the Soviets had begun development of greatly improved missile systems, notably the SA-5 'Gammon'. The Soviet Air Defence Forces (Protivo-Vozdushnaya Oborona, PVO) cleared the SA-5 for service in 1967; if deployed to Vietnam, it would have provided an additional risk to the A-12. The final Black Shield mission over North Vietnam and the Demilitarized Zone (DMZ) was flown on 8 March 1968. Good quality photography was obtained of Khe Sanh and the Laos, Cambodia, and South Vietnamese border areas. No usable photography was obtained of North Vietnam due to adverse weather conditions. There was no indication of a hostile weapons reaction and no ECM systems were activated. Final missions over North Korea In 1968, three missions were flown over North Korea. The first mission occurred during a very tense period following seizure of the Navy intelligence ship Pueblo on 23 January. The aim was to discover whether the North Koreans were preparing any large scale hostile move following this incident and to actually find where the Pueblo was hidden. The ship was found anchored in an inlet in Wonsan Bay attended by two North Korean patrol boats and guarded by three Komars. Chinese tracking of the flight was apparent, but no missiles were fired at the Oxcart. The second mission on 19 February 1968, was also the first two-pass mission over North Korea. The Oxcart vehicle photographed 84 primary targets plus 89 bonus targets. Scattered clouds covered 20 percent of the area, concealing the area in which the USS Pueblo was photographed on the previous mission. One new SA-2 site was identified near Wonsan. Retirement Even before the A-12 became operational, its intended purpose of replacing the U-2 in overflights of the Soviet Union had become less likely. Soviet radar systems increased their blip-to-scan ratios, which rendered the A-12 vulnerable. In any event, President Kennedy had stated publicly that the United States would not resume such missions. By 1965, moreover, the photoreconnaissance satellite programs had progressed to the point that manned flights over the Soviet Union were unnecessary to collect strategic intelligence. The A-12 program was ended on 28 December 1966 – even before Black Shield began in 1967 – due to budget concerns and because of the SR-71, which began to arrive at Kadena in March 1968. The twin-seat SR-71 was heavier and flew slightly lower and slower than the A-12. Ronald L. Layton flew the 29th and final