The official ceiling of the Lightning was kept secret. Low security RAF documents often stated "in excess of ". In September 1962, Fighter Command organised interception trials on Lockheed U-2As at heights of around , which were temporarily based at RAF Upper Heyford to monitor Soviet nuclear tests. Climb techniques and flight profiles were developed to put the Lightning into a suitable attack position. To avoid risking the U-2, the Lightning was not permitted any closer than and could not fly in front of the U-2. For the intercepts, four Lightning F1As conducted 18 solo sorties. The sorties proved that, under GCI, successful intercepts could be made at up to . Due to sensitivity, details of these flights were deliberately avoided in the pilot log books.