In aviation, the impact of threats is becoming increasingly significant, particularly for global navigation satellite system (GNSS). Two relevant GNSS threats are represented by jamming and spoofing. In order to evaluate the technological solutions to counter GNSS attacks, such attacks should be assessed by means of a proper GNSS threat simulator. This work shows the implementation and the testing results of a GNSS security impact simulator which injects the desired threat scenarios as a deviations on the GNSS actual measurements. The proposed simulator can be integrated in both real- and fast-time simulation environments. The provided results confirm the effectiveness of the simulator, and include in-flight demonstrations by means of a flight experimental vehicle.