[VLM]

You've sort of described kind of how some auth systems work around MITM by having a bidirectional conversation with salt while sharing a the same clock and talking about timestamps during their bidirectional conversations. That doesn't work very well in a broadcast environment where your only source of timestamps is the MITM and technology exists such that the MITM sounds just as good, but louder, than the genuine other guy.

You'd be surprised how many people think GPS is a bidirectional protocol like DME/TACAN or an aircraft radar transponder. Its actually a heck of a lot more like the old fashioned TRANSIT sats or VOR or LORAN or OMEGA, with a thin smear of spread spectrum on top to reduce the impact of simplistic jamming and it sends more metadata on top of the nav data than pretty much anything ever invented.

[shabble]

Speaking of OMEGA, there's a Navy training film from 1969 on Youtube[1] which explains some of the theory and is helpful in understanding where GPS came from.

[1] https://www.youtube.com/watch?v=7mFAemn1pSw

[jevinskie]

Wow, that is a really elegant, quite low-tech way of positioning!