GPS Mil vs. Consumer tech.
Mil sets work on a different frequency and use a highly encrypted PN (Pseudo Noise) code, which, if not known by the receiver, prevents use of the inherently higher accuracy mil signals. The consumer grade PN code is, I think, 1023 bits long, while the mil one is "months" long, and is changed at shorter intervals than that, so there are never any repeated binary streams that would give a clue to decrypting it.
On the Civilian (consumer) side, there was something called "selective availability (SA) which caused the satellites to "lie" about what time it was, on a purely random basis, thus increasing the amount of error introduced at a receiver. One of the recent Presidents had that removed from service, primarily to allow aviation to have the best possible (non-mil) GPS accuracy for nav purposes.
SA was turned off, by the way, during the Desert Storm operation, to allow Civilian sets to be used by tanks, as there was a shortage of mil GPS receivers for deployment. And there are other stories and anecdotes, of course.
The phase-tracking used in attitude detection is a different animal. The wavelength at GPS frequencies is roughly (very) 25cm, so the theory predicts that one receiver moving vertically (e.g.) relative to a second one, toward or away from the satellite(s), would experience a detectable phase difference in the order of centimeters. Needs superfast processor and GPS chips, but it works....