the reach of existing land-based aircraft that the Allies had; this was dubbed the Mid-Atlantic gap. At first, the British developed temporary solutions such as CAM ships and merchant aircraft carriers. These were superseded by mass-produced, relatively cheap escort carriers built by the United States and operated by the US Navy and Royal Navy. There was also the introduction of long-ranged patrol aircraft. Many U-boats feared aircraft, as the mere presence would often force them to dive, disrupting their patrols and attack runs. The Americans favored aggressive hunter-killer tactics using escort carriers on search and destroy patrols, whereas the British preferred to use their escort carriers to defend the convoys directly. The American view was that defending convoys did little to reduce or contain U-boat numbers, while the British were constrained by having to fight the battle of the Atlantic alone for the early part of the war with very limited resources. There were no spare escorts for extensive hunts, and it was only important to neutralize the U-boats which were found in the vicinity of convoys. The survival of convoys was critical, and if a hunt missed its target a convoy of strategic importance could be lost. The British also reasoned that since submarines sought convoys, convoys would be a good place to find submarines. Once America joined the war, the different tactics were complementary, both suppressing the effectiveness of and destroying U-boats. The increase in Allied naval strength allowed both convoy defense and hunter-killer groups to be deployed, and this was reflected in the massive increase in U-boat kills in the latter part of the war. The British developments of centimetric radar and the Leigh Light, as well as increased numbers of escorts, reached the point of being able to support U-boat hunting towards the end of the war, while earlier on, the advantage was definitely on the side of the submarine. Commanders such as F. J. "Johnnie" Walker of the Royal Navy were able to develop integrated tactics which made the deployment of hunter-killer groups a practical proposition. Walker developed a creeping attack technique, where one destroyer would track the U-boat while another attacked. Often U-boats would turn and increase speed to spoil the depth charge attack, as the escort would lose sonar contact as it steamed over the submarine. With the new tactic, one escort vessel would attack while another would track the target. Any course