The normal ure accuracy envelope to avoid a false

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Unformatted text preview: Failure magnitude and behavior. • Distribution of user population around the globe. A-4.2.1 Failure Frequency Estimate The GPS satellite service failure history over the past several years indicates a very low service failure rate. However, when a service failure does occur, it can result in extremely large range and/or range rate errors. This behavior will typically persist until action is taken to remedy the problem. Based upon an historical assessment of Block II satellite and Control Segment failure characteristics, GPS should experience no more than an average of three service failures per year. This failure rate estimate is conservative – expectations are on the order of one per year, based upon projected navigation payload component reliabilities and the assumption that action will be taken to switch redundancy configurations if early indications of an imminent failure are detected. An allocation of three per year allows for a possible increase in service failures as the Block II satellites reach the end of their operational life expectancy. Page A-19 October 2001 GPS SPS Performance Standard A-4.2.2 Failure Duration Estimate The duration of a failure is a function of the following factors: • Control Segment monitor station coverage, • Control Segment monitor station, communications and Master Control Station availability, • Master Control Station failure detection efficiency and timeline, • Timeline for correcting the problem or terminating the failed satellite's service, and • Control Segment ground antenna coverage and availability. The combination of these factors results in a conservative maximum failure response timeline on the order of no more than six hours. In most cases the response to a failure will be much more prompt, but with any complex system such as the Control Segment, allowances must be made for varying system resource status and operational conditions. The nominal failure response time, taking into consideration a favorable combination of the above factors, is on the order of 30-45 minutes. A-4.2.3 Failure Magnitude and Behavior GPS is designed to be fault tolerant -- most potential failures are either caught before they manifest themselves or their effects are compensated for by the system. The only failures to which the system seems susceptible are of three types: • Insidious, long-term (day or more to manifest themselves) performance deviations, • Catastrophic, almost instantaneous failures, or • Short-term transients. Insidious failures do not propagate very quickly -- failures of this type experienced to date have not affected the GPS ability to support SPS accuracy performance standards. Insidious failures are typically due to a problem in the ephemeris state estimation process. Catastrophic failures are due almost exclusively to satellite code and carrier generation hardware failures. These failures, in general, result in very rapid ranging signal error growth -- range errors can grow to sever...
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This document was uploaded on 09/26/2013.

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