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Note.— GPS SA was discontinued at midnight on 1 May 2000.
2. General
Standards and Recommended Practices for GNSS contain provisions for the elements identified in Chapter 3, 3.7.2.2.
Note.— Except where specifically annotated, GBAS guidance material applies to GRAS.
3. Navigation system performance requirements
3.1 Introduction
3.1.1 Navigation system performance requirements are defined in the Manual on Required Navigation Performance
(RNP) (Doc 9613) for a single aircraft and for the total system which includes the signal-in-space, the airborne equipment
and the ability of the aircraft to fly the desired trajectory. These total system requirements were used as a starting point to
derive GNSS signal-in-space performance requirements. In the case of GNSS, degraded configurations which may affect
multiple aircraft are to be considered. Therefore, certain signal-in-space performance requirements are more stringent to take
into account multiple aircraft use of the system.
3.1.2 Two types of approach and landing operations with vertical guidance (APV), APV-I and APV-II, use vertical
guidance relative to a glide path, but the facility or navigation system may not satisfy all of the requirements associated with
precision approach. These operations combine the lateral performance equal to that of an ILS Category I localizer with
different levels of vertical guidance. Both APV-I and APV-II provide access benefits relative to a non-precision approach,
ANNEX 10 — VOLUME I ATT D-1 23/11/06
Annex 10 — Aeronautical Communications Volume I
and the service that is provided depends on the operational requirements and the SBAS infrastructure. APV-I and APV-II
exceed the requirements (lateral and vertical) for current RNAV approaches using barometric altimetry, and the relevant onboard
equipment will therefore be suitable for the conduct of barometric VNAV APV and RNAV non-precision approaches.
3.2 Accuracy
3.2.1 GNSS position error is the difference between the estimated position and the actual position. For an estimated
position at a specific location, the probability should be at least 95 per cent that the position error is within the accuracy
requirement.
3.2.2 Stationary, ground-based systems such as VOR and ILS have relatively repeatable error characteristics, so that
performance can be measured for a short period of time (e.g. during flight inspection) and it is assumed that the system
accuracy does not change after the test. However, GNSS errors change over time. The orbiting of satellites and the error
characteristics of GNSS result in position errors that can change over a period of hours. In addition, the accuracy itself (the
error bound with 95 per cent probability) changes due to different satellite geometries. Since it is not possible to continually
measure system accuracy, the implementation of GNSS demands increased reliance on analysis and characterization of errors.
Assessment based on measurements within a sliding time window is not suitable for GNSS.
3.2.3 The error for many GNSS architectures changes slowly over time, due to filtering in the augmentation systems
and in the user receiver. This results in a small number of independent samples in periods of several minutes. This issue is
very important for precision approach applications, because it implies that there is a 5 per cent probability that the position
error can exceed the required accuracy for an entire approach. However, due to the changing accuracy described in 3.2.2, this
probability is usually much lower.
3.2.4 The 95 per cent accuracy requirement is defined to ensure pilot acceptance, since it represents the errors that will
typically be experienced. The GNSS accuracy requirement is to be met for the worst-case geometry under which the system
is declared to be available. Statistical or probabilistic credit is not taken for the underlying probability of particular ranging
signal geometry.
3.2.5 Therefore, GNSS accuracy is specified as a probability for each and every sample, rather than as a percentage of
samples in a particular measurement interval. For a large set of independent samples, at least 95 per cent of the samples
should be within the accuracy requirements in Chapter 3, Table 3.7.2.4-1. Data is scaled to the worst-case geometry in order
to eliminate the variability in system accuracy that is caused by the geometry of the orbiting satellites.
3.2.6 An example of how this concept can be applied is the use of GPS to support performance required for nonprecision
approach operations. Assume that the system is intended to support non-precision approaches when the horizontal
dilution of precision (HDOP) is less than or equal to 6. To demonstrate this performance, samples should be taken over a
long period of time (e.g. 24 hours). The measured position error g for each sample i is denoted gi. This error is scaled to the
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