Refer to figure.
NAVSTAR GPS is the original GNSS (Global Navigation Satellite System). It uses a constellation of 24 satellites in 6 different orbital planes, orbiting at 20,200 km.

For a receiver to work correctly, it must be able to "see" 4 satellites, in that the signals from the satellites are line-of-sight only, and therefore cannot go over the horizon or through mountains, thick obstructions, etc. 4 satellites give enough "pseudorange" rings to get a suitable fix in 3D space of a receiver's position. Within low and mid latitudes there are usually more than 4 satellites in view of any receiver, however, as you get closer to the poles, the satellites in view become fewer and quickly drop below the 4 required. Extra range can be gained when at high altitude, as the horizon is much less restrictive there.

This reinforces the (nicely vague) answer of this question, that the "visibility varies dependent on the time and position of the receiver". It is not always best at the equator, nor is it the same everywhere around the globe.