had—contrary to the CIA's expectations—successfully tracked the aircraft worried Eisenhower, but he approved the first Soviet overflight, Mission 2013 on 4 July. U-2 Article 347's main targets were the Soviet submarine construction program in Leningrad, and counting the numbers of the new Myasishchev M-4 "Bison" bomber. Soviet radar monitored the U-2 incursion into Soviet airspace in real-time, with radar tracking starting from the time the aircraft crossed into East German airspace. Soviet leader Nikita Khrushchev was informed immediately and was quite upset, believing correctly that the United States violation of sovereign Soviet airspace was casus belli. While contemplating appropriate retaliatory steps, he ordered Soviet Ambassador to Washington, Georgi Zaroubin, to protest vehemently to the U.S. State Department that very day, explaining that the recent trust-building to ease tensions between the two countries was undermined by the overflight provocations. A second flight on 5 July continued searching for Bisons, took photographs of Moscow (the only ones taken by the program), and flew over cloud-covered rocket factories at Kaliningrad and Khimki. Eisenhower knew from the earlier overflights that his hope of no Soviet detection was unrealistic, but ordered that the overflights stop if the aircraft could be tracked. The CIA found that the Soviets could not consistently track the U-2s and therefore did not know that Moscow and Leningrad had been overflown. The aircraft's photographs showed tiny images of MiG-15s and MiG-17s attempting and failing to intercept the aircraft, proving that the Soviets could not shoot down an operational U-2. Knutson recalled that the "constant stream of Russian fighters" trying to shoot down the U-2 during overflights was sometimes "so thick" that they interfered with photographs. Repeatedly failing for years to stop the aircraft embarrassed the USSR, which made diplomatic protests against the flights but did not publicize the penetration of Soviet territory. U-2 missions from Wiesbaden would depart westward in order to gain altitude over friendly territory before turning eastward at operational altitudes. The NATO Air Defence mission in that area included No. 1 Air Division RCAF (Europe), which operated the Canadair Sabre Mark 6 from bases in northeastern France. This aircraft had a service ceiling of 54,000 feet and numerous encounters between the U-2 and RCAF 'ZULU' alert flights have been recorded for posterity. "Bomber gap" disproven On 10 July, the Soviets protested what they described as overflights by a USAF "twin-engine medium bomber", apparently believing that it was a