 Major differences from the MiG-19S included RP-1 "Izumrud" radar in the aircraft's nose, with small radomes in the centre and on the top lip of the air intake and an armament of two cannon in the aircraft's wing roots.From 1957, production of all weather fighters switched to the missile equipped MiG-19PM, with an armament of four K-5M air-to-air missiles, with the cannon removed.In 1955, following American introduction of high-altitude reconnaissance balloons and overflights by British Canberra aircraft, which could not be intercepted by existing aircraft, together with intelligence reports of the development of the Lockheed U-2 with an even greater ceiling, development began on a specialist high-altitude version of the Mig-19, the Mig-19SV, which entered limited production.