Symmetricom ET6000, User Manual

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GENERAL INFORMATION
If in the D (Dynamic) mode, the unit will use the last position data stored in battery backed RAM as
a starting point to begin its systematic search for satellites. It will continue to do three dimensional
position fixes (latitude, longitude, and altitude) upon acquiring four or more satellites until the
mode is changed. If in the “STATIONARY” mode and a known position has been entered, the unit
will use the position information stored in battery backed RAM as a starting point to begin its
systematic search for satellites.
1.6.2 SIGNAL INTERRUPTION
During GPS ExacTime operation, should the signal from the satellites be interrupted, the antenna
disconnected or blocked, the reacquisition time is dependent upon events during the interruption.
For the first minute of the interruption, the GPS RPU continues to search for the last satellite signals
to which it was locked. If the signal is regained during this minute, reacquisition will be almost
immediate if the users velocity has not changed by more than fifty meters per second.
If the velocity has changed, the Doppler frequency has shifted. The GPS RPU must finish its search
of previous satellite signals and will then expand the search to reacquire. The search time will
depend on the amount of velocity change, but it is usually within fifteen seconds.
If the signal is regained within one minute, the expanding frequency search will already have begun
cycling. In this case, reacquisition may require a few minutes depending upon where the RPU is in
the frequency search when the signal is regained.
If the signal is regained within one hour, the same search must take place, then the new ephemeris
data must be collected. In this case, reacquisition will occur within a few minutes.
The user should realize that obstructions, shading of the antenna, and satellite transmission
interruptions can degrade the signal reception and length of acquisition times.
1.6.3 POSITION AND VELOCITY SOLUTION
The position and velocity, along with the time tag of the measurement, are digitally output from the
RPU to the GPS ExacTime Processor. The position data is three dimensional and available in a
latitude, longitude, and altitude (WGS-84) coordinate frame. The GPS solutions are computed at
typically less than one second intervals.
Symmetricom Inc ET6xxx ExacTime GPS TC & FG (Rev C) 1-15