11.4  PRECISE TIME AND GPS

11.4.1  Introduction

Precise time is important for a growing number of military, civilian, and scientific

applications.  Precise time references accurate to a few milliseconds or better have

historically been complicated and costly to obtain, but GPS will afford the means to do

it very simply and economically.  Navstar GPS provides precise time, globally, to an

absolute accuracy of approximately 200 nanoseconds (ns) relative to UTC (USNO). 

(This figure and others given in 11.3 and its subsections are subject to implemen tation

factors and might be considered usual values; with careful implementations and under

circumstances, much better accuracies are possible.)

11.4.2  Applications of Precise Time

Both scientific and civilian precise  time interests can be served by GPS.  Some

examples of civilian/scientific applications are described below:

1.

Simultaneous observations of space objects from observatories

2.

Use by national standards laboratories

3.

Research into the theory of general relativity

4.

Development and calibration of frequency standards

5.

Use of Time Division Multiplexed (TDM) and other communications

disciplines requiring precise time coordination between sites.

11.4.3  Interrelationship Between Different Definitions of Time

A number of different time definitions will be described here.

11.4.3.1  Time Based on the Rotation of the Earth On Its Axis

There are several definitions of time based on the rotation of the earth, but they are all

interrelated (see Figure 11 2).

1.

Universal Time (UT)

UT is mean solar  time on the  Greenwich meridian.  It is  used in the

application of astronomical navigation.

2.

Universal Time 0 (UT 0)

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