The combination of these factors results in a conservative system operator 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.

6.1.4.5  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 two types:

Insidious, long term (day or more to become evident) performance deviations,

or

Catastrophic, almost instantaneous failures

Insidious failures do not propagate very quickly   failures of this type experienced to

date have not affected the GPS ability to support 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 frequency generation

hardware failures. These failures in general result in very rapid ranging error growth  

range errors can grow to several thousand metres in a very short period of time.

Typically, a failure of this type will begin with a phase jump of indeterminate magnitude,

followed by a large ramp or increased noise consistent with the behavior of a quartz

oscillator.

6.1.4.6  User Global Distribution and Failure Visibility

For the purposes of reliability performance standard definition, the effect of a service

failure is not weighted based upon user distribution   a uniform distribution of users

over the globe is assumed.

Given a maximum failure duration of six hours, approximately 63% of the Earth s

surface will have a failed satellite in view for some portion of the failure.  The average

amount of time that the failed satellite will be in view for those locations which can see

it is approximately three hours.

6.1.4.7  Satellite Use in the Position Solution

Given a 24 satellite constellation, an average of eight satellites will be in view of any

user on or near the Earth.  The satellite visibility distribution for the nominal 24 satellite

constellation is shown in Figure 6 1.  With all satellites weighted equally, the probability

of a failed satellite being in the position solution of any user located within the failure

visibility region is 50%.  Equal weighting is considered to be a reasonable assumption

for use in global reliability computations.  However, in the worst case individual site

computation it must be assumed that the receiver is tracking and using the failed

satellite for the duration of  the satellite visibility window.

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