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necessarily transmitting, a usable ranging signal. At least 21 satellites in the 24 nominal plane/slot positions must be set
healthy and must be transmitting a navigation signal with 0.98 probability (yearly averaged).
4.1.5 Reliability. Reliability is the percentage of time over a specified time interval that the instantaneous SPS SIS
URE is maintained within the range error limit, at any given point within the coverage area, for all healthy GPS satellites.
The reliability standard is based on a measurement interval of one year and the average of daily values within the coverage
area. The single point average reliability assumes that the total service failure time of 18 hours will be over that particular
point (3 failures each lasting 6 hours).
4.1.6 Major service failure. A major service failure is defined to be a condition over a time interval during which a
healthy GPS satellite’s ranging signal error (excluding atmospheric and receiver errors) exceeds the range error limit. As
defined in Chapter 3, 3.7.3.1.1.3 a), the range error limit is the larger of:
a) 30 m; or
b) 4.42 times the URA, not to exceed 150 m.
ATT D-7 23/11/06
Annex 10 — Aeronautical Communications Volume I
4.1.7 Coverage. The SPS supports the terrestrial coverage area, which is from the surface of the earth up to an altitude
of 3 000 km.
4.2 GLONASS
Note.— Additional information concerning GLONASS can be found in the GLONASS Interface Control Document
published by Scientific Coordination Information Center, Russian Federation Ministry of Defence, Moscow.
4.2.1 Satellite selection. The performance standard definitions are based upon the assumption that the channel of
standard accuracy (CSA) receiver will select satellites based on the minimum PDOP every 5 minutes, or whenever a satellite
used in the position solution sets below the mask angle.
4.2.2 Accuracy. Accuracy is conditioned by coverage, availability and reliability standards, and a measurement interval
of 24 hours for any point on the earth.
4.2.3 Time transfer accuracy. Time transfer accuracy is conditioned by coverage, availability and reliability standards
and a measurement interval of 24 hours for any point on the earth. It is based upon the GLONASS CSA receiver time as
computed using the output of the position solution. Time transfer accuracy is defined with respect to Universal Coordinated
Time as maintained by the National Time Service of Russia (UTC(SU)).
4.2.4 Availability. Availability is conditioned by coverage and a typical 24-hour interval (defined using an averaging
period of 60 days) averaged over the earth.
4.2.5 Reliability. Reliability is conditioned by coverage, service availability, 200 m not to exceed predictable horizontal
reliability threshold, a maximum of 18 hours of major service failure behaviour over the sample interval, and a measurement
interval of one year, average of daily values over the earth.
4.2.5.1 Major service failure. A major service failure is defined as a departure from the normal ranging signal
characteristics in a manner that can cause a reliability or availability service failure. Specifically, a major service failure is
defined as a departure from the normal ranging signal characteristics in one of the following ways:
a) a statistical departure from nominal system ranging accuracy that causes the CSA instantaneous ranging error to
exceed 70 m; or
b) a fault in a CSA ranging signal, navigation message structure or navigation message content that impacts the CSA
receiver’s minimum ranging signal reception or processing capabilities.
4.2.6 Coverage. Coverage is conditioned by the probability of four or more satellites in view over any 24-hour interval
averaged over the earth, a PDOP of 6 or less from four satellites, a 5-degree mask angle, and a constellation of twenty-four
operational satellites as defined in the almanac.
4.2.7 GLONASS time. GLONASS time is generated based on GLONASS Central Synchronizer time. Daily instability
of the Central Synchronizer hydrogen clock is not worse than 5 × 10–14. The difference between GLONASS time and
UTC(SU) is within 1 millisecond. The navigation message contains the requisite data to relate GLONASS time to UTC(SU)
within 0.7 microsecond.
4.2.7.1 Transformation of GLONASS-M current data information into common form. A satellite navigation message
contains current data information in NT parameter. It could be transformed into the common form by the following algorithm:
a) Current year number J in the four-year interval is calculated:
23/11/06 ATT D-8
Attachment D Annex 10 — Aeronautical Communications
If 1 ≤ NT ≤ 366; J = 1;
If 367 ≤ NT ≤ 731; J = 2;
If 732 ≤ NT ≤ 1096; J = 3;
If 1097 ≤ NT ≤ 1461; J = 4.
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