Viavi 87339, Руководство по эксплуатации

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Principals of Operation
6 -18
Troposphere:
The troposphere is the lower part of the earth's atmosphere (0 -15km), that encompasses
our weather. It's full of water vapor and varies in temperature and pressure and causes a
variable but predictable delay. This delay is corrected using a simple model based on
pressure, temperature and altitude.
The errors of the GPS system are summarized in table 6-1. The individual values are no
constant values, but are subject to variances, all numbers are approximate values.
Altogether this sums up to an error of between ± 12 to ± 15 meters.
Table 6-1 PS Positional Error Sources
6.1.5 GPS Timekeeping
Most clocks are synchronized to Coordinated Universal Time (UTC) however, the atomic
clocks on the satellites are set to GPS time. GPS time is not corrected to match the
rotation of the Earth, so it does not contain leap seconds or other corrections that are
periodically added to UTC. GPS time was set to match Coordinated Universal Time (UTC)
in 1980, but has since diverged. The lack of corrections means that GPS time remains at
a constant offset with International Atomic Time (TAI) (TAI - GPS = 19 seconds). Periodic
corrections are performed on the on-board clocks to correct relativistic effects and keep
them synchronized with ground clocks.
The GPS navigation message data includes the difference between GPS time and UTC,
which as of January 1, 2017 is 18 seconds due to the Leap second added to UTC
December 31, 2016. Receivers subtract this offset from GPS time to calculate UTC and
specific time zone values. New GPS units may not show the correct UTC time until after
receiving the UTC offset message.
The GPSG-1000 updates the values related to leap second changes that are within the
navigation message data every 60 minutes. The timing of the leap second value updates
are based on the simulation start time. Values related to the timing of upcoming leap
seconds are stored in a data file within the GPSG-1000. Values related to announcing
upcoming leap seconds to receivers are transmitted 60 days prior to the leap seconds
rollover date, and are maintained for 60 days after the event.
Type of Time Delay Error Positional Error
Ephemeris Errors ± 2.5 m
Satellite Clock Errors ± 2 m
Multi-path Delays ± 1 m
Receiver Clock Inaccuracies and
Rounding Errors:
± 1 m
Ionospheric Delays ± 5 m
Tropospheric Delays ± 0.5 m