Ascension Island is an isolated volcanic island, 7°56′ south of the Equator in the South Atlantic Ocean. It is about from the coast of Africa and from the coast of South America. It is governed as part of the British Overseas Territory of Saint Helena, Ascension and Tristan da Cunha, of which the main island, Saint Helena, is around to the southeast. The territory also includes the sparsely populated Tristan da Cunha archipelago, to the south, about halfway to the Antarctic Circle. Ascension Island was discovered by the Portuguese navigator João da Nova in 1501 and named Conception Island, but was rediscovered in 1503 on Ascension Day by Alphonse de Albuquerque and renamed. Ascension Island was garrisoned by the British Admiralty from 22 October 1815 to 1922 and was an important refueling stop for ships and commercial airliners in the days of international air travel by flying boats. During World War II, it was an important naval and air station, especially providing antisubmarine warfare bases in the Battle of the Atlantic. The island is the location of RAF Ascension Island, which is a Royal Air Force station, a European Space Agency rocket tracking station, a British-American signals intelligence facility and the BBC World Service Atlantic Relay Station. The island was used extensively as a staging point by the British military during the Falklands War. Ascension Island hosts one of four ground antennas that assist in the operation of the Global Positioning System (GPS) navigational system (the others are on Kwajalein Island, Diego Garcia, and Cape Canaveral). NASA operates a Meter Class Autonomous Telescope (MCAT) on Ascension Island for tracking orbital debris, which is potentially hazardous to operating spacecraft and astronauts, at a facility called the John Africano NASA/AFRL Orbital Debris Observatory. The island's discovery is disputed. An undocumented claim states that it was discovered in 1501, either on 25th March or 21st May, by João da Nova, on his outward voyage to India.