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form an Augmentation System. Ground-based differential stations with a datalink may be used at
specific locations to augment the system locally. The aircraft will select which elements of the
system are to be used depending on the operations to be undertaken.45
Europe’s primary contribution to GNSS-1 will involve signal relay transponders carried on
geostationary satellites, and a network of ground stations. Together they are to provide a regional
augmentation service for GPS and GLONASS signals over Europe and cover all the countries
belonging to the European region. Two Inmarsat-3 satellites will be used, one stationed over the
eastern part of the Atlantic Ocean and the other over the Indian Ocean. A third satellite, Artemis,
designed by ESA, will be launched in early 2000 and stationed over Africa. This augmentation is
called an “overlay”, and the European programme is known as the European Geostationary
Navigation Overlay Service, or EGNOS.
A monitoring and interpretation service should be established to monitor the status of all
elements of the GNSS and interpret this information in a manner that provides relevant
information to pilots and ATC. The information disseminated from the monitoring service or
displayed at controller positions must be expressed in operational terms.
ATC procedures must be established for the use of GNSS and must cover the failure or
degradation of the system. When ATC is informed of a change in the status of the GNSS by the
monitoring service or by display equipment, specific procedures associated with that change
must be implemented. Should it not be possible to achieve the RNP in an airspace, an
alternative RNP should be declared.
4. Area Augmentation Systems
4.1 LAAS/GBAS
Local Area Augmentation System is the ICAO definition ground based augmentation for Satellite
Navigation. Ground Based Augmentation System is the European application of LAAS. The time
schedule foresees that it could be operational by 2003 including CAT 1 approaches.
4.2 SBAS
SBAS is a generic term for GPS and GLONASS augmentations such as WAAS, EGNOS and
MSAS, which use geostationary satellites to broadcast information to users over a large
geographical service.
Satellite Based Augmentation System. NATS and other European air traffic service providers
(AENA, DFS, ENAV, ANA, Swisscontrol and the Defence Nuclear Agency) have formed the
EGNOS Operators and Infrastructure Group (EOIG). SBAS uses the transmission of a GPS
look-alike signal from the SBAS geostationary satellite to further augment the GPS system
performance. The first full SBAS flight trial was planned for June 1999. EOIG is validating for
ICAO the SARPS that define SBAS signal in space performance. SARPS are urgently required
to define the SBAS navigation service in adequate detail to prove full interoperability between
signals from separately developed systems.
4.2.1 WAAS6
There have been a number of proposed systems that could be used to improve the
4 IFATCA technical manual section 13 / chapter 1
5 IFATCA technical manual section 13 / chapter 1
6 IFATCA technical manual Section 8 / Chapter 3
Document prepared by EVP Europe, August 1999 Page 6 of 14
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.
WAAS consists of two basic elements. The first is a network of differential ground-stations that
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.
This situation highlights the importance of the GNSS receiver in the aircraft being able to detect
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