The Harrier GR.1 made its first flight on 28 December 1967. It officially entered service with the RAF on 1 April 1969 and the Harrier Conversion Unit at RAF Wittering received its first aircraft on 18 April. The aircraft were built in two factories—one in Kingston upon Thames, southwest London, and the other at Dunsfold Aerodrome, Surrey—and underwent initial testing at Dunsfold. The ski-jump technique for launching Harriers from Royal Navy aircraft carriers was extensively trialled at RNAS Yeovilton from 1977. Following these tests ski-jumps were added to the flight decks of all RN carriers from 1979 onwards, in preparation for the new variant for the navy, the Sea Harrier.