The basic sensor package at each observatory consists of two magnetometers
- a tri-axial fluxgate magnetometer for gathering vectorial data (data with a directional component and a numerical component)
- a proton magnetometer that measures the total intensity of the magnetic field (F)
The vector-component data collected by the fluxgate magnetometer typically includes:
- horizontal intensity (H)
- declination (D)
- vertical component (Z)
The redundancy between these two measurement systems allows for consistency checks that are useful for troubleshooting. Since it is important that the directional information be precise, both the fluxgate magnetometer and the theodolite are supported by piers that are firmly anchored into the ground so that they cannot move once they are oriented properly. The two instruments are sensitive to temperature and magnetic interference; as a result the buildings at each observatory are situated on plots of land of sufficient size to help isolate the operations from outside magnetic interference. To reduce the baseline drift of the instruments due to temperature, the sensors and electronics are housed in well-insulated, thermostatically-controlled buildings, but this doesn't completely eliminate the baseline drift. Each observatory is visited by a Program employee or contractor about once a week so that 'absolute' measurements can be made using a theodolite coupled to a small magnetometer. These absolutes data are used during data processing to make final adjustments to the data baselines. USGS Geomagnetism Program observatory standards are consistent with those set by Intermagnet, an international consortium which promotes the worldwide collection of high-quality, ground-based magnetometer data, and within which the USGS Geomagnetism Program has an important leadership role.