Guralp Systems CMG-DCM build <10,000 - Manual (Page 90)

CMG-EAM (Platinum Firmware)
8 Digitiser Synchronisation
Accurate time-stamping of samples is essential to seismology. Güralp
Systems Ltd recommend the use of GPS receivers for generating clock
signals wherever possible: they are the most accurate time sources
available for all practical purposes. Where GPS receivers cannot be
used but an internet connection is available, Network Time Protocol
(NTP) can produce acceptable results. Platinum firmware can produce
NTP-synchronised NMEA output for use with GPS-capable devices.
CMG-EAMs, CMG-DCMs and CMG-NAMs have an internal clock
which is used to time-stamp log-file entries (but not data samples).
This lock is managed by the NTP subsystem. CMG-DAS units and
Authenticated Digitisers have two clocks: the digitiser clock and the
EAM/DCM clock: the former is used to time-stamp data samples and
the latter to time-stamp log-file entries. The EAM/DCM clock is also
managed by NTP. The two clocks can be synchronised in a number of
ways.
8.1 RTSTATUS packets
Where a CMG-EAM or CMG-DCM is used with an external GPS-
synchronised GSL digitiser, the digitiser can emit special
synchronisation packets called RTSTATUS packets. These are
transmitted along the same link as the data packets. Platinum units
can use these as a time source for NTP: see section 6.3 on page 64 for
more details.
90 Issue C