Please see

L.-H. Lyu: SPACE WEATHER STUDY USING MULTIPOINT TECHNIQUES (Elsevier, UK, 2002).

Standard Positioning Service (SPS) is the level of GPS service intended for civilian applications, while Precise Positioning Service (PPS) is for military applications. PPS uses additional signals and was originally about 10 times more accurate than PPS (and has other performance advantages); however, it is only available to authorized users (requires keys). Industry has largely overcome the accuracy limitations of SPS; meanwhile the military will be transitioning to a new signal (M-code) in the next few years which will further improve its performance.