CHAPTER FIVE
5.3 THEORY OF OPERATION
5.3.1 GPS SYSTEM OPERATION
Before going into the Theory of Operation of the ExacTime GPS Time and Frequency Generator we
must first understand the operation of the GPS system and what the Satellites are providing. The
GPS system is a ranging system, in which satellites inform the ground user equipment where they
are located and give the time information. The function of the user equipment is to determine its
position (X, Y, and Z) as well as the time (T). This is accomplished through the use of simultaneous
equations. Since there are four unknowns it requires at least four satellites to solve the equations. At
power-on the user equipment has no idea where it is located or what time it is. Once it has acquired
(TRACKED) four satellites, it can begin the process of solving the equations. Once a position has
been determined, it can be loaded into memory and used to solve for the time only. Another aspect
of the GPS system that is in need of some explanation is a deterrent function known as Selective
Availability (SA). This is a purposeful degradation of the GPS system’s performance so that an
enemy of the United States can not use the system accurately against us in time of a war. Although
SA is not currently activated, it can be at any time. The degradation that takes place is from twenty-
five meters to one-hundred meters Spherical Error of Probability (SEP) for position information and
one-hundred nanoseconds to 300 nanoseconds 95% of the time for Time information.
5.3.2 MODES OF OPERATION
Now that we understand that a GPS is a ranging system and the user equipment must calculate its
position as well as time, we will now take a look at the different MODES of operation of the
equipment and understand how it affects the operation of the equipment. There are several modes of
operation for the ExacTime (Auto, Stationary, Dynamic, and Flywheel). We will start off with the
Flywheel mode and work our way back to the normal Auto mode.
A. FLYWHEEL MODE
Flywheel mode is a new, optional mode in which the unit is powered-on and does not
operate from GPS information. The operator can set the time and the unit will function as a
stand-alone clock and frequency source. No correction to the time or frequency are done by
the unit, it simply continues from the last information in terms of the DAC value, position
data, output selection, etc.
5-6 ET6xxx ExacTime GPS TC & FG (Rev C) Symmetricom Inc
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