5700MSC-IP
IP Network Grand Master Clock & Video Master Clock System
Revision 0.2 Page - 53
Each GPS satellite Space Vehicle (SV) contains four onboard atomic clocks. These clocks are
averaged and are used to transmit GPS time, along with the positions of all satellites (the almanac)
and fine tracking data (the ephemeris). To acquire GPS time, the receiver only needs to track one
satellite. The position of the satellite is known and the transmission delay can be subtracted to obtain
GPS time with millisecond accuracy.
In order to obtain a three-dimensional position, the ranges from multiple satellites must be compared.
If the receiver had perfect timing, three satellite ranges would be sufficient to calculate a position. In
real-world applications, the clock in the receiver will not be an atomic clock and the smallest error in
frequency will cause huge errors in positioning. Consequently, a fourth satellite range is incorporated
to solve for timing error in the receiver. This allows the receiver to correct its clock and synchronize it
to within nanoseconds of the GPS constellation. The 5700MSC-IP is more involved with this clock
correction/synchronization feature of the GPS process than it is with the positioning aspect.
3.5.2. GNSS Lock Operation
The GNSS receiver is a Trimble Acutime 360 which is designed for maximum accuracy in static
(antenna not moving) mounting applications. The receiver will also function when mobile with
decreased precision.
When the GNSS receiver is first connected, it will try to track satellite signals using its stored almanac.
The status display will report “GNSS Searching” during this initialization stage. If the receiver has been
powered off for a while, or has been moved a significant distance, it may need to download a new
almanac. This can require up to 15 minutes of continuously tracking a satellite to complete.
It should be noted here that when GNSS is being used for a time reference only, contact with just a
single satellite is enough to provide accurate time. A GNSS time reference will start working as soon
as a single satellite is tracked. For frequency synchronization, a more complex process is involved.
When the almanac is determined to be valid, the receiver will attempt to complete a self-survey. This is
done by averaging up to 300 positional readings to obtain precise locational data for the antenna. This
process can take up to 5 minutes.
Obtaining a position requires the receiver to track at least four satellites. Satellites which do not meet
the minimum quality criteria (elevation, signal strength, dilution of precision) cannot be used during a
self-survey. The status display will read “GNSS Poor Signal” if four healthy satellite signals cannot be
found. These requirements are more stringent when performing a self-survey than they are when in
normal operation.
When the self-survey is complete, the GNSS receiver will begin to send out a 1pps pulse to the
5700MSC-IP. The 5700MSC-IP will frequency-lock its ovenized oscillator to the pulse edges, and
phase-lock to GNSS time. Once the GNSS receiver knows its precise location, it can substitute its
position into the equation and solve only for clock error and clock bias. In this mode, it can operate
with just one satellite and still provide accurate timing to the 5700MSC-IP.The status of the GNSS lock
progress can be monitored in the Lock Status and Inputs status screens.
As more satellite signals are detected, the receiver will enter over determined clock mode. Up to 12
satellites can be tracked, using their signal strength and position in the sky to give them more or less
weight in contributing to the solution. The quality criteria are more relaxed than when performing a self-
survey and each available satellite will improve timing precision. If the receiver is moving, such as in a
mobile application, the receiver will enter dynamic mode and still provide reasonably high precision.
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