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worst-case geometry as 6 × gi/HDOP. Ninety-five per cent of the scaled errors must be less than 220 m for the system to
comply with the non-precision accuracy requirement under worst-case geometry conditions. The total number of samples
collected must be sufficient for the result to be statistically representative, taking into account the decorrelation time of the
errors.
3.2.7 A range of vertical accuracy values is specified for Category I precision approach operations which bounds the
different values that may support an equivalent operation to ILS. A number of values have been derived by different groups,
using different interpretations of the ILS standards. The lowest value from these derivations was adopted as a conservative
value for GNSS; this is the minimum value given for the range. Because this value is conservative, and because GNSS error
characteristics are different from ILS, it may be possible to achieve Category I operations using larger values of accuracy and
alert limits within the range. The larger values would result in increased availability for the operation. The maximum value in
the range has been proposed as a suitable value, subject to validation.
23/11/06 ATT D-2
Attachment D Annex 10 — Aeronautical Communications
ATT D-3 23/11/06
3.2.8 Specific alert limits have been defined for each augmentation system. For GBAS, technical provision has been
made to broadcast the alert limit to aircraft. GBAS standards require the alert limit of 10 m. For SBAS, technical provisions
have been made to standardize the alert limit through an updateable database (see Minimum Operational Performance
Standards for Global Positioning System/Wide Area Augmentation System (GPS/WAAS) Airborne Equipment (RTCA/DO-
229C)).
3.2.9 The GPS SPS position error (Chapter 3, 3.7.3.1.1.1) accounts for the contribution of the space and control
segment to position errors (satellite clock and ephemeris errors) only; it does not include the contributions of ionospheric and
tropospheric delay model errors, errors due to multipath effects, and receiver measurement noise errors (Attachment D, 4.1.2).
These errors are addressed in the receiver standards. The user positioning error at the output of ABAS-capable equipment is
mainly driven by the GNSS receiver used.
3.2.9.1 For Basic GNSS receivers, the receiver qualification standards require demonstration of user positioning
accuracy in the presence of interference and a model of selective availability (SA) to be less than 100 m (95 per cent of time)
horizontally and 156 m (95 per cent of time) vertically. The receiver standards do not require that a Basic GNSS receiver
applies the ionospheric correction described in Appendix B, 3.1.2.4.
Note.— The term “Basic GNSS receiver” designates the GNSS avionics that at least meet the requirements for a GPS
receiver as outlined in Annex 10, Volume I and the specifications of RTCA/DO-208 as amended by United States Federal
Aviation Administration (FAA) TSO-C129A, or EUROCAE ED-72A (or equivalent).
3.2.9.2 Since the discontinuation of SA, the representative user positioning accuracy of GPS has been conservatively
estimated to be as shown in Table D-0. The numbers provided assume that the worst two satellites of a nominal 24 GPS
satellite constellation are out of service. In addition, a 7 m (1 σ) ionospheric delay model error, a 0.25 m (1 σ) residual
tropospheric delay error, and a 0.80 m (1 σ) receiver noise error are assumed. After discontinuation of SA (Attachment D, 1.),
the dominant pseudo-range error for users of the GPS Standard Positioning Service is the ionospheric error that remains after
application of the ionospheric corrections. This error is also highly variable and depends on conditions such as user
geomagnetic latitude, level of solar activity (i.e. point of the solar cycle that applies), level of ionospheric activity (i.e.
whether there is a magnetic storm, or not), elevation angle of the pseudo-range measurement, season of the year, and time of
day. The ionospheric delay model error assumption reflected in Table D-0 is generally conservative; however, conditions can
be found under which the assumed 7 m (1 σ) error during solar maximum would be inadequate.
Table D-0. GPS user positioning accuracy
GPS user positioning accuracy
95% of time, global average
Horizontal position error 33 m (108 ft)
Vertical position error 73 m (240 ft)
3.2.10 SBAS and GBAS receivers will be more accurate, and their accuracy will be characterized in real time by the
receiver using standard error models, as described in Chapter 3, 3.5, for SBAS and Chapter 3, 3.6, for GBAS.
Note 1.— The term “SBAS receiver” designates the GNSS avionics that at least meet the requirements for an SBAS
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