The stationary observation balloon, or aerostat, had an advantage over aircraft in that it had a direct telephone line to the artillery battery, giving near real-time reconnaissance feedback during an attack. The armies deployed them no closer than three miles from the front, and two observers normally ascended to 3,600 feet. While fixed-wing aircraft used square maps, the balloon used fan-shaped maps due to their specific field of view. Armies did not begin an offensive without their balloon intelligence sensors on station. The old term “when the balloon goes up” meant an engagement was about to begin. Because it was a very important static reconnaissance platform with a telephone directly to the artillery batteries, they became important targets for fighters.