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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.