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positioning service can be predicated only in the coverage area(s) (where the field strength requirements are satisfied) within
the Dmax range.
7.3.4 As the desired coverage area of a GBAS positioning service may be greater than that which can be provided by a
single GBAS broadcast station, a network of GBAS broadcast stations can be used to provide the coverage. These stations
can broadcast on a single frequency and use different time slots (8 available) in neighbouring stations to avoid interference or
they can broadcast on different frequencies. Figure D-4A details how the use of different time slots will allow a single
frequency to be used without interference subject to guard time considerations noted under Table B-59. For a network based
on different VHF frequencies, guidance material in 7.17 should be considered.
7.4 Data structure
A bit scrambler/descrambler is shown in Figure D-5.
Note.— Additional information on the data structure of the VHF data broadcast is given in RTCA/DO-246B, GNSS
Based Precision Approach Local Area Augmentation System (LAAS) — Signal-in-Space Interface Control Document (ICD).
7.5 Integrity
7.5.1 Different levels of integrity are specified for precision approach operations and operations based on the GBAS
positioning service. The signal-in-space integrity risk for Category I is 2 × 10-7 per approach. GBAS ground subsystems that
are also intended to support other operations through the use of the GBAS positioning service have to also meet the signal-inspace
integrity risk requirement specified for terminal area operations, which is 1 × 10-7/hour (Chapter 3, Table 3.7.2.4-1).
Therefore additional measures are necessary to support these more stringent requirements for positioning service. The signalin-
space integrity risk is allocated between the ground subsystem integrity risk and the protection level integrity risk. The
ground subsystem integrity risk allocation covers failures in the ground subsystem as well as core constellation and SBAS
failures such as signal quality failures and ephemeris failures. The protection level integrity risk allocation covers rare faultfree
performance risks and the case of failures in one of the reference receiver measurements. In both cases the protection level
equations ensure that the effects of the satellite geometry used by the aircraft receiver are taken into account. This is described
in more detail in the following paragraphs.
7.5.2 The GBAS ground subsystem defines a corrected pseudo-range error uncertainty for the error relative to the
GBAS reference point (σpr_gnd) and the errors resulting from vertical (σtropo) and horizontal (σiono) spatial decorrelation. These
23/11/06 ATT D-22
Attachment D Annex 10 — Aeronautical Communications
uncertainties are modelled by the variances of zero-mean, normal distributions which describe these errors for each ranging
source.
7.5.3 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. General methods for
determining that the model variance is adequate to guarantee the protection level integrity risk are described in Section 14.
The lateral protection level (LPL) provides a bound on the lateral position error with a probability derived from the integrity
requirement. Similarly, the vertical protection level (VPL) provides a bound on the vertical position. For Category I precision
approach and APV, if the computed LPL exceeds the lateral alert limit (LAL) or the VPL exceeds the vertical alert limit
(VAL), integrity is not adequate to support the operation. For the positioning service the alert limits are not defined in the
standards, with only the horizontal protection level and ephemeris error position bounds required to be computed and applied.
The alert limits will be determined based on the operation being conducted. The aircraft will apply the computed protection
level and ephemeris bounds by verifying they are smaller than the alert limits. Two protection levels are defined, one to
address the condition when all reference receivers are fault-free (H0 – Normal Measurement Conditions), and one to address
the condition when one of the reference receivers contains failed measurements (H1 – Faulted Measurement Conditions).
Additionally an ephemeris error position bound provides a bound on the position error due to failures in ranging source
ephemeris. For Category I precision approach and APV, a lateral error bound (LEB) and a vertical error bound (VEB) are
defined. For the positioning service a horizontal ephemeris error bound (HEB) is defined.
7.5.4 Ground system contribution to corrected pseudo-range error (σpr_gnd). Error sources that contribute to this error
　
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