The aim of the invention is to position an unmanned aircraft (1) that is used as a platform (10) for telecommunication or other scientific purposes at a predetermined altitude in the stratosphere. Said aim is achieved by an unmanned aircraft (1) comprising a pressurized balloon (11) which is filled with gas and supports the platform (10). The pressurized balloon (11) is arranged within an outer balloon (12; 12') which is provided with an aerodynamic external shape in the stratosphere and forms a low-pressure or high-pressure insulating chamber (20; 50) around the pressurized balloon (11), saidinsulating chamber (20; 50) being optionally filled with a medium.; Furthermore, heating and cooling means are provided for the medium circulating in the insulating chamber (20; 50) while means encompassing electrically driven propellers (15) or similar located outside the outer balloon (12, 12') are supplied for maintaining the position of the platform relative to the Earth. The negative effectsthat 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 materialandthe service life thereof is effectively extended.