CMG-RTM
Real-time Clock/
GPS Emulator
Operator's Guide
Part No. MAN-RTM-0003
Designed and manufactured by
Güralp Systems Limited
3 Midas House, Calleva Park
Aldermaston RG7 8EA
England
Proprietary Notice: The information in this manual is
proprietary to Güralp Systems Limited and may not be
copied or distributed outside the approved recipient's
organisation without the approval of Güralp Systems
Limited. Güralp Systems Limited shall not be liable for
technical or editorial errors or omissions made herein,
nor for incidental or consequential damages resulting
from the furnishing, performance, or usage of this
material.
Issue B 2015-04-15
CMG-RTM Real-time Clock / GPS Simulator
Table of Contents
1 Preliminary Notes..................................................................................................... 3
1.1 Proprietary Notice............................................................................................... 3
1.2 Warnings, Cautions and Notes........................................................................... 3
1.3 Manuals and Software........................................................................................3
2 Introduction............................................................................................................... 4
3 Operation................................................................................................................... 5
4 Connector pin-outs..................................................................................................10
4.1 GPS Output, Power Input (right-hand connector)...........................................10
4.2 GPS Input (left-hand connector)......................................................................11
5 Specifications.......................................................................................................... 12
6 Revision history.......................................................................................................13
2 Issue B
Operator's Guide
1 Preliminary Notes
1.1 Proprietary Notice
The information in this document is proprietary to Güralp Systems
Limited and may be copied or distributed for educational and
academic purposes but may not be used commercially without
permission.
Whilst every effort is made to ensure the accuracy, completeness and
usefulness of the information in the document, neither Güralp
Systems Limited nor any employee assumes responsibility or is liable
for any incidental or consequential damages resulting from the use of
this document.
1.2 Warnings, Cautions and Notes
Warnings, cautions and notes are displayed and defined as follows:
1.3 Manuals and Software
All manuals and software referred to in this document are available
from the Güralp Systems website: www.guralp.com unless otherwise
stated.
April 2015 3
Warning: A black cross indicates a chance of injury or death
if the warning is not heeded.
Caution: A yellow triangle indicates a chance of damage to
or failure of the equipment if the caution is not heeded.
Note: A blue circle indicates indicates a procedural or
advisory note.
CMG-RTM Real-time Clock / GPS Simulator
2 Introduction
The CMG-RTM is a battery backed, high-precision, thermallycompensated, real-time clock module with simulated GPS output. It
obtains its initial synchronisation from a GPS receiver and, once
locked, starts generating NMEA sentences and a PPS (pulse per
second) signal, making it suitable for use as a time-source for GPSsynchronised equipment in situations where GPS receivers are
impractical. A large liquid crystal display provides status information.
z
Once the initial synchronisation source is removed, the CMG-RTM
continues to run under battery power, maintaining timing through the
use of a high-precision, thermally-compensated, crystal-controlled
oscillator. The CMG-RTM can then be transported to the deployment
site and connected to a digitiser, from which it subsequently draws
power. Once powered, it will generate the NMEA and PPS signals
which are required to synchronise the connected digitiser.
Each unit is individually factory-trained to calculate a temperature
compensation table with 0.1°C granularity, ensuring stability of
operation in a wide range of environments.
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Operator's Guide
3 Operation
The CMG-RTM has two 10-pin bayonet connectors: one, on the right,
for attachment to a power supply (during synchronisation) or to a
digitiser (during operation) and one, on the left, for attachment to a
GPS receiver.
If the unit has been disconnected from power for some time, the
internal battery will be discharged. The unit automatically shuts
down under these circumstances.
The unit should first be connected to either a digitiser or power
supply, using the right-hand connector. The following screen will be
displayed:
The second line shows the date and time, according to the internal
clock. The third line shows that no GPS data are currently being
received.
A GPS receiver should then be attached to the left-hand connector.
The third line of the display will change to show that GPS signals are
being received.
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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 subsecond 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
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Operator's Guide
view of the sky. For more information about the GPS system and
optimum receiver placement, please see our GPS installation guide at
http://www.guralp.com/ howtos/ setting-up-gps-equipmen t.shtml.
When sufficient satellites have been in view for enough time, the GPS
receiver will establish a full three-dimensional fix (A3D) and the
display will change to the following:
Here, five satellites are in view (5 SVs). After a minute or so with a
stable three-dimensional fix, the clock control system will start the
synchronisation process and the display will change to the following:
The internal clock will be coarsely adjusted until it is in line with the
GPS time, and then the offset (the difference between internal clock
time and GPS time) and drift (the first derivative of the offset) are
monitored while the pulse width modulation value is altered to
minimise both. This process is known as training.
In the display above, the first line gives the drift, or frequency error
(-5E-8) as -5×10-8. Note that the drift value is unit-less, representing
seconds per second. The third line shows the elapsed time since
synchronisation (0d00:03), which is zero days, zero hours and three
minutes, and the current offset from PPS, which is 12.3µs. The last
line shows the date and time.
The CMG-RTM should now be left until both offset and drift reach
sensibly small values. In a thermally stable environment with good
satellite visibility, this process should take less than an hour.
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CMG-RTM Real-time Clock / GPS Simulator
The GPS receiver and power supply (or digitiser) can then be
disconnected. The display will change to show that GPS data are no
longer being received:
The second line shows the elapsed time since synchronisation
(0d01:23) and the offset from GPS time, which is now an estimate
(EST). The bottom line shows the state of charge of the internal
battery. If the battery were fully charged, the display would look like
this:
The unit can now be completely disconnected from external power. It
enters a low-power sleep mode, turns off the display back-light and
stops transmitting GPS signals.
The display shows the time since synchronisation and the estimated
offset from GPS time; the date and time from the internal clock; the
battery status; the sleep indicator and the time during which it has
been sleeping. If the battery is fully charged, the unit will run for
approximately eight days in this mode.
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Operator's Guide
The unit is now free-running under battery power and can be
transported to the deployment site, where the target digitiser should be
plugged into the right-hand connector, which will then provide power.
The CMG-RTM will provide accurate timing to the digitiser.
April 2015 9
Note: The CMG-RTM will not provide positional
information to the digitiser. Only timing information is
generated.
CMG-RTM Real-time Clock / GPS Simulator
4 Connector pin-outs
4.1 GPS Output, Power Input (right-hand connector)
This is a standard 10-pin “mil-spec” plug,
conforming to MIL-DTL-26482 (formerly
MIL-C-26482). A typical part-number is
02E-12-10P although the initial “02E” varies
with manufacturer.
Suitable mating connectors have partnumbers like ***-12-10S and are available
from Amphenol, ITT Cannon and other
manufacturers.
Pin Function
A Power 0 V input
B Power +5 V to +36 V DC input
C 1pps signal output to digitiser
D not connected
E Console RS232 TxD (for factory use only)
F Console RS232 RxD (for factory use only)
G Ground (for factory use only)
H not connected
J not connected
K NMEA RS232 transmit to digitiser
Wiring details for the compatible socket,
***-12-10S, as seen from the cable end.
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Operator's Guide
4.2 GPS Input (left-hand connector)
This is a standard 10-pin “mil-spec” plug,
conforming to MIL-DTL-26482 (formerly
MIL-C-26482). A typical part-number is
02E-12-10P although the initial “02E” varies
with manufacturer.
Suitable mating connectors have partnumbers like ***-12-10S and are available
from Amphenol, ITT Cannon and other
manufacturers.
Pin Function
A Power 0 V output
B Power +V output
C 1pps signal input
D not connected
E not connected
F not connected
G Ground
H not connected
J not connected
K NMEA RS232 receive from GPS
Wiring details for the compatible socket,
***-12-10S, as seen from the cable end.
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CMG-RTM Real-time Clock / GPS Simulator
5 Specifications
Physical
Length (casing) 150 mm
Length (including connectors) 162 mm
Width 80 mm
Depth 45 mm
Material Polystyrene
Display 21 character, 4 line
Weight 305 g
Electrical
Power supply input voltage 5 V to 36 V DC
Power supply input current
(without receiver connected)
20 mA at 12 V DC
Battery capacity 1.84 Ah
Battery backup 7 – 10 days
Timing
NMEA output sentences $GPGSA and $GPZDA
Trained Temperature range -10 °C to +50 °C
in 1 °C increments
(extrapolated for out-of-range values).
Operating Temperature range -10 °C to +60 °C *
* Battery and display performance are
both compromised at the extremes of
this range but timing performance is
unaffected.
Accuracy <3×10
-8
(<100 ms per month)
12 Issue B
Operator's Guide
6 Revision history
2015-03-31 A New document
2015-04-15 B Revised graphics and improved specifications
April 2015 13
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