GPS has some intentional noise introduced into its signal to limit the accuracy of receivers - a cheap GPS unit would be able to say "I'm within a 100-foot circle of here". Military gear had the capability of removing the noise, so GI's could be "here" within a much smaller circle of error.
In 2000 or so, the military removed the noise from the signal so everyone could be "here", closer together.
BTW, GPS stands for "gets people somewhere".
There are encrypted versions of GPS.