Israel Aircraft Industries (IAI) plans to develop a geostationary balloon as a platform for ballistic-missile detection sensors. An unnamed US company will probably team with IAI. The idea is to develop a balloon able to operate at an altitude of 65,500-82,000ft (20-25,000m) with solar panels supplying power for an engine to drive a propeller keeping the balloon in place. The sensors would allow detection of missiles launched during or immediately after boost phase.
Source: Flight International