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GSM-19M Shallow Tow Marine Magnetometer


  • Sensitivity = 0.01 nT
  • Omnidirectional Sensor
  • Advanced Overhauser Effect Sensor
  • DGPS option


The GSM-19M Marine Magnetometer utilizes Overhauser effect sensors to create a shallow tow magnetometer system that is extremely sensitive, yet easily deployed. The Overhauser type sensor delivers sensitivities an order of magnitude better than standard proton sensors. It also is much more efficient, requiring only a fraction of the power required by proton sensors. The proton sensor uses a large DC magnetic field to polarize the sensor, whereas the Overhauser sensor is polarized with a low power, high frequency magnetic field.

Overhauser sensors can measure the magnetic field while polarizing. As a consequence the GSM-19M magnetometers can sample at one sample per second with virtually no loss of sensitivity. Even at rates of four samples per second the noise envelope is typically less than 0.1 nT.

Overhauser sensors do not differ from the Heading Errors typically associated with Cesium magnetometers. With cesium type magnetometers the sensor must be oriented properly so that "dead zones" are avoided. Even when oriented correctly Cesium magnetometers have a heading error of about 0.25 nT that cannot be eliminated. The GSM-19M does not have this problem. The sensor is completely omni-directional, even in low latitudes.

The GSM-19M magnetometers consists of a surface console with rechargeable battery, a 100 meter tow cable, and a fish housing the sensor. The console is used to communicate with the magnetometer, and to display and store data. Only the sensor is contained in the fish. Should the fish be lost, replacement costs are lower than systems that contain both electronics and the sensor in the fish.

The GSM-19M console features a real time graphic display of the current profile. In addition digital display of the current reading, current position, and warning messages are provided. The console design, with internal rechargeable battery pack, allows the unit to be completely sealed against the elements.

Tuning is automatic worldwide, with provision for manual override. In high gradient conditions the GSM-19M monitors the signal decay rate and displays a warning message when the gradient becomes too great. Filters for rejection of 50 or 60 Hz noise are provided.

The RS-232 port on the GSM-19M will output data as it is collected. This allows interface to GPS loggers that will accept RS-232 data. The standard GSM-19M may be operated in a remote mode via computer. Memory storage is 512 K in the standard unit, and may be upgraded to 2 MB.

Integrated DGPS

With the GPS Log Option the GSM-19M magnetometers will display and store GPS data using standard NMEA format. Position accuracy is dependant on the user's DGPS system.

Also offered is an internally mounted GPS board that may be integrated with radio modem for DGPS mode. A range of GPS boards are offered to meet customer specified accuracy. These are quoted on a case by case basis to take advantage of current technology. Complete systems, with base station, and DGPS software are provided.

Cable Weak Link Option

The cable may be installed with a weak link at the shipboard end of the cable, and a buoy attached. In case of a snag the buoy will support the end of the cable, and recovery of the fish is probable.

The GSM-19M Overhauser system is a robust package, free of heading errors, that is ready for surveys anywhere in the world. Its 0.01 nT resolution and high absolute accuracy deliver data quality that can be relied on, without worry of heading error corrections. For diurnal corrections (base stations) on offshore surveys the GSM-19 Overhauser should be considered.




Range: 20,000 to 150,000nT
Gradient Tolerance: Over 10,000nT/m
Operating Modes, Single Reading: 1) contact cl. 2) ext. trigger CR (ASCII)
Cycling Time: 0.2s
Analog Output: 2 channels (prog. range)
Digital Output:


Visual Output: 4 x 20 dot matrix LCD
Power Requirements: 24V, 30mA stb, 0.2A max current
Operating Temperature: -40C to +60C
Fish Dimensions: 9cm dia 60cm long
Fish weight: 3.5kg
Cable Length: 100m max
Console: 7 x 22.5 x 24cm

Standard Components

Fish sensor, console, 100m cable and instruction manual.