Magnetic sensors are used to measure the Earth magnetic field. These sensors enable a compassing capability by providing direction to the magnetic North.
Usually magnetic field is expressed in nano Teslas or Gauss. However, compass applications do not really require specific units since only the direction of magnetic field matters. Thus, a normalized vector (with a norm equal to 1.0) can perfectly describe the direction to Magnetic North.
Ellipse magnetometers are Anisotropic Magneto Resistance (AMR) based sensors. These are constituted of a tiny Silicon board and layer of ferromagnetic alloy (see picture below). Resistance depends on the angle between the current direction and magnetization direction, which is influenced by magnetic field.
The advantages of AMR magnetometers are good sensitivity, and good bias stability.
Need of a magnetic calibration
The Earth magnetic field is particularly weak (typically in the range of 250 to 650 mGauss).
In addition, the materials located in the vicinity may greatly affect the local magnetic field. In particular hard irons (magnets) can create local deviations of the magnetic field, orders of magnitude higher than Earth magnetic field. Soft Iron are ferromagnetic objects that react to their local magnetic field by changing its direction and can also disturb the local magnetic field.
In order to distinguish the weak Earth magnetic field, from the local disturbances, it becomes necessary to perform an in-situ calibration to map correctly all disturbances and compensate them.
For more information on Ellipse magnetometers calibration, please read the following document: Magnetic calibration
Unlike typical low cost magnetic sensors which require regular re-calibration, SBG Systems select only sensors that don't get magnetized over time and thus, don't require periodic calibration.