In this question, I asked about a helmet with built-in HUD, and have a follow-up question:
What methods are used (or have been used) to perform head-tracking in the cockpit?
The website for that particular helmet (which I'm surprised exists like it does ... could I simply order one? doubt it ...) has the following (edit: here is additional information for that helmet):
High-speed tracker with a full-spherical Field of Regard(FOR); choice of Hybrid-Inertial or Magnetic tracker