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availability may still be sufficient for approval of operations.
6.2.6 Before publishing procedures based on SBAS signals, a State is expected to provide a status monitoring and
NOTAM system. To determine the effect of a system element failure on service, a mathematical service volume model is to
be used. The State can either obtain the model from the SBAS operator or develop its own model. Using the current and
forecast status data of the basic system elements, and the locations where the State has approved operations, the model would
identify airspace and airports where service outages are expected, and it could be used to originate NOTAMs. The system
element status data (current and forecast) required for the model could be obtained via a bilateral arrangement with the SBAS
service provider, or via connection to a real time “broadcast” of the data if the SBAS service provider chooses to provide data
in this way.
* All figures are located at the end of the attachment.
ATT D-11 23/11/06
Annex 10 — Aeronautical Communications Volume I
6.2.7 Participating States or regions will coordinate through ICAO to ensure that SBAS provides seamless global
coverage, taking into account that aircraft equipped to use the signal could suffer operational restrictions in the event that a
State or region does not approve the use of one or more of the SBAS signals in its airspace. In such an event, the pilot may
have to deselect GNSS altogether since the aircraft equipment may not allow deselection of all SBAS or a particular SBAS.
6.2.8 As the SBAS geostationary orbit satellite coverages (footprints) overlap, there will be interface issues among the
SBASs. As a minimum, the SBAS airborne receivers must be able to operate within the coverage of any SBAS. It is possible
for an SBAS provider to monitor and send integrity and correction data for a geostationary orbit satellite that belongs to
another SBAS service provider. This improves availability by adding ranging sources. This improvement does not require
any interconnection between SBAS systems and should be accomplished by all SBAS service providers.
6.2.9 Other levels of integration can be implemented using a unique connection between the SBAS networks
(e.g. separate satellite communication). In this case, SBASs can exchange either raw satellite measurements from one or more
reference stations or processed data (corrections or integrity data) from their master stations. This information can be used to
improve system robustness and accuracy through data averaging, or integrity through a cross check mechanism. Availability
will also be improved within the service areas, and the technical performance will meet the GNSS SARPs throughout the
entire coverage (i.e. monitoring of satellites ephemeris would be improved). Finally, SBAS control and status data could be
exchanged to improve system maintenance.
6.3 Integrity
6.3.1 The provisions for integrity are complex, as some attributes are determined within the SBAS ground network and
transmitted in the signal-in-space, while other attributes are determined within the SBAS equipment on the aircraft. For the
satellite status and basic corrections functions, an error uncertainty for the ephemeris and clock corrections is determined by
the SBAS ground network. This uncertainty is modelled by the variance of a zero-mean, normal distribution that describes
the user differential range error (UDRE) for each ranging source after application of fast and long-term corrections and
excluding atmospheric effects and receiver errors.
6.3.2 For the precise differential function, an error uncertainty for the ionospheric correction is determined. This
uncertainty is modelled by the variance of a zero-mean, normal distribution that describes the L1 residual user ionospheric
range error (UIRE) for each ranging source after application of ionospheric corrections. This variance is determined from an
ionospheric model using the broadcast grid ionospheric vertical error (GIVE).
6.3.3 There is a finite probability that an SBAS receiver would not receive an SBAS message. In order to continue
navigation in that case, the SBAS broadcasts degradation parameters in the signal-in-space. These parameters are used in a
number of mathematical models that characterize the additional residual error from both basic and precise differential
corrections induced by using old but active data. These models are used to modify the UDRE variance and the UIRE variance
as appropriate.
6.3.4 The individual error uncertainties described above are used by the receiver to compute an error model of the
navigation solution. This is done by projecting the pseudo-range error models to the position domain. The horizontal
protection level (HPL) provides a bound on the horizontal position error with a probability derived from the integrity
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