What spacing is required when separating aircraft from prominent obstructions 40 miles or more from the radar antenna?

The correct spacing required for separating aircraft from prominent obstructions when they are 40 miles or more from the radar antenna is 5 miles. This requirement is based on ensuring sufficient horizontal separation to mitigate the risk of collisions that could occur due to radar inaccuracies, terrain, or the natural curvature of the earth, especially at such distances. The 5-mile standard is established to provide a buffer zone that enhances safety and provides adequate reaction time for both air traffic controllers and pilots when navigating around these obstructions.

In contrast, smaller spacings such as 2 miles, 3 miles, or 4 miles do not offer the same level of safety at extended ranges, where the precision of radar data may be less reliable due to signal attenuation or other factors. Therefore, the larger separation distance of 5 miles is crucial for maintaining safe operations in the vicinity of significant obstacles.