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determined through design, analysis and modelling, rather than measurement. The availability model should account for the
ionospheric, tropospheric and receiver error models used by the receiver to verify integrity (e.g. HPL, LPL and VPL
calculations). The availability specified in Chapter 3, 3.7.2.4, applies to the design availability.
Note.— Additional guidance material pertaining to reliability and availability of radio communications and navigation
aids is contained in Attachment F.
4. GNSS core elements
4.1 GPS
Note.— Additional information concerning GPS can be found in the Global Positioning System Standard Positioning
Service — Performance Standard, October 2001, and Interface Control Document (ICD)-GPS-200C.
4.1.1 The performance standard is based upon the assumption that a representative standard positioning service (SPS)
receiver is used. A representative receiver has the following characteristics: designed in accordance with ICD-GPS-200C;
uses a 5-degree masking angle; accomplishes satellite position and geometric range computations in the most current
realization of the World Geodetic System 1984 (WGS-84) Earth-Centred, Earth-Fixed (ECEF) coordinate system; generates
23/11/06 ATT D-6
Attachment D Annex 10 — Aeronautical Communications
a position and time solution from data broadcast by all satellites in view; compensates for dynamic Doppler shift effects on
nominal SPS ranging signal carrier phase and C/A code measurements; excludes GPS unhealthy satellites from the position
solution; uses up-to-date and internally consistent ephemeris and clock data for all satellites it is using in its position solution;
and loses track in the event that a GPS satellite stops transmitting C/A code. The time transfer accuracy applies to a stationary
receiver operating at a surveyed location. A 12-channel receiver will meet performance requirements specified in Chapter 3,
3.7.3.1.1.1 and 3.7.3.1.2. A receiver that is able to track four satellites only (Appendix B, 3.1.3.1.2) will not get the full
accuracy and availability performance.
4.1.2 Accuracy. The accuracy is measured with a representative receiver and a measurement interval of 24 hours for
any point within the coverage area. The positioning and timing accuracy are for the signal-in-space (SIS) only and do not
include such error sources as: ionosphere, troposphere, interference, receiver noise or multipath. The accuracy is derived
based on the worst two of 24 satellites being removed from the constellation and a 6-metre constellation RMS SIS user range
error (URE).
4.1.3 Range domain accuracy. Range domain accuracy is conditioned by the satellite indicating a healthy status and
transmitting C/A code and does not account for satellite failures outside of the normal operating characteristics. Range
domain accuracy limits can be exceeded during satellite failures or anomalies while uploading data to the satellite.
Exceedance of the range error limit constitutes a major service failure as described in 4.1.6. The range rate error limit is the
maximum for any satellite measured over any 3-second interval for any point within the coverage area. The range
acceleration error limit is the maximum for any satellite measured over any 3-second interval for any point within the
coverage area. The root-mean-square range error accuracy is the average of the RMS URE of all satellites over any 24-hour
interval for any point within the coverage area. Under nominal conditions, all satellites are maintained to the same standards,
so it is appropriate for availability modelling purposes to assume that all satellites have a 6-metre RMS SIS URE. The
standards are restricted to range domain errors allocated to space and control segments.
4.1.4 Availability. Availability is the percentage of time over any 24-hour interval that the predicted 95 per cent
positioning error (due to space and control segment errors) is less than its threshold, for any point within the coverage area. It
is based on a 36-metre horizontal 95 per cent threshold; a 77-metre vertical 95 per cent threshold; using a representative
receiver; and operating within the coverage area over any 24-hour interval. The service availability assumes the worst
combination of two satellites out of service.
4.1.4.1 Relationship to augmentation availability. The availability of ABAS, GBAS and SBAS does not directly relate
to the GPS availability defined in Chapter 3, 3.7.3.1.2. States and operators must evaluate the availability of the augmented
system by comparing the augmented performance to the requirements. Availability analysis is based on an assumed satellite
constellation and the probability of having a given number of satellites. Twenty-four operational satellites are available on
orbit with 0.95 probability (averaged over any day), where a satellite is defined to be operational if it is capable of, but is not
　
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