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GPS jamming defense in the US is mostly provided by the FCC bringing the hammer of God down on people who run GPS jammers. The penalties for interfering with navigation signals are severe.

HAMMER OF GOD[1][2][3][4][5]

[1] https://www.fcc.gov/general/jammer-enforcement

[2] https://www.fcc.gov/document/fcc-fines-florida-driver-48k-ja...

[3] https://www.cnet.com/culture/truck-driver-has-gps-jammer-acc...

[4] https://www.fcc.gov/document/fcc-upholds-fine-jammer-used-bl...

[5] https://docs.fcc.gov/public/attachments/DOC-339560A1.pdf

GPS signal is comparatively weak. The satellites are far away, and terrestrial signal sources can be much stronger.

> They use GPS for precise positioning in flight

Large airliners, for the most part, don't. They use inertial navigation. GPS is a source of calibration for their inertial navigation system. There are some (non-precision) approaches that use GPS.