The sailors of old typically used flat maps to find their position, so three reference points would suffice. Since GPS devices move in three dimensions, you need a fourth satellite so that you can ...

GPS III space vehicles will introduce new capabilities to meet higher demands of both military and civilian users. It brings the full capability to use M-Code in support of warfighter operations.

GPS IIF satellite vehicles continue to modernize the GPS constellation while bringing new capabilities to both military and civilian users. GPS IIF SVs will bring into service the dedicated civil ...

These satellites will augment the 31-satellite GPS constellation currently in orbit, with the goal of ensuring global navigation coverage amid concerns about potential threats to space assets from ...