Washington DC, U.S.A. and compares the GPS time and UTC (USNO) time trans mitted

by the satellites with the (USNO) Master Clock.  Based on this compari son USNO

determines the accuracy of the GPS/UTC time information provided by each GPS

satellite and transfers this information to the MCS (see Figure 11 3).  This GPS to UTC

time bias and drift offsets, as well as the number of leap seconds, are uploaded in the

satellite almanac data message. This  information is used in the GPS receiver

algorithms to determine UTC (USNO) time from GPS.  The result is a world   wide time

reference system for UTC (USNO) available to every user of GPS (see Figure 11 4). 

The absolute time accuracy available to the user depends on several factors described

in Table 11 1, but the relative time accuracy between two GPS users can be much

better than the absolute time accuracy.  If the stations simultaneously  track  the  same

 GPS  satellites  for  time  dissemination,  then the effects of certain Control Segment

and satellite induced errors on the relative time accuracy are much reduced, and

relative time accuracy can be as good as 10  20 ns.  Almost all users employ loc al

clocks or oscillators of some kind to satisfy system requirements for long   and

short term accuracy and stability, or to avoid the need for continuous updates from an

external reference, such as GPS.  Slaving the clocks too tightly to GPS time would

impart to them the shorter term instability associated with reception and interpretation

of GPS signals and with the instabilities previously mentioned.  Longer  term

measurements that are required to obtain an accurate rate or frequency would not

enjoy the short term advantage of simultaneous tracking, since over a period of time,

most of the space and Control Segment  functions would effect the stability of the

dissemination function.

Figure 11 3.   Determination of GPS UTC (USNO) Time Difference

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