Guralp Systems CMG-DCM build <10,000 - Manual (Page 120)

Acquisition Modules and Platinum Firmware Digitiser Synchronisation
9 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 (but see below: GPS is best used indirectly). 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.
All acquisition modules have an internal clock which is used to time-stamp
log-file entries (but not data samples). This clock is managed by the NTP
subsystem but need not necessarily use Internet NTP servers (and normally
doesn't).
DAS units, Cylindrical Digitisers and Integrated Instruments have two clocks:
the digitiser clock and the acquisition module clock: the former is used to
time-stamp data samples and the latter to time-stamp log-file entries. The
acquisition module clock is also managed by NTP. The two clocks can be
synchronised in a number of ways.
9.1 Overview and important notes
Acquisition modules can synchronise their internal clocks to three different
time sources:
• special timing signals, known as RTSTATUS packets, produced by a
digitiser
• internet NTP
• signals from a GPS receiver
GPS receivers produce two signals:
• NMEA, which is a serial, ASCII data stream consisting of a sequence of
“sentences” which convey information such as the time (to the nearest
second), the position, the number of satellites visible and much more
• PPS, which is a one-pulse-per-second signal which accurately signals the
start of each second
GSL digitisers, such as the DMG-DM24, use both signals to accurately
synchronise their internal clocks.
Because of a limitation in the Linux kernel on ARM processors, the
CMG-DCM and CMG-EAM cannot make use of the PPS signal. This means
that a DCM or EAM synchronised directly to GPS can be several hundreds of
milliseconds adrift. While this is not crucial - the clock is only used to
MAN-EAM-0003 120 Issue E - February 2014