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6. Satellite-based augmentation system (SBAS)
6.1 An SBAS is made up of three distinct elements:
a) the ground infrastructure;
b) the SBAS satellites; and
c) the SBAS airborne receiver.
6.1.1 The ground infrastructure includes the monitoring and processing stations that receive the data from the
navigation satellites and compute integrity, corrections and ranging data which form the SBAS signal-in-space. The SBAS
satellites relay the data relayed from the ground infrastructure to the SBAS airborne receivers that determine position and
time information using core satellite constellation(s) and SBAS satellites. The SBAS airborne receivers acquire the ranging
and correction data and apply these data to determine the integrity and improve the accuracy of the derived position.
6.1.2 The SBAS ground network measures the pseudo-range between the ranging source and an SBAS receiver at the
known locations and provides separate corrections for ranging source ephemeris errors, clock errors and ionospheric errors.
The user applies a tropospheric delay model.
6.1.3 The ranging source ephemeris error and slow moving clock error are the primary bases for the long-term
correction. The ranging source clock error is adjusted for the long-term correction and tropospheric error and is the primary
basis for the fast correction. The ionospheric errors among many ranging sources are combined into vertical ionospheric
errors at predetermined ionospheric grid points. These errors are the primary bases for ionospheric corrections.
23/11/06 ATT D-10
Attachment D Annex 10 — Aeronautical Communications
6.2 SBAS coverage area and service areas
6.2.1 It is important to distinguish between the coverage area and service areas for an SBAS. A coverage area
comprises one or more service areas, each capable of supporting operations based on some or all of the SBAS functions
defined in Chapter 3, 3.7.3.4.2. These functions can be related to the operations that are supported as follows:
a) Ranging: SBAS provides a ranging source for use with other augmentation(s) (ABAS, GBAS or other SBAS);
b) Satellite status and basic differential corrections: SBAS provides en-route, terminal, and non-precision approach
service. Different operations (e.g. RNP types) may be supported in different service areas;
c) Precise differential corrections: SBAS provides APV and precision approach service (i.e. APV-I, APV-II and
precision approach may be supported in different service areas).
6.2.2 Figure D-1* shows the initial coverage areas and approximated initial service areas for three SBASs: the Wide
Area Augmentation System (WAAS), the European Geo-stationary Navigation Overlay Service (EGNOS) and the
Multifunction Transport Satellite (MTSAT) Satellite-based Augmentation System (MSAS).
6.2.3 An SBAS may provide accurate and reliable service outside the defined service area(s). The ranging, satellite
status and basic differential corrections functions are usable throughout the entire coverage area. The performance of these
functions may be technically adequate to support en-route, terminal and non-precision approach operations by providing
monitoring and integrity data for core satellite constellations and/or SBAS satellites. The only potential for integrity to be
compromised is if there is a satellite ephemeris error that cannot be observed by the SBAS ground network while it creates an
unacceptable error outside the service area. For alert limits of 0.3 NM specified for non-precision approach and greater, this
is very unlikely.
6.2.4 Each State is responsible for defining SBAS service areas and approving SBAS-based operations within its
airspace. In some cases, States will field SBAS ground infrastructure linked to an existing SBAS. This would be required to
achieve APV or precision approach performance. In other cases, States may simply approve service areas and SBAS-based
operations using available SBAS signals. In either case, each State is responsible for ensuring that SBAS meets the
requirements of Chapter 3, 3.7.2.4, within its airspace, and that appropriate operational status reporting and NOTAMs are
provided for its airspace.
6.2.5 Before approving SBAS-based operations, a State must determine that the proposed operations are adequately
supported by one or more SBASs. This determination should focus on the practicality of using SBAS signals, taking into
account the relative location of the SBAS ground network. This could involve working with the State(s) or organization(s)
responsible for operating the SBASs. For an airspace located relatively far from an SBAS ground network, the number of
visible satellites for which that SBAS provides status and basic corrections would be reduced. Since SBAS receivers are able
to use data from two SBASs simultaneously, and to use autonomous fault detection and exclusion when necessary,
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