GPS Simultaneous Uploads and GPS Constellation Fault Probability Determination

Norbert Suard

Abstract:	As defined in the Milestone III report of the ARAIM EU-US technical subgroup [EU-US ATSG 2016], the constellation fault probability is defined as: “The probability that, at any given time and due to a common cause, any subset of two or more satellites within constellation j is in a fault state is no greater than Pconst.” Faults involving several satellites on a common cause are also known as wide faults (WF). One example is blundered navigation data broadcast by multiple satellites, with a common cause originating at the Constellation Service Provider (CSP) ground segment. Simultaneous uploads in a constellation constitute therefore a direct way to propagate such common satellite faults to user algorithms. On another hand, in [Kovach 2015], GPS Pconst is assumed to be close to 1 e-7/hour and could be decreased to less than 1 e-8/hour if an additional check is inserted while for RAIM or HARAIM, this probability is considered to be null (assertion 1 of [EU-US ATSG 2016]). Based on such considerations, several analyses were performed at CNES to observe in details how the GPS navigation data are broadcast after upload by each GPS satellite and in particular if the first broadcast after uploads occurs at the same time on different satellites. These observations and analyses have put in evidence the existence of simultaneous uploads in GPS with at least one event par day. Consequently, if the fault probability of the ground segment is known or if it is observed at least one event where navigation data were faulty computed on ground and broadcasted for at one satellite, it is then possible to derive a minimum Pconst for the GPS constellation. Such event may have occurred in 2012 with PRN 19 ephemeris update on June 17 [WAAS Team 2012]. Based on this deduction and considering that these two types of events are independent, the GPS Pconst derivation is therefore of (1/24)*(1/(25*365*24)) for an earth ground user, which is approximately equal to 2 e-7/hour. This paper will present the different steps of these analyses and the obtained results.
Published in:	Proceedings of the 30th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2017) September 25 - 29, 2017 Oregon Convention Center Portland, Oregon
Pages:	1019 - 1026
Cite this article:	Suard, Norbert, "GPS Simultaneous Uploads and GPS Constellation Fault Probability Determination," Proceedings of the 30th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2017), Portland, Oregon, September 2017, pp. 1019-1026. https://doi.org/10.33012/2017.15173
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