Comment Re:GPS? (Score 1) 294
All a GPS repeater tells you is where the repeater receiving antenna is. Only good for rough positioning, but still very good for timing. This is why they are used in hangars and bus tunnels -- near enough is good enough.
TBMs are generally navigated by laser surveying instruments. This is a real example of the surveyor's craft, and even in ancient times (i.e. pre laser) tunnels generally met in the middle.