Inside the DM24
7.1.2.2 Trigger notifications
When a trigger occurs, a message is immediately saved to the current status block.
You may not see this message straight away, however, since the status block must be
full before the digitiser will send it.
2006 1 18 14:48:36 SOFTWARE Trigger : Trigger# 22
The trigger type is mentioned, as well as the sequence number (for later retrieval, if
necessary, using the
EVENTSLIST
command). When the trigger condition ends, the
system reports
2006 1 18 14:49:10 End of Trigger
7.1.2.3 Environmental measurements
These are measured every 10 minutes (at 5, 15, 25, 35, 45 and 55 minutes past the
hour), and are given in the same form as for heartbeat messages:
2006 1 18 14:45:00 External supply : 13.0V Temperature 24.62'C
7.2 GPS and Synchronisation
7.2.1 Overview
The DM24 has two internal clocks: a hardware real-time clock (RTC), which runs
continuously (even when the unit is powered down) and a software clock that is
used for time-stamping digitised data. The software clock uses a voltage-controlled
crystal oscillator (VCXO) to provide a time reference which is stable to below 0.1 ppm.
The software clock is set from the RTC at boot-up: this is the only time that the
system consults the RTC.
The software clock is normally synchronised to GPS time, as reported by an attached
Güralp GPS receiver. After power-up, the output from the GPS receiver is monitored
until a stable fix is obtained, at which point both clocks are synchronised to GPS
time. A sophisticated algorithm then monitors the frequency and phase difference
between the GPS's PPS signal and the VCXO's output and adjusts the voltage input to
the VCXO so that the software clock precisely tracks the average GPS time. (Recall
that GPS time includes deliberate fluctuations to reduce the instantaneous accuracy
for non-military users.)
If the GPS signal becomes too weak to use or if the receiver is deliberately powered
down to reduce the power consumption, the software clock will continue to run with
the same VCXO setting until a stable GPS signal is received again. At this point, the
software clock will normally be slewed to bring it back into precise alignment with
GPS time. If the difference between GPS time and the software clock after a period of
free-running is too large, a step-change is performed and both clocks are re-set to
GPS time. This is reported in the status stream as a resynchronisation.
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Issue U - December, 2021
