For this, the attack is from the operator of the GPS signals. There is no defense; the attacker could digitally sign the signals or even shut off the satellite.

For this, the attack is from the operator of the GPS signals. There is no defense; the attacker could digitally sign the signals or even shut off the satellite. For an attack in a foreign country, digital signatures only prevent false information. A digital signature does you no good if the interfering signal is far stronger than the desired signal. The desired signal may even go missing if the enemy attacks the satellite. Military users can choose to use encrypted GPS signals. This at least lets them know to ignore an attacker, but doesn't help with hearing the legit signal. Military users sometimes use highly directional antennas to avoid picking up jammers near the ground, but that is no help against an enemy that can put a jammer in their own satellite.