Spoofing feeds in a fake GPS signal that looks genuine. The receiver notices nothing and believes it is somewhere else — and that makes spoofing more dangerous than jamming.
In practiceIn 2017 more than twenty ships in the Black Sea reported that their GPS placed them at an airfield inland. In the war in Ukraine spoofing is used to make attack drones and cruise missiles stray from their targets (Lessons from Ukraine (open source)).
Jamming makes GNSS unusable; spoofing is more insidious — it makes the receiver calculate a wrong position or time while it believes everything is fine. It is the navigation counterpart of deceptive jamming (Deceptive jamming): not noise, but credible lies.
- Spoofing: the attacker transmits counterfeit GNSS signals with correct codes but manipulated time/position data. The advanced variant first lines up with the real signals (same position, slightly stronger), "captures" the receiver's tracking loops and then slowly drags the calculated position or time away — exactly the mechanism of a range gate pull-off (Range gate pull-off), but in the navigation domain.
- Meaconing: the attacker picks up real GNSS signals and retransmits them with a delay. He does not need to know the codes (so it works even against encrypted P(Y)/M!), but he can only impose an offset, not a completely arbitrary position.
A jammed system knows that it has no GPS and can fall back on alternatives. A spoofed system trusts a wrong position or time blindly — a weapon flies to the wrong coordinates, a drone is sent "home", or an entire network that relies on GPS time is thrown out of step without anyone noticing. Spoofing is therefore often the greater threat than jamming.
What helps against spoofing: signal authentication (Galileo OSNMA, and the encrypted military codes — what cannot be reproduced cannot be spoofed), checking the direction of arrival with a CRPA (CRPA — Controlled Reception Pattern Antenna — real satellites arrive from different directions overhead, a spoofer usually from one direction), power and plausibility checks, and cross-checking with an INS or a second navigation source.