Aerostat
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An aerostat or lighter-than-air aircraft is an aircraft that relies on buoyancy to maintain flight. Aerostats include unpowered balloons and powered airships.
A modern aerostat used by the U.S. Department of Homeland Security, the Tethered Aerostat Radar System (TARS)
A free-flying hot air balloon
The Goodyear blimps are non-rigid airships.
Observation balloon
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An observation balloon is a type of balloon that is employed as an aerial platform for gathering intelligence and spotting artillery. The use of observation balloons began during the French Revolution
British observation balloon from 1908, typical of pre-WWI observation balloons
The battle of Fleurus, 26 June 1794, saw the first military use of an aircraft (L'Entreprenant).
German Parseval-Siegsfeld type balloon at Équancourt (September 1916). The rear "tail" fills with air automatically through an opening facing the wind.
Caquot type kite balloon, used by the Allies in the mid-latter part of WWI