Technical data

Tables
It is a combination of the Universal Co-ordinated Time (UTC) traceable
signal transmitted by the GPS satellites, used in conjunction with the
Symmetricom™ GPS Timing Receiver, which has enabled an extremely
accurate timing system to be developed.
GPS satellites transmit a code for timing purposes, in addition to a
‘Navigation message’ which includes their exact orbital location and
system integrity data. Receivers use this information, together with data
from their internal almanacs, to precisely establish the satellite location.
A receiver determines position by measuring the time taken for these
signals to arrive.
Signals from at least four satellites are required to resolve the
calculations for latitude, longitude, altitude, and time (GPS-3D).
Subsequent operation of the GPS Receiver in Time Transfer Mode (GPS-
TT) requires only the continued reception of signals from one satellite.
Precise Timing
In precise timing applications, the Symmetricom™ GPS timing receiver
system mediates the multiple GPS satellites available by using its own
GPS Timing Receiver, together with an ovenized oscillator. GPS Receiver
manages the received signal from each satellite in view and qualifies
their performance using the Receiver Autonomous Integrity Monitoring
(RAIM) algorithm. Accurate timing information is thereby provided, and
also UTC information for output via the Time Port.
The individual performance of each satellite being tracked is observed
and compared to all the others. Unacceptable performance data is
discarded and replaced by the ‘next best’, if one exists. Up to twelve
satellites can be tracked at any one time.
GPS Receiver Positioning and Navigation
The Symmetricom™ GPS Receiver needs to be able to see at least 4
satellite vehicles (SV’s) to obtain an accurate 3-D position fix.
The receivers antenna must have a clear view of the sky to acquire
satellite lock. Remember always, it is the location of the antenna which
will be given as the position fix. If the antenna is mounted on the roof of
a building, pole, or other structure, allowance for this must be made for
the antenna height above ground when using the solution.
GPS Timing Receiver Installation and Operation Manual
D-2