution. Most commercially available GPS receivers can do this. With regard to internet connectivity, because the (compressed) EDAS messages are compact, the mean bandwidth needed is around 500 bits/s. Therefore, an Internet connection via GSM or GPRS is amply sufficient.
This section details the implementation of the GPS/EDAS correction module. A full description of the algorithms is given in the DO-229D document. In addition, ESA has placed all the publications describing the use of EDAS on its website at http://www.gsa.europa.eu/egnos/edas
The GPS/EDAS correction module must carry out the following operations:
- Open the connection to the receiver and configure it so that it does not use EGNOS (or SBAS in general).
- Open a connection to the EDAS server. The server uses a standard TCP/IP connection and the information exchanged is in plain text.
Once the receiver has a valid GPS position, the module must carry out the following operations
- Decode the GPS position and the list of satellites used from the NMEA datastream,
- Transform this position into pseudoranges (for each satellite). This entails the reverse process to the one used by the receiver to calculate a position. This is possible because the EDAS server makes available the satellite ephemeris data and Klobuchar parameters (through the GPS message data) so as to ignore the rough ionospheric corrections produced by the GPS receiver.
- Retrieve the EGNOS message, decompress it and apply the corrections to each pseudorange.
- Recalculate a positioning solution using the corrected pseudoranges and the ephemeris data.
- Reformat the result in the NMEA standard and, transfer it to the application using the integrity service.
More sample applications
Precision farming with EGNOS
Creating a time distribution system
Using EGNOS through the EDAS service
Using EGNOS's integrity mechanisms