to fly in (perennially bad) conditions off the Grand Banks Coastal Command would never have attempted, U-boats could trail convoys beginning very soon after departure from Halifax. Without air-to-surface vessel radar (ASV), the almost "perpetual fog of the Grand Banks also allowed pack operations to penetrate within a couple of hundred miles of Newfoundland, while aircraft patrolled harmlessly above", visual detection impossible. A means of detecting surfaced submarines at night, when they were at their most vulnerable, recharging batteries, and felt most safe, was a top priority for Coastal Command. ASV gave it to them. The previous AI.II (Mark 2 Airborne Interception) radar became ASV.II (Air to Surface Vessel Mark 2) fitted in Coastal Command aircraft. Coastal Command priority for it, however, ranked behind Fighter Command's night fighter units. ASV.II's 1½-metre wavelength (actually 1.7 m, 176 MHz), mid-VHF band emissions meant however, that a submarine was usually lost in sea return before it came in visual range, at around a mile (1,850 m), by which time it was already diving. In response, the Leigh light was developed. Though it had to overcome Air Ministry indifference, and only entered service in June 1941, it proved very successful. This, however, required a large aircraft, such as the Wellington or Liberator, to carry the generator needed to power the light, and most of Coastal Command's aircraft were incapable of it, nor were Bomber Command inclined to turn over anything better. Moreover, the Germans developed Metox, which picked up ASV's radar pulses before it was able to detect a submarine at all, rendering it useless. The appearance of H2S three gigahertz-frequency (10 cm) radar changed that, and the combination of H2S (as ASV.III) and Leigh light proved lethal to U-boats. Harris, however, denied Coastal Command any allocation of H2S systems, claiming Bomber Command needed it to find targets, in preference to Gee and Oboe, while arguing Coastal Command might lose it to the Germans. Churchill backed him up. Marshal John Slessor, head of Coastal Command, countered Bomber Command also risked having it fall in enemy hands, and having the Germans produce a countermeasure against it, before Coastal Command ever got to use it. In the event, this was exactly what happened. The first ASV.III was fitted to a Coastal Command Wellington at Defford in December 1942, with twelve based at Chivenor by February 1943, while a copy of H2S was lost 2/3 February when