Original sentence: A proposed alternative to the geostationary relay is a special  solar-powered ultralight airplane, which would fly along a circular path above a fixed ground location, operating under autonomous computer control at a height of approximately 20.000 meters
Edited sentence: A proposed alternative to the geostationary relay is a special  solar-powered ultralight airplane, which would fly along a circular path above a fixed ground location, operating under autonomous computer control at a height of approximately 20,000 meters