35TH ANNUAL CONFERENCE, Tunis, Tunisia, 15-19 April 1996

WP No. 87



There have been a number of proposed systems that could be used to improve the accuracy and integrity of the Global Positioning System (GPS) to enable it to be used for precision approach and landing operations. The USA has decided to develop a system called the Wide Area Augmentation System (WAAS).

The FAA has awarded a contract for the construction of the system. Initial suggestions that it might be available as early as 1997 appear to have been over-optimistic and it is now expected that the system will be fully operational by 2001. This system could be very significant for the future of satellite navigation as it appears that other regions of the world will develop similar, compatible systems that will eventually build up to provide complete world-wide coverage.

The WAAS consists of two basic elements. The first is a network of differential ground stations which receive the GPS signals and calculate differential correction signals. 35 ground stations are required to cover the USA. These differential corrections are then transmitted to the second element of the system, which are WAAS transponders on a number of Inmarsat geostationary communications satellites. The differential signals are then transmitted from the communication satellites to the aircraft. In addition, the communication satellites also transmit integrity information about the performance of the GPS satellites and a signal similar to a GPS satellite. This GPS type signal is used for navigation and gives the appearance of an additional GPS satellite being present.

The system is expected to meet the RNP for Cat 1 precision approaches. It is expected that further augmentations, such as differential ground stations on the airport will be required for Cat II/III operations.

The WAAS has the advantage of providing continuous information on the accuracy and integrity of the GPS system over a wide area and also of increasing the number of satellites available.

There are also some outstanding problems to be resolved. ICAO SARPS need to be developed for the system. Until this has been done, it is not certain that the system will meet the RNP. There are currently not enough Inmarsat satellites to cover the entire USA with 2 satellites in order to provide a backup. The WAAS will initially only be available along the East and West coasts, and a corridor through the central USA will not be covered. The Inmarsat satellite will not provide coverage above or below 70 of latitude and the service will not be available in these areas.

The European Union, European space Agency, and Eurocontrol have formed a group to provide a similar, compatible, service in Europe called the European Geo-stationary Overlay System (EGNOS). This is planned to be operational in 2002. There are also proposals to provide a similar service in the Asia / Pacific region.

Last Update: February 12, 2020  

February 12, 2020   54   Jean-Francois Lepage    1996    

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