United States Department of Transportation
i
Reliability of Navigation Service Provided by the Global Positioning System
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1985-09-01
[PDF-3.02 MB]
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Abstract:The planned NAVSTAR/GPS satellite constellation of 18 satellites plus 3 active will provide excellent coverage over the continental United States (CONUS) if all are operating properly. This report examines the coverage under conditions of one satellite failure. It turns out that the failure of any satellite results in service outages of up to half an hour somewhere in the CONUS. While altimeter aiding eliminates most of these outages, still remain 14 outage events each day. Furthermore, 'coasting' the navigation solution with a stable clock is not effective in handling these outages. One technique which does appear to be effective is using a mask angle of 5 degrees or less. At these low angles the signal is down in amplitude due to antenna rolloff, and it is more affected by tropospheric refraction and multipath, but it is better to use even a corrupted signal than to suffer a service outage. Also examined in this report is the impact of adding satelites to the constellation. It is shown that a 24-satellite consellation still provides 100% availability in the CONUS when one satellite fails. Keywords include: Navigation, GPS, Satellites, Avionics, Aviation, Ranging systems, Altimeters, Stable oscillators, and Dilution of precision.
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Content Notes:Introduction, Approach, GPS service reliability under conditions of unaided receiver operation, GPS service reliability under conditions of altimeter aiding of user receiver, Clock quality required for "coasting" during outages, Conclusions and recommendations, Appendices
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