The landings would be supported by the largest invasion fleet in history – 7,016 vessels in total. The Royal Canadian Navy contributed 121 vessels to the armada, including destroyers, frigates, corvettes, landing ships, minesweepers and torpedo boats. Four Canadian Tribal-class destroyers were in the Royal Navy's 10th Destroyer Flotilla, which joined other RN units in keeping the English Channel near Normandy clear of German naval units. Naval Force J had begun intense training for the invasion with the 3rd Canadian Infantry Division in February 1944, with a full-scale simulation of the invasion carried out on 4 May in Exercise Fabius. On D-Day itself, Force J, commanded from , was to bombard German defensive positions along the landing zone with everything from heavy-calibre cruiser guns to self-propelled artillery attached to landing craft. According to Canadian Army Historian C. P. Stacey, a light bombardment of the landing zone would commence "30 minutes before H Hour and continue for 15 minutes; heavy bombing would then begin on the flanks of the divisional attack, lasting until H Hour". Additional cover would be provided by Royal and Royal Canadian Air Force squadrons both before and on D-Day. A successful surprise invasion required total air superiority over the English Channel and Normandy. In the months preceding D-Day, the RAF Second Tactical Air Force attacked airfields, coastal garrisons, radar, railway lines and transport routes in order to soften the beach defences, as well as prevent the German Luftwaffe from mounting a serious challenge to air superiority over Normandy. By dawn, on 6 June, the RAF tactical air forces had 2,434 fighter and fighter-bomber aircraft with approximately 700 light and medium bombers to support them.