What distance signal must be used by vessels to indicate their position in fog?

Vessels in foggy conditions are required to use a specific sound signal to indicate their presence and position for safety reasons. The choice that specifies alternating short-long blasts serves as a crucial signal that indicates a vessel's location while navigating in reduced visibility situations, such as fog.

When a vessel uses alternating short and long blasts, it communicates its movements effectively, allowing nearby vessels to ascertain its position and direction. This practice helps in preventing collisions and ensures that all vessels are aware of one another's proximity. The alternating pattern is distinctive and easily recognizable, making it useful in conditions where visual contact is limited.

This signal is clearly outlined in the International Regulations for Preventing Collisions at Sea (COLREGs), which emphasizes the importance of sound signals in navigation safety, particularly in low-visibility scenarios.