@jmpowell, TheUnnamedNewbie
Thank you for your interest. I failed to mention that this is for outdoor use, although predictable behavior in the presence of chain link fences and brick walls is desirable. Sunlight and darkness would cripple optical systems, as would rain. Ultrasonic systems have very poor off axis results, and whilst I could rotate the sensor, response would be very slow.
I can see that a 'square chunks of ferrite with orthogonal windings' would give very good results, especially for close distances. Your references to head tracking systems was most useful, although I am only sensing in one (horizontal) plane.
From the helmet tracking info it would at least seem that the idea is viable, which is a good start.
Many thanks
Best Regards
picclock