The GPS need to be linked to satellites to exchange signals with them and know your position and how fast it changes.
The more satellites you get at your disposal, the better positioning accuracy you get (you need at least three to position you, and one more to eliminate the errors).
However, their position is of importance, too: the more evenly they're distributed in relation to the receiver (your phone, in this case), the more accurately they're able to position you.
The official US Government information states
that well-designed GPS receivers have been achieving horizontal accuracy of 9.8 ft. (3 m.) or better 95% of the time, while a regular smartphone is typically accurate
to within a 16 ft. (4.9 m) radius under open sky
Which takes us to the next point.