Editing Battle of the Atlantic (section)

===Metox receiver===
[[File:Depth charges explode astern of HMS STARLING of the 2nd Escort Group in the Atlantic, January 1944. A21992.jpg|thumb|upright|Depth charges detonate astern of the sloop {{HMS|Starling|U66|6}}. She participated in the sinking of 14 U-boats throughout the war]]

By August 1942, U-boats were being fitted with radar detectors to enable them to avoid ambushes by radar-equipped aircraft or ships. The first such receiver, named [[Metox]] after its French manufacturer, could pick up the metric radar bands used by the early radars. This enabled U-boats to avoid detection by Canadian <!--and American? cite them having obsolete radars, please--> escorts, which were equipped with obsolete radar sets,<ref>''North Atlantic Run''; ''The Great Naval Battle of Ottawa''</ref>{{page needed|date=May 2013}} and allowed them to track convoys where these sets were in use.

It also caused problems for the Germans, as it sometimes detected stray radar emissions from distant ships or planes, leading U-boats to submerge when they were not in danger, preventing them from recharging batteries or using their surfaced speed.

Metox provided the U-boat commander with an advantage that had not been anticipated by the British.  The Metox set beeped at the pulse rate of the hunting aircraft's radar, about once per second.  When the radar operator came within {{convert|9|miles|km}} of the U-boat, he changed the range of his radar.  With the change of range, the radar doubled its pulse frequency and as a result, the Metox beeping frequency also doubled, warning the commander that he had been detected and that the approaching aircraft was nine miles away.