Guralp Systems CMG-DM24S12AMS - Manual (Page 51)

CMG-DM24S12AMS Operator's guide
The internal time synchronisation and control software will wait for the
GPS unit to report a good position fix (this requires 3 satellites) before
beginning. (The system actually waits for good reception of 6
consecutive messages, which normally occur every 10 to 20 seconds.)
If the GPS unit maintains a good fix from the satellites available, the
system will then switch on the control process and set the internal clock.
It will also re-synchronise the Analogue to Digital Converters so that the
data is accurately time-stamped to the new reference. Any data
transmitted up to this point will be stamped with the time from the
internal backup clock, which is set to the new accurate time at the end of
this process. The re-synchronisation will result in a discontinuity in the
data received.
From this point, the control process will continually attempt to keep the
internal time-base synchronised to the GPS 1 pulse per second output by
adjusting its voltage-controlled crystal oscillator. The control algorithm
has two stages: initially it compares its internal 1 Hz timebase with the
GPS signal and adjusts the voltage control to minimise the error. Once
this has been achieved it then controls the crystal to minimise both the
‘phase error’ (i.e. the offset between its internal 1 Hz signal and the GPS)
and the drift (frequency error relative to GPS.) During the control
process the system reports the measured errors and the control signal
applied, as a PWM (Pulse Width Modulation) value.
During the initial, coarse adjustment stage only the coarse voltage
control is used and no drift calculation is made. If the system is
operating in a similar environment to that when the system was last
powered (most importantly, the same temperature) the saved control
parameters will be appropriate and the system should rapidly switch to
the ‘fine’ control mode. The system reports its control status and
parameters each minute, with error measurements given in nominal
timebase units. In a stable temperature environment the system should
soon settle down showing an offset error of only a few thousand
(average error < 100 microseconds) and a drift rate under 100 counts (< 1
in 10
-6
).
March 2004 51