Unformatted text preview: ce for at least a minimum percentage
of the time. The predicted error level is scaled to a 95% level. Another refinement to the
Page A-2 October 2001 GPS SPS Performance Standard availability concept was to establish two availability definitions: one for horizontal performance
(combination of horizontal position solution geometry and URE) and one for vertical (combination
of vertical position solution geometry and URE).
A-22.214.171.124 Service Reliability GPS can be used anywhere within the global service volume. A failure in a system with a global
service volume affects a large percentage of the globe. A natural concern about using GPS is
whether or not it provides a satisfactory level of service reliability. Service reliability as it is used
in a GPS context is somewhat more restrictive than the classical definition, which includes times
that the service is available as well as when it is performing within specified tolerances. GPS
service reliability is viewed as a measure only of how well GPS maintains Signal-in-Space User
Range Errors (SIS UREs) within a specified reliability error threshold, given the satellite is healthy
and not broadcasting a URA indicating possible performance over the error threshold. A service
reliability of 100% is achieved when no healthy satellite’s SPS SIS URE exceeds the reliability
error threshold over the sample interval without an indication to the user.
A-126.96.36.199 Accuracy GPS position solution accuracy represents how consistently the receiver's output conforms to an
expected solution. Users view accuracy in many different ways, depending on their application.
To accommodate the majority of users' needs, GPS positioning accuracy is defined in the
Performance Standard from three different perspectives:
• Positioning Accuracy, • Time Transfer Accuracy, and • Ranging Signal Accuracy (represented in this standard as User Range Error (URE)) Positioning accuracy represents how well the position solution conforms to "truth". Truth is defined to be any specified user location where the position is known, within acceptable error
tolerances and with respect to an accepted coordinate system, such as the World Geodetic
System 1984 (WGS84) Earth-Centered, Earth-Fixed (ECEF) Coordinate System. Factors which
affect positioning accuracy include geometry and URE variations unique to a given user location
and the sample interval over which measurements are taken.
Time transfer accuracy represents how well a time transfer service user can relate receiver time
to Coordinated Universal Time (UTC) as disseminated by the United States Naval Observatory
(USNO). The definition of time transfer has been modified for this edition of the SPS Performance
Standard to represent performance from a time transfer user’s perspective, as opposed to a
positioning service user. The primary distinction is that a time transfer user operates from a
known location and directly ensembles the measured range residuals from all satellites in view
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- Fall '09
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