The strength of the total field can be measured down to about 0.1 nT. Like fluxgate magnetometers, proton precession magnetometers show no appreciable instrument drift with time. One of the important advantages of the proton precession magnetometer is its ease of use and reliability. Sensor orientation need only be set to a high angle with respect to the Earth's magnetic field. No precise leveling or orientation is needed. Finally, because the signal generated by precession is small, this instrument can not be used near AC power sources.
Precession is motion like that experienced by a top as it spins.
Because of the Earth's gravitational field, a spinning top not only spins about its axis of rotation, but the axis of rotation rotates about vertical. This rotation of the top's spin axis is referred to as precession.