Jing, the main tracker unit transmits a magnetic field that essentially goes through all things non-metalic. The actual tracker picks up on the magnetic field and is able to send an orientation back to the main unit based off the physics of the magnetic field. Yes, magnetic trackers are still around (in fact Ascension still sells the Flock of Birds system). Many tracking systems purchased these days are usually purchased either to capture user motion (like for movies) or for simulation (like for military purposes). Magnetic trackers can be used for this *as long as there aren't many metallic sources near by* (it throws off the whole system)! So, many places that use tracking may have to resort to other options (such as optical tracking). However, magnetic tracking is still pretty common.

As for your second question, yes other things can be used as landmarks. In fact, this is what I'm doing in my research and I'll talk about it tomorrow.

Sunny, to answer your question, think about what would happen if you looked at a perfect square box with only one landmark. How would you know what side is which? So basically, the reason why they use multiple landmarks is to help know which side is which. The system could treat multiple landmarks on the same objects as connected but there is really no need to do this. For this particular system, the landmarks behave independently. So, to answer your question, if there are multiple landmarks on the same object, then they are seen as two different landmarks even though they are on the same object.