Guralp Systems CMG-RTM - Manual (Page 6)

CMG-RTM Real-time Clock / GPS Simulator
The GPS receiver produces two sets of outputs: a stream of textual data
know as the NMEA (because it is in NMEA 0183v1.5 format) and a
one-Hertz pulse train known as the PPS. The NMEA provides coarse
timing (to the nearest second) and the PPS provides accurate sub-
second timing.
Both signals are required for correct operation. If either is missing,
there is a problem with the receiver, the cable or the CMG-RTM itself.
If the NMEA is missing, the following screen is displayed:
If the PPS is missing, the following screen is displayed:
If you see either of these screens, please check the GPS cable and
connections. If no fault is apparent, please contact GSL technical
support for advice.
While the GPS receiver is looking for satellite signals, the display will
change:
In the illustration above, the receiver does not know where it is
(No Fix) and it can see no satellites (SVs stands for “Space Vehicles”,
which is standard GPS terminology for the satellites used by the GPS
system). As a result, the system is not synchronised (Not Sync).
Over the next minute or so, the receiver will start to receive data from
more satellites. It will first calculate an automatic two-dimensional fix
(A2D). This is not sufficient for synchronisation purposes and if this
condition persists, the receiver should be moved to obtain a better
6 Issue B