deriving the defined frequency.  UTC is a type of atomic time maintained by the U.S.

Naval Observatory (USNO), and others.  UTC is occasionally adjusted in steps (leap

seconds) to maintain agreement with UT  1 to within 0.9 seconds.  Leap seconds are

necessary because of the effects on UT  1 of the irregular rotation of th e earth over

time.  The International Earth Rotation Service in Paris, France determines when step

adjustments are necessary.  A number of observatories/ laboratories maintain atomic

clocks as very precise time references. They usually synchronize these clocks to UTC,

which is the commonly used reference time. UTC repre sents an average of time from

58 different laboratories around the world.  Each major country  maintains its own

version of UTC and defines national standards of time. Therefore, there is no one

"Coordinated Universal Time".  Instead, there is an Inter national Atomic Time (TAI),

kept in Paris by the International Bureau of Weights and Measures (BIPM), and several

versions of UTC.  The TAI is a weighted average of the times kept by the 58

laboratories which cooperate with BIPM to form this average.  For the past few years,

the majority of time comparisons used to form TAI have been done using GPS.  The

difference between TAI and the various national UTC time references are closely

monitored and are therefore well known.  National UTC references will therefore be

steered to TAI when necessary and for GPS users, steering of UTC (USNO) will be

experienced once every couple of years.  For U.S. agencies, UTC is maintained by the

U.S. Naval Observatory (USNO) in Washington, D.C.  GPS time is required by the U.S.

DoD to be referenced to UTC (USNO).

11.4.3.3  GPS Time

The internal reference time used by the three segments (Space  , Control  and User 

Segment) in the GPS system is called GPS time.  GPS time is a continuous time count,

with no discontinuities, from the GPS epoch.  GPS time is estimated and main tained by

the MCS by estimating the ensemble satellite and monitor station time off sets.  To aid

USNO in providing a stable and accurate reference to the system, an ensemble of

cesium beam clocks is also maintained at the GPS Monitor Station that is collocat ed

with the MCS.  As a Precise Time Reference Station, it main tains time and rate very

accurately traceable to UTC (USNO).  It normally maintains a UTC (USNO) reference

to an accuracy of a few nanoseconds.  GPS time will normally be steered to within 30

nanoseconds of UTC (USNO) after accounting for the leap seconds which have

accumulated in UTC since the GPS epoch of 0 hours 6 January 2025 (UTC).  The

remaining difference between GPS time and UTC (USNO) is trans mitted in the NAV

msgs from the satellites.  The relationship between GPS time and UTC is:

GPS time = UTC time + DUTC time

where, DUTC time = Number of leap seconds + GPS to UTC bias

As of May 1995 the leap second difference between GPS and UTC is 10 seconds.  The

GPS receiver uses the NAV msg data to provide UTC (USNO) time outputs.

11.4.4  Precise Time Dissemination from GPS

GPS satellites have highly stable atomic clocks onboard with a known or predictable

offset from GPS time.  USNO monitors all the satellites when in view of the USNO in

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