INTRODUCTION TO GEOMAGNETISM
The geomagnetic field protects the Earth's atmosphere against charged particles from the solar wind. Rapid and long-term changes in the field are monitored by low-orbiting satellites and a global network of about 200 magnetic observatories. Field models, computed from these measurements, provide the direction and strength of the magnetic field at any desired location. Magnetometers (or a simple compass) can then be used to compute pointing directions, complementing the location information provided by the global positioning system (GPS).