Terrain contour matching systems compare their surroundings to what?

The correct answer focuses on the functionality of terrain contour matching systems, which utilize stored radar maps as a reference to compare the aircraft's surroundings. These systems employ radar to gather data about the terrain directly beneath the aircraft, ensuring that this information aligns with pre-existing terrain maps stored in the system. By matching the radar returns from the terrain to these stored maps, the system can accurately determine the aircraft's position and maintain safe navigation.

Stored radar maps are crucial because they contain detailed topographical information about the geography, including elevation changes, that the aircraft may encounter. By using these maps, the system enhances situational awareness and aids pilots in avoiding obstacles, especially in low visibility conditions or challenging terrains.

Other options, such as external human input, real-time satellite data, and open-source mapping data, do not function in the same manner as stored radar maps within the context of terrain contour matching systems. While they might serve in various navigational and tracking capabilities, they do not provide the specific, precise terrain information that radar-based systems utilize for effective contour matching.