Although not directly involved in the fighting, the Intelligence Corps is considered one of the arms because it provides much of the information that a commander uses to make decisions on the battlefield. Intelligence personnel are responsible for compiling and analysing information about the enemy and their positions, movements, equipment and tactics, the terrain, and the weather. This information is gathered from reports from the fighting units, aerial photographs, prisoners of war, captured documents and equipment and radio intercepts. Intelligence personnel are also responsible for ensuring the security of their own units so the enemy has as little opportunity as possible to gain information about them. Jokes are often made about military intelligence being a contradiction in terms; intelligence soldiers are known as "spooks".