Heading-Error-Free Scalar Magnetometry for Navigation*

Inertial navigations systems (INS) rely on measurements of platform accelerations to provide continuous updates to the change in position and operate regardless of whether GPS or other references are available. The accuracy in position inevitably degrades as sensor errors compound with successive measurements.  The resulting position uncertainty can be constrained if periodic absolute updates can be provided through a known reference. The Earth’s magnetic anomaly field is a reference that provides a topographic map of magnetic features that is globally available. Improved navigation performance has been demonstrated using commercial magnetic sensors; however, these demonstrations have required extensive platform calibrations as well as compensation for sensor dead zones and heading errors. 

Magnetometry of Earth’s anomaly field can be exploited for the development of navigation systems that are GPS-independent. We describe our progress developing an atom-based sensor capable of producing accurate readings of the total scalar magnetic field while operating in any orientation in the geomagnetic field with heading error significantly less than traditional optically pumped magnetometers (3-5 nT). Non-degeneracies in the Zeeman splittings of the hyperfine state’s magnetic suborbitals, as well as inefficiencies in optical pumping, contribute to these orientation-induced errors, and are known as heading errors. Our compact, heading-error-free scalar (CHEFS) magnetometer is based on the electromagnetically induced absorption of a 795-nm, optical pumping laser resonant with the buffer-gas-broadened ⁸⁷Rb D₁ line. Zeeman spectroscopy is performed by scanning the frequency of a microwave magnetic field resonant with the F=1→F=2 M1 transition of  ⁸⁷Rb, tracking individual hyperfine transition frequencies to extract a heading error free measurement of the magnetic field magnitude.