Metox

Metox, named after its manufacturer, was a pioneering high frequency very sensitive radar warning receiver (RWR) manufactured by a small French company in occupied Paris, which could detect ASV (Air to Surface Vessel radar) transmissions from patrolling Allied aircraft. It is not clear whether the design was German or French or both. It was installed on German U-boats during World War II from 1942.

From July 1940 onwards, the British fitted the RAF Mk II AI (Airborne Interception) radar into Coastal Command aircraft for use as the MkII "1½-metre ASV". The radar's known AI weaknesses — problems due to land clutter and inability to determine height effectively, which caused its failure in night fighters — were no handicap in this new role. With two range scales, 0–9 and 0–36 miles, it could detect surfaced U-boats at up to 12 miles (19 km) and land at up to 70 miles, though a typical U-boat detection range was 5 miles. The radar had a fairly crude display by today's standards, but was able to give the range and an approximate direction within an arc either side of the aircraft heading. Returns were lost in sea clutter once the aircraft was within about 1 mile of the U-boat, but usually by then, the aircraft was within visual range—and the U-boat was well into a crash dive.

To counter this, Wing Commander Humphry de Verde Leigh (later OBE DFC AFC) developed the Leigh light, effectively a powerful floodlight steered by the ASV radar. This allowed ASV radar equipped aircraft to search for U-boats at night. The U-boat was initially tracked by the radar with the light following the radar track but switched off. Once the returns were lost, the light would be switched on and the U-boat would be bathed in light and very vulnerable. The first successful attack was on the U-502 on 5 July 1942. The sudden light was often the first indication that the U-boat had been found and the Leigh light was initially very successful, particularly in the Bay of Biscay.

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