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faulty satellites and discard them from the position calculation. The FAA claim that GPS
receivers have always detected the failure of a GPS satellite and that an undetected failure has
never occurred. The use of WAAS considerably enhances this situation, as it provides a means
to independently detect faulty satellite signals and pass this integrity information back to aircraft
receivers.7 (Sic. a kind of a flag in the instrument to show a system failure).
4.2.2 MSAS
Japan is implementing the Multi Satellite-based Augmentation System (MSAS – Japanese
Definition) that will provide correction to GPS only. The SBAS system planned by Japan.
4.2.3 EGNOS
The space-borne segment of EGNOS will initially be composed of navigation transponders
carried on two satellites owned by the International Maritime Satellite Organisation (Inmarsat).
These are Inmarsat-3 series satellites that are positioned above the Indian Ocean at 64° East,
and over the Atlantic Ocean at 15.5° West. The Inmarsat-3 satellites operate from geostationary
orbits at 36,000km above the Equator. Since their orbit speed matches that of the Earth’s
rotation, the spacecraft appear to be stationary above the same area of Earth at all times.
Inmarsat provides the EGNOS transponder capacity on their two spacecraft under the terms of a
lease contract running for five years, with a possible five-year extension. The Inmarsat-3 satellite
positioned over the Indian Ocean region was launched in April 1996, while the satellite above the
Atlantic Ocean was placed in orbit four months later, in August 1996.
The EGNOS ground network will provide the backbone for three navigation services: ranging,
integrity monitoring and wide-area differential corrections.
The ranging service will enable the EGNOS transponders to broadcast GPS-like navigation
signals. As a result, these satellites become two more sources of space-based navigation data
for users. This is important because neither the GPS or GLONASS systems can guarantee that
the minimum number of six satellites required for safety-critical applications, like aircraft
navigation, is in view at all times and all locations world-wide. Therefore, EGNOS will help fill this
important gap with its navigation signals.
7 IFATCA technical manual Section 13 / Chapter 1
Document prepared by EVP Europe, August 1999 Page 7 of 14
The ranging system initially consists of four reference stations (two for each EGNOS
transponder), one Mission Control Centre and two navigation uplink stations (one per satellite).
The four reference stations perform ranging measurements to determine the precise location of
each Inmarsat-3 satellite. This ranging information is then sent to the Mission Control Centre
where the satellites’ positions are calculated, and synchronisation is performed with respect to
the GPS system. The Mission Control Centre then composes a navigation message for the
EGNOS transponders. These messages are relayed to the navigation uplink stations for each
satellite and transmitted to the spacecraft for re-broadcast as a navigation signal available for
users in the coverage area.
Ranging services should be available as the first step under the multi-step introduction of
EGNOS, starting early 1998.
The second step in the development of EGNOS is the integrity service; through which range
error estimates for each GPS, GLONASS or EGNOS navigation signal are broadcast. The
EGNOS integrity service will enable users to know within 10 seconds (or 6 secs?) whether a
navigation satellite signal is out of tolerance, allowing action to be taken before any critical
situation arises. This is particularly important in applications such as civil aviation landing
approaches, where many lives count on the accuracy of satellite signals. Without this integrity
service notification of abnormal performance or failure of the various satellites could take 215
minutes or more to reach the users.
The third function of EGNOS is known as the wide-area differential service, which broadcasts
correction signals to improve the precision of satellite navigation. With the wide-area differential
service, the satellite navigation precision will dramatically increase to 5 or 10 metres – well
above the approximately 100 metres for the currently available non-encrypted signals from GPS.
For very specific applications, local area augmentations are required. In this case an interface
with EGNOS is foreseen. This also will improve EGNOS coverage in the northern latitudes.
A gradual deployment of EGNOS with adequate redundancies in the number of Control Centres
and other system elements is planned. It began with the start of the ranging service early in 1998
(for tests only). The integrity and wide-area differential services is being introduced gradually
　
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