There is cryptographic hash so that you can't send false data, but replaying the existing data but time delayed is still possible. The timing is how the triangulation fix is done. More sources of data, the harder it is to spoof. If you only have 3 or 4 fixed points, it's easy to spoof. 6 to 10 and it becomes extremely difficult.
Also military GPS is far more accurate than civilian GPS by a factor of 10x.
I'm speaking out of ignorance here, but why couldn't you just use a quantitized time value (to account for transmission lag) as a part of the data that's hashed? It works great to prevent replay attacks with OAuth.
nope. it's time that is the factor for triangulation. you always take the first time you received from the one source and you can discard certainly something way out side of what you expect. It's how you defeat the concreate canyon effect in big cities where signals are bouncing off everything.
However if you have a low strength signal from one source and you receive a frame that is a second or a two off, it can cause you to think you are miles away from where you think you supposed to be.
"military GPS is far more accurate than civilian GPS" - is this still true? I don't know how my iPhone GPS triangulates me, but it usually has me located to within 3-5' at all times when I'm walking home.
That has nothing to do with why the iphone is so fast.
Civilian transmission rates on L1 are 10x slower than the military L2 frequency. It takes a full 6 seconds to get a single frame downloaded and that is also hoping your almanac is up to date. (Thankfully AGPS can provide the almanac data by other menas.)
The military version it takes .6 seconds to do the same it takes us civilians to do in 6 seconds. The SA drift is gone now but still the slower speed makes fixing a perfect position slightly more difficult and less accurate.
Also military GPS is far more accurate than civilian GPS by a factor of 10x.