Unformatted text preview: al thousand meters in a very short period of time. One example of 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.
Short-term transients can last a few seconds and are characterized either by a short transition
into and out of non-standard code, or by rapid changes in ranging signal phase. Transient
behaviors are discussed in more detail in Section A-4.4.
A-4.2.4 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
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 that can see it is approximately three hours. 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. Page A-20 October 2001 A-4.3 GPS SPS Performance Standard Expected Service Reliability Characteristics When the system is performing nominally, no satellite’s instantaneous URE will ever reach the
service reliability threshold. Service reliability on those days where GPS does not experience a
service failure is 100%.
The estimated maximum of three 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. 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 99.94%. A-4.4 Current Experience with Transient Satellite Behaviors GPS in general experiences steady state operations with very low levels of range rate and range
acceleration errors. Infrequently, steady state operations are disturbed. For instance, whenever
an upload occurs an instantaneous change in transmitted satellite position will cause an
instantaneous offset in the apparent range to the satellite, sometimes by several meters. Other
transients can also occur, and are the focus of discussions in this section.
GPS Block II and Block IIA satellites occasionally exhibit anomalous behavior in signal
transmission that directly affects GPS SPS users. The anomalies occur as a result of a timing
problem in the Block II and IIA satellites and occur on approximately 36% of all Block II navigation
uploads. Occurrence on Block IIA satellites is rare, happening approximately once per month.
The anomaly has not occurred on Block IIR satellites. When the anomaly occurs, the GPS
satellite typically broadcasts Non-...
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- ........., SPS, Global Positioning System