You need more advanced antennas and other methods to protect against jamming. These multi-element controlled-reception pattern antennas are larger and more expensive.
No one can check any signatures, if the signed message cannot reach an audience.
Perhaps an aperture could prevent omni-directional cross-talk and interference, forcing a receiver to only listen from a specific, selected line-of-sight source, but to simulate warfare, what if the enemy really does manage to completely destroy a resource?
A genuine disruption keeps everyone honest in the spirit of the exercise. It’s really down. Not just “what if”? No peeking. No cheating.
GPS is operated and maintained by the USAF. https://www.gps.gov
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.