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either replaced the existing navigation aid infrastructure or where no infrastructure previously existed.
3.4.3 Approach and landing
3.4.3.1 For approach and landing operations, continuity of service relates to the capability of the navigation system to
provide a navigation output with the specified accuracy and integrity during the approach, assuming that it was available at
the start of the operation. The occurrence of navigation system alerts, either due to rare fault-free performance or to failures,
constitute continuity failures. In this case, the continuity requirement is stated as a probability for a short exposure time.
3.4.3.2 The continuity requirements for approach and landing operations represent only the allocation of the
requirement between the aircraft receiver and the non-aircraft elements of the system. In this case, no increase in the
requirement is considered necessary to deal with multiple aircraft use of the system. The continuity value is normally related
only to the risk associated with a missed approach and each aircraft can be considered to be independent. However, in some
cases, it may be necessary to increase the continuity values since a system failure has to be correlated between both runways
(e.g. the use of a common system for approaches to closely-spaced parallel runways).
3.5 Availability
3.5.1 The availability of GNSS is characterized by the portion of time the system is to be used for navigation during
which reliable navigation information is presented to the crew, autopilot, or other system managing the flight of the aircraft.
3.5.2 When establishing the availability requirements for GNSS, the desired level of service to be supported should be
considered. If the satellite navigation service is intended to replace an existing en-route navigation aid infrastructure, the
availability of the GNSS should be commensurate with the availability provided by the existing infrastructure. An assessment
of the operational impact of a degradation in service should be conducted.
3.5.3 Where GNSS availability is low, it is still possible to use the satellite navigation service by restricting the
navigation operating times to those periods when it is predicted to be available. This is possible in the case of GNSS since
unavailability due to insufficient satellite geometry is repeatable. Under such restrictions, there remains only a continuity risk
associated with the failure of necessary system components between the time the prediction is made and the time the
operation is conducted.
3.5.4 En-route
3.5.4.1 Specific availability requirements for an area or operation should be based upon:
a) traffic density and complexity;
b) alternate navigation aids;
c) primary/secondary surveillance coverage;
d) air traffic and pilot procedures; and
e) duration of outages.
3.5.4.2 For this reason, the GNSS SARPs specify a range of values for availability requirements. The requirements
support GNSS sole-means operations in airspace with various levels of traffic and complexity. The lower end of the range is
only sufficient for providing sole means of navigation in a low traffic density and complexity airspace.
ATT D-5 23/11/06
Annex 10 — Aeronautical Communications Volume I
3.5.4.3 While augmentations can reduce the dependency of the GNSS on a particular core element, they do not provide
usable service without the core elements. The requirement for the availability of a particular augmentation in an area should account
for potential degradation in the GNSS core elements (i.e. the minimum constellation of core elements (number and diversity of
satellites) that is expected). Operational procedures should be developed in case such a degraded configuration occurs.
3.5.5 Approach
3.5.5.1 Specific requirements for an area should be based upon:
a) traffic density and complexity;
b) procedures for filing and conducting an approach to an alternate airport;
c) navigation system to be used for an alternate airport;
d) air traffic and pilot procedures;
e) duration of outages; and
f) geographic extent of outages.
3.5.5.2 When developing operating procedures for GNSS approach systems, the duration of an outage and its impact
on the alternate airport should be considered. Although GNSS outages can occur which affect many approaches, the approach
service can be restored without any maintenance because of the orbiting of the satellites.
3.5.6 Determining GNSS availability
The availability of GNSS is complicated by the movement of satellites relative to a coverage area under consideration and the
potentially long time needed to restore a satellite in the event of a failure. Accurately measuring the availability would require
many years to allow for a measurement period longer than the MTBF and repair times. The availability of GNSS should be
　
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