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ENRI PAPER 103 May 2003

Reliability of GPS Ranging Signal

Takeyasu Sakai and Kazunobu Koremura

Abstract:
The paper describes reliability of ranging signal broadcast from GPS (Global Positioning System)satellites to users. GPS user equipment computes and fixes its position solution with measuring ranges to satellites. The interruption of ranging signal thus results in degradation of reliablity of GPS navigation systems,which must be sufficiently reliable to navigate the user to the intended destination safely.

We have evaluated the frequency and the duration of interruptions of GPS ranging signal for the global coverage through public notice advisory information as well as continously observed raw data. Both of malfunctions and maintainances of satellites are considered equally.Based on the public advisories “issued for 2.5years MTBF of ranging signal was computed as 3to 4months per satellite.Our analysis of continuous observation data,however, indicates very short-term cracking of ranging signal occurs frequently on Block II satellites once every115hours, which may cause serious impacts on reliability of GPS navigation systems. This result implies the cracking of ranging signal is very popular event so that it dominates, MTBF of ranging signal, although it has never been notified to users.

Computation examples of GPS navigation system reliability based on our results are also shown, assuming navigation requirements corresponding to the precision approach phase of commercial aircraft.According to the examples it should be noted that the cracking has a certain impact on continuity of navigation systems in critical situation, probably raising continuity broken risk 10times as large.

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(The body of this paper is only in Japanese)
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