OpenRA - Red Alert Wiki

The Visibility mechanic determines what the player can see. There are two visibility impairing mechanics: Fog of War and Shroud

At game start, the map is covered by pitch-black shroud. The shroud is permanently removed for any area explored once by any of the player's units (or teammate units). Many players prefer to disable the shroud mechanism by ticking the "Explored Map" map option.

The Fog of War is a semi-transparent cover of the map. The fog of war is present wherever the player has no units.

Terrain can be seen through the fog of war, but enemy units cannot be seen. Units remove the fog of war in a circle around them, which has radius as big as their sight distance. This makes scouting important. When friendly units leave an area, the fog of war always returns.

The GPS allows players to see through the Fog of War and eliminates all shroud.

The gap generator can generate artificial shroud which is immune to the GPS.

The spy plane allows the player to temporarily see through the shroud and the fog of war on a chosen area on the map.

The communications center reveals the area around it in a 10-cell radius.