The use and testing of compasses and magnetometers at the National Data Buoy Center

The buoys deployed by the National Data Buoy Center (NDBC) measure many environmental variables, both oceanographical and meteorological. Three of these measurements in particular-atmospheric winds, ocean currents, and surface water waves-require that a horizontal direction reference be used. Although there are several types of instruments and methods that can provide this direction reference, NDBC has found compasses and magnetometers to be the best choice for the application. The characteristics of the Earth\´s magnetic field at the location where a buoy is deployed are usually well known. For most of the cases, a computer model of the Earth\´s magnetic-field vector produces its magnitude values in the three-dimensional directions (components) at the geographic coordinates (latitude and longitude) of the buoy station. At other times a direct in-situ measurement of the Earth\´s magnetic-field vector is obtained. A magnetic compass or a triaxial magnetometer (and sometimes both) installed on the buoy provides the values of the magnetic heading of the buoy\´s hull and superstructure. This is the direction reference that the buoy\´s other instrumentation needs in order to provide the angular orientation relative to Earth for the other measured variables. The propeller-vane anemometers deployed by NDBC measure wind direction relative to the buoy\´s hull. The magnetic heading of the hull, as determined by a magnetic compass, and appropriate calculations by the buoy\´s data processor permit reporting wind-direction values referenced to Magnetic North. When the local magnetic variation is known, one further calculation relates wind direction to Geographic North. Similarly, the sensors that measure water currents and surface-water waves need the buoy-hull magnetic heading values for referencing their measurements to an Earth-fixed angular orientation. In the case of sea surface waves, a magnetometer is used to measure the buoy\´s pitch and roll motion as well as buoy azimu- > - > th. The measurement of the buoy\´s heave, along with the pitch and roll angles determined from the measurement of magnetic field components provide all the information necessary to estimate wave directionality. Many different designs of electronic remote-reading magnetic compasses and magnetometers have been used at NDBC. Flux-gate, Hall-effect, magnetoresistive, and magnet/rotating coded card designs, to name a few, have been deployed. The testing of these magnetic instruments is routinely done at NDBC\´s "compass-rose" test stand. More recently a triaxial Helmholtz coil test fixture installed inside a walk-in environmental temperature chamber has been placed into operation.