War II era there had been a long debate about the relative merits of daylight versus night-time bombing. At night the bomber is virtually invulnerable (until the introduction of radar) but finding its target was a major problem. In practice, only large targets such as cities could be attacked. During the day the bomber could use its bombsights to attack point targets, but only at the risk of being attacked by enemy fighters and anti-aircraft artillery. During the early 1930s the debate had been won by the night-bombing supporters, and the RAF and Luftwaffe started construction of large fleets of aircraft dedicated to the night mission. As "the bomber will always get through", these forces were strategic in nature, largely a deterrent to the other force's own bombers. However, new engines introduced in the mid-1930s led to much larger bombers that were able to carry greatly improved defensive suites, while their higher operational altitudes and speeds would render them less vulnerable to the defences on the ground. Policy once again changed in favour of daylight attacks against military targets and factories, abandoning what was considered a cowardly and defeatist night-bombing policy. In spite of this change, the Luftwaffe continued to put some effort into solving the problem of accurate navigation at night. This led to the Battle of the Beams during the opening stages of the war. The RAF returned in force in early 1942 with similar systems of their own, and from that point on, radio navigation systems of increasing accuracy allowed bombing in any weather or operational conditions. The Oboe system, first used operationally in early 1943, offered real-world accuracies on the order of 35 yards, much better than any optical bombsight. The introduction of the British H2S radar further improved the bomber's abilities, allowing direct attack of targets without the need of remote radio transmitters, which had range limited to the line-of-sight. By 1943 these techniques were in widespread use by both the RAF and USAAF, leading to the H2X and then a series of improved versions like the AN/APQ-13 and AN/APQ-7 used on the Boeing B-29 Superfortress. These early systems operated independently of any existing optical bombsight, but this presented the problem of having to separately calculate the trajectory of the bomb. In the case of Oboe, these calculations were carried out before the mission at the ground bases. But as daylight visual bombing was still