Unmanned aircraft used as a platform for telecommunication or other scientific purposes at a predetermined altitude in the stratosphere. The unmanned aircraft includes a pressurized balloon filled with gas and that supports the platform. The pressurized balloon is arranged within an outer balloon provided with an aerodynamic external shape in the stratosphere and forms a low-pressure or high-pressure insulating chamber around the pressurized balloon. A heating and cooling system circulates medium in the insulating chamber and electrically driven propellers are located outside the outer balloon to maintain the position of the platform relative to the Earth. The negative effects that extreme differences in temperature have on the gas pressure in the pressurized balloon are thus largely cancelled such that the pressurized balloon can be made of a lighter and cheaper material and the service life thereof is effectively extended.