Fundamentally, the basic GPS calculation is finding the intersection of 4 hyperspheres. Picturing this is obviously a nightmare so consider the much simpler case: You’re on the ground listening to two town bells that ring exactly at noon and looking at your watch to see when you hear them. You hear the first bell at 12:00:02 and thus conclude you are 2 seconds of sound travel from that bell. You hear the second at 12:00:03 and thus conclude you are 3 seconds of sound travel from that bell. Draw that on a map–you have two circles. Unless they are only barely touching two circles intersect in **two** points and you have no way to know which of the two it is.

GPS must also find altitude so you need another signal source and GPS requires precision far, far beyond what any watch can deliver so you must also figure out where you are in time, thus requiring another signal source. As with the simple case this still gives two possible locations. Fortunately, one of those points is far from the Earth and thus rejected, you take the answer that’s on Earth. I can easily see a sloppy programmer rejecting the solution with a negative altitude as not being on Earth.

Now, with 5 or more satellites there’s normally (degenerate cases can occur, if more than two satellites are in a perfect line you only get two satellites worth of information from them–but note that while line-ups do occur they don’t persist) only one solution–but if the receiver solved it for 4 and then simply used the others to refine the answer….