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This is why missiles don't use GPS. (at least U.S. ones as far as I know)

They use a highly precise inertial measurement unit to track their location since launch.

They use GPS and a lot of different technique because the IMU need regular correction. But the main source of trust is the IMU. By jamming the GPS signal you can induce more drifting, but you would have to jam it for a long time to induce a very big drifting (and even them, it might be compensated by the other technique employed).

They use a combination of GPS, terrain recognition, and IMUs.

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