Guralp Systems CMG-RTM User Manual
Page 7
Operator's Guide
view of the sky. For more information about the GPS system and
optimum receiver placement, please see our GPS installation guide at
When sufficient satellites have been in view for enough time, the GPS
receiver will establish a full three-dimensional fix (A3D) and the
display will change to the following:
Here, five satellites are in view (5 SVs). After a minute or so with a
stable three-dimensional fix, the clock control system will start the
synchronisation process and the display will change to the following:
The internal clock will be coarsely adjusted until it is in line with the
GPS time, and then the offset (the difference between internal clock
time and GPS time) and drift (the first derivative of the offset) are
monitored while the pulse width modulation value is altered to
minimise both. This process is known as training.
In the display above, the first line gives the drift, or frequency error
(-5E-8) as -5×10
-8
. Note that the drift value is unit-less, representing
seconds per second. The third line shows the elapsed time since
synchronisation (0d00:03), which is zero days, zero hours and three
minutes, and the current offset from PPS, which is 12.3µs. The last
line shows the date and time.
The CMG-RTM should now be left until both offset and drift reach
sensibly small values. In a thermally stable environment with good
satellite visibility, this process should take less than an hour.
April 2015
7