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| Universal time |
Local solar mean time at Greenwich meridian. Some commonly
used versions of universal time are:
UT0: Universal time as deduced directly from observations.
The fixed numerical relationship between universal and sidereal
time is 3 minutes 56.555 seconds.
UT1: UT0 corrected for polar motion.
UT2: UT1 corrected for seasonal variations in the earth's
rotation rate.
UTC: Universal time co-ordinated; uniform atomic time system
kept very close to UT2 by offsets. Maintained by the U.S.
Naval Observatory.
GPS time is directly relatable to UTC:
UTC - GPS = x seconds. (This constant changes slowly, but
= 8 seconds in 1995.)
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| Universal time coordinated
(UTC) |
An international, highly accurate and stable uniform atomic
time system kept very close, by offsets, to the universal time
corrected for seasonal variations in the earth's rotation rate.
Maintained by the U.S. Naval Observatory. GPS time is directly
relatable to UTC: UTC-GPS = seconds. (The changing constant
= 5 seconds in 1988.) |
| URA |
See user range accuracy |
| USC |
United States Code |
| USCG |
United States Coast Guard |
| User interface |
The hardware and operating software by which a receiver operator
executes procedures on equipment (such as a GPS receiver) and
the means by which the equipment conveys information to the
person using it: the controls and displays. |
| User range
accuracy (URA) |
The contribution to the range-measurement error from an individual
error source (apparent clock and ephemeris prediction accuracys),
converted into range units, assuming that that error source
is uncorrelated with all other error sources. Values of 32 indicate
that Selective Availability is active. Values less than 10 generally
indicate that SA is inactive. |
| User segment |
The part of the GPS system that includes the receivers of
GPS signals. |
| UTC |
See universal time coordinated |
| UTM |
Universal Transverse Mercator Map Projection. A special case
of the Transverse Mercator projection. Abbreviated as the UTM
Grid, it consists of 60 north-south zones, each 6 degrees wide
in longitude. |
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