6.1.4.8  Failure Effect on Position Solution

Given the nature of catastrophic failures, it must be assumed that the inclusion of the

satellite in the position solution will induce a service reliability failure independent of

the satellite s geometric contribution.  Some receivers will be capable of detecting and

rejecting large instantaneous changes in a range residual which are indicative of a

major service failure.  The minimum  receiver represented in the Signal Specification is

not however, required to have this capability.  For the purposes of service reliability

standard definition, it must be assumed that if the receiver is capable of tracking the

failed satellite and it supports the nominal position solution geometry, the receiver will

use it in the position solution.

6.1.4.9  Expected Service Reliability Characteristics

When the system is performing nominally and the receiver design meets the minimum

usage conditions established in Section 2.2 of the Signal Specification, predictable

horizontal error will never reach the service reliability threshold.  Service reliability on

those days where GPS does not experience a major service failure will be 100%.

The estimated maximum of three major service failures per year, coupled with a

maximum duration of six hours each, yields a maximum of 18 service failure hours per

year.  The worst case site on the globe will be the place where all 18 service failure

hours are observed and the failed satellites are used in the position solution.  For this

worst case condition, the daily average service reliability over a one year period will be

no worse than 99.79%.  The equivalent global daily average will be no worse than

99.97%.

6.1.5  Additional Commentary

(The following commentary is not derived from the GPS SPS Performance Stan dard.)

It should be noted that several criteria used as conditions and constraints in the

performance standards may not be applicable to many user applications.  As examples,

the coverage standard is based upon 24 operational satellites, a four satellite position

solution, a PDOP of 6 or less, and a 5 degree mask angle; the service availability

standard is based on a "normal" operating scenario; and the service reliability standard

is based on the assumption that the user does not perform integrity checking.

6.1.5.1  24 Operational Satellites and Service Availability

The assumption of 24 operational satellites may be optimistic rather than conservative.

In the long term, the GPS constellation will be in a continuous cycle of satellite end of 

life failures and corresponding launch of replacements.  It is expected that three to four

satellites will reach end of life each year, based on experience with the Block I

satellites and considering design improvements to the Block II satellites.  This means

that service coverage can change every few months, although end of life failures can

be anticipated to some degree and some launches can be made prior to the actual

failure.  A number of studies have been conducted to determine the probability of a

specific number of satellites in service at any given time, including some studies

conducted for the U.S. DoD to help determine  satellite replenishment strategies.  One

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