GPS is a passive system. The satellites in orbit emit a constant location and identifier signal perceivable from the ground. A GPS positioning device looks for

GPS is a passive system. The satellites in orbit emit a constant location and identifier signal perceivable from the ground. A GPS positioning device looks for and reads those signals and then performs a triangulation calculation. Spoofing these signals means outputting identical but incorrect ones either from your own constellation of satellites or from ground-based arrays/emitters. Typically military hardware expects a certain level of spoofing and has it's own built-in countermeasures/security checks so effective jamming/spoofing means successfully navigating these countermeasures either through brute force or accurate intelligence.