Geometrics G-824A User manual

G-824A CESIUM MAGNETOMETER
Operation Manual
COPYRIGHT © 2014
P/N 27731-OM
Revision A7
GEOMETRICS, INC.
2190 Fortune Drive, San Jose, CA 95131 USA
Phone: [phone]
Fax: [phone]
EMAIL: [email protected]m

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TABLE OF CONTENTS
1.0 INTRODUCTION...............................................................................................................1
2.0 SYSTEM COMPONENTS..................................................................................................2
3.0 PERFORMANCE ...............................................................................................................4
4.0 INSTALLATION .................................................................................................................5
4.1 Sensor orientation.........................................................................................................................................................6
4.2 Sensor attachment ......................................................................................................................................................11
Vibration........................................................................................................................................................................12
Electronic/electromagnetic ............................................................................................................................................12
Temperature...................................................................................................................................................................13
5.0 OPERATION....................................................................................................................13
6.0 TROUBLE SHOOTING....................................................................................................13
7.0 G-824A MAGNETOMETER.............................................................................................15
7.1 CM-321 Output Format.............................................................................................................................................17
7.2 Installing Driver for USB to Serial Converter ........................................................................................................21
7.2.1 Configuring the FTDI Driver...............................................................................................................................21
7.2.2 Installing Tera Term Terminal Emulator.............................................................................................................24
7.2.3 Serial Data Interface ............................................................................................................................................25
7.3 Detailed configuration descriptions:.........................................................................................................................25
7.3.1 Tera Term, Terminal Emulator Program Setup ...................................................................................................25
7.3.2 ASCII Data Output Format with 250 Hz Output Rate Configuration Example...................................................27
7.3.3 SET CYCLE TIME (Data Output Rate)...............................................................................................................28
7.3.4 SET FORMAT OUTPUT OF DATA...................................................................................................................29
7.3.5 SAVING SETTINGS IN MEMORY ...................................................................................................................32
7.3.6 START G-824A DATA TRANSMISSION .........................................................................................................33
7.3.7 Setting the Power Up Mode:..................................................................................................................................33
7.3.8 XS3 Data Output Format with 1000 Hz Output Rate Example ...........................................................................34
7.3.9 PPS Synchronization...........................................................................................................................................38
7.3.10 Magnetometer Operational Status.....................................................................................................................39
7.4 XS3 Data Output Format..........................................................................................................................................40
7.5 G-824A Warm-up Time...........................................................................................................................................41
7.6 Power-up Initialization..............................................................................................................................................41
7.7 Hemisphere Locking Switch......................................................................................................................................41

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8.0 G-824B MAGNETOMETER.............................................................................................42
9.0 ACCESSORY SOFTWARE ............................................................................................43
9.1 Cesium Sensor Azimuth Program - CsAz .............................................................................................................44
APPENDIX A1. MODEL G-824A SPECIFICATIONS ............................................................45
APPENDIX A2. MODEL G-824B SPECIFICATIONS ............................................................46
APPENDIX A3. SENSOR INSTALLATIONS.........................................................................47
APPENDIX A4. CESIUM-VAPOR SENSOR THEORY..........................................................52
APPENDIX A.6 G-824A GENERAL BLOCK DIAGRAM OF SYSTEM COMPONENTS.......56
APPENDIX A.7 G-824B GENERAL BLOCK DIAGRAM OF SYSTEM COMPONENTS.......57
LIST OF FIGURES AND TABLES
FIGURE 1SENSOR,SENSOR CABLE AND SENSOR DRIVER MODULE............................................................2
FIGURE 2G-824 SYSTEM AND ACCESSORIES............................................................................................3
FIGURE 3ACTIVE ZONE FOR THE CESIUM-VAPOR SENSOR..........................................................................4
FIGURE 4SHOWING INCLINATION OF EARTH'S MAGNETIC FIELD ...............................................................5
FIGURE 5MAGNETIC FIELD INTENSITY AT THE EARTH'S SURFACE............................................................6
FIGURE 6MID-LATITUDE ZONE.................................................................................................................8
FIGURE 7POLAR ZONE SENSOR ORIENTATION ..........................................................................................9
FIGURE 8EQUATORIAL LATITUDE...........................................................................................................10
FIGURE 9SENSOR MOUNT THAT PROVIDES FOR TWO AXES OF ADJUSTMENT.P/N 27530-04...................11
FIGURE 10 BLOCK DIAGRAM OF G-824, POWER,PPS, AND COMPUTER CONNECTIONS ...........................18
FIGURE 11. SCHEMATIC DIAGRAM OF DAISY-CHAINED CM-321COUNTER MODULES.................................20
FIGURE 12. WIRING DIAGRAM OF INTERFACE CABLE................................................................................20
TABLE 1. G-824A STANDARD COMPONENT WEIGHTS AND DIMENSIONS ....................................................3
TABLE 2. TROUBLE SHOOTING.................................................................................................................13
TABLE 3. G-824A STANDARD COMPONENT WEIGHTS AND DIMENSIONS ..................................................15
TABLE 4. G-824A INTERFACE CABLE PIN-OUT.........................................................................................15
TABLE 5. EXAMPLE OF CM-321 DEFAULT OUTPUT DATA STREAM...........................................................19

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1.0 INTRODUCTION
Congratulations on your purchase or rental of one of the finest cesium vapor
magnetometers ever produced. This manual will provide you with an understanding of
alkali vapor magnetometer technology and will give you instructions for installing the
system for airborne, land or marine use. It is not meant to be exhaustive for every case as
there are many situations and applications in which magnetometers can be used. Please
contact the factory if you have specific questions relating to your application.
The Geometrics G-824 magnetometer employs an optically pumped Cesium-vapor
atomic magnetic resonance system that functions as the frequency control element in an
oscillator circuit. The G-824 contains counter circuitry that internally converts the
magnetometer’s analog signal into a digital RS−232 output.
The frequency of the magnetometer electrical oscillator is known as the Larmor
frequency and it varies directly with the ambient magnetic field at the sensor (white
cylinder shown in Fig. 1). When this frequency is accurately measured it provides an
extremely precise measurement of the earth’s total magnetic field (better than 1 part in
108). The design of the sensor optical package uses a split-beam design which eliminates
the need to optimize sensor orientation to obtain precise measurements.
As shown in Figure 1, the G-824 magnetometer consists of two interconnected modules:
a sensor module and sensor driver module. The G-824 relies on counter circuitry that is
internal to the sensor driver module using the Geometrics CM-321 SupremaC counter.
Due to licensing concerns we do not offer the G-824 with analog Larmor output for
external counting at this time. The system comes with a sensor, sensor driver module
with counter, power/data cable (25 feet or 7.6 m), and an RS-232 output cable in a rugged
shipping case. Optional accessories include MagLog logging software, power/data
junction box, USB-Serial convertor, 110/220 VAC 50/60 Hz 28VDC power supply for
lab use, birds, stingers and wing-tip sensor housing aircraft installation kits with internal
orientation platforms.
A basic understanding of the physics employed in the G-824 and optically pumped
resonance magnetometers in general is valuable for troubleshooting new installations and
for achieving optimum results in the field. This information can be found in the Appendix
A4 of this manual.

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2.0 SYSTEM COMPONENTS
The basic system components of the G-824A consist of the sensor module, a sensor
driver module, and an interconnect cable that is permanently attached to the sensor
module and detachable from the sensor driver module. These components are identified
in Figures 1 and 2. The weights, dimensions, and connector’s specifications for these
components and ancillary system components are listed below in Table 1.
Figure 1 Sensor, sensor cable and Sensor Driver Module

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Table 1. G-824A standard component weights and dimensions
Description
Weight
Dimensions
Cesium sensor package
P/N 27516-04 (9 ft.) or -21 (14 ft.)
1lb. 8.25 oz.
(0.69 kg.)
2’3/8”x5-1/2” (61.9 cm x 13.97 cm) cylindrical housing with
109” (276.86 cm) or 168” (426.72 cm) cable. Termination:
Burndy 0119 G6JF12-88 PNE
G-824A Sensor driver module
P/N 27735-02
4 lbs. 6.5 oz.
(2 kg.)
18” (45.72 cm) long including connectors.
16”(40.64 cm) long x 2-1/2” (6.35 cm) square housing
Carrying Case
P/N 27750-01
10 lbs.
(4.54 kg.)
18l” x 9.5w” x 15h” (459.74 cm W x 24.13 cm H x 38.1 cm L)
A multi-conductor cable connects the sensor driver module to the junction box and
carries power and digital data from the module. The multi-conductor cable may be as
long as 200ft (61m) for interconnection with a power/data junction box (e.g., Geometrics
Cable P/N 27560-01)
Figure 2 G-824 System and Accessories

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3.0 PERFORMANCE
Geometrics G-824A magnetometer produces an RS-232 digital data transmission based
on the Cesium Larmor frequency of 3.498572 Hz per nT (nanoTesla). Nano-Tesla refers
to the magnetic field strength also known as one gamma or 10-5 gauss. At the earth’s
surface in a nominal 50,000 nT field the Larmor frequency is about 175 kHz which is
converted using a proprietary counting system in the internal CM-321 SupremaC counter.
The output of the G-824A is an RS-232 transmission at up to 1000 samples per second.
Figure 3 Active zone for the cesium-vapor sensor.
Sensor Active Zone extends from 15º to
75º referenced to the cylindrical axis (for
both hemispheres.)

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The G-824A is primarily intended for use in airborne and land/ base station applications,
and operates over the earth's magnetic field range of 17,000 to 100,000nT. Absolute
accuracy (as compared to an international standard) is rated at ± 2nT over the earth’s
magnetic field range independent of orientation and does not drift with time. The
accuracy of the internal counter's time base is extremely high and with oversampling
techniques used can give noise levels in the range of 0.4pT/Hz RMS or 10 times better
than the CM-201 counter in the G-823 and G-882. Error due to orientation of the
G-824A does not exceed ±0.3nT or 0.6 nT peak-to-peak (p-p) in spin and tumble
throughout the active zones shown in Figure 3. Environmental conditions for proper
operation are -35 to +50°C (-31 to +122° F), humidity to 99 percent (non-condensing),
and an altitude range of 0 to 30,000 feet.
Like all magnetometers, performance of the G-824A is primarily dependent upon the
counting circuitry employed and the quality of the installation procedures. Compensation
and/or noise reduction techniques must normally be used to minimize the magnetic effect
of the platform (aircraft, UAV or AUV) and its motion. Navigational and positional
errors, radiated electromagnetic noise and heading errors from the aircraft's induced and
remnant magnetic fields are typically the major contributors to noise in the survey results.
4.0 INSTALLATION
The particular installation requirements for each system component must be met in order
to obtain the best performance from the system. It is important to remember that the
sensor driver circuit receives a signal from the sensor whose amplitude is normally one
Figure 4 Showing inclination of earth's magnetic field
Epoch 2000, 2º contour interval.

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milli-volt as it delivers both heater and lamp oscillator power to the sensor. Anything
that increases the cross-talk between the power and signal circuits or introduces noise
into the power circuit can degrade the sensor output signal and affect system
performance.
4.1 Sensor orientation
Although the G-824A measures the total intensity of the earth’s magnetic field, magnetic
fields in general are vector fields. The total field is the sum of the three components as
projected onto the earth’s field vector which is roughly vertical at the poles and
horizontal at the magnetic equator. At any point the field is defined by its magnitude and
direction. Unless the sensor is very near highly magnetic objects, the local magnetic field
will be almost entirely due to the earth’s magnetic field. In order for the G-824A to
accurately measure the local magnetic field magnitude, it must be properly oriented
relative to the local magnetic field direction.
The sensor head must be oriented so that the local field flux vector impinges at an angle
of from 15º to 75º to the cylindrical axis of the sensor, for all platform attitudes
encountered during survey. Alignments that produce a field/axis angle less than 15º
place the magnetic field within the sensor’s "polar dead zone". Similarly, alignments that
produce a field/axis angle greater than 75º place the magnetic field within the sensor’s
Figure 5 Magnetic field intensity at the Earth's surface.
Epoch 2000, 1000 nT contour interval.