20 kW, and a pulse repetition frequency of 500 Hz. It had a maximum range of only , making it inferior to Chain Home. Furthermore, it could not accurately determine altitude, but it was a fully steerable and semi-mobile system. Freya was first successfully used on December 18, 1939 when two stations detected an approaching daytime raid on Wilhelmshaven by 22 RAF Vickers Wellington bombers at a range of 113 km and guided fighter planes toward them via radio. Only half of the Wellingtons returned to Britain undamaged, but the German fighters only reached the bomber after they had made their bombing run on ships in harbour. The performance of Freya left the Luftwaffe so impressed that, by the Spring of 1940, eleven Freya stations were installed to guard Germany's western border. After the invasion of France in 1940, additional Freya stations were built along the Atlantic coast. When Britain started its bombing raids, Hermann Göring ordered Colonel (later General) Josef Kammhuber to install an efficient air defence. This led to the so-called Kammhuber Line into which more Freya stations were incorporated. In the later course of the war, Freya devices turned out to be vulnerable to chaff, along with other countermeasures, which meant they could still be used for early warning, but no longer for guiding fighter planes. British bombing raids could also be organized such that the Kammhuber Line could be overwhelmed in massed raids. British intelligence One of the first to give British intelligence any details about the Freya Radar was a young Danish Flight Lieutenant, Thomas Sneum, who, at great risk to his life, photographed radar installations on the Danish island of Fanø in 1941. He brought the negatives to Britain in a dramatic flight which is fictionalized in Ken Follett's novel Hornet Flight. Sneum's deed is also mentioned in R. V. Jones's Most Secret War as a 'most gallant exploit' and is one of the featured stories in Courage & Defiance by Deborah Hopkinson. Further development FuMG 401: For experiments with beam reflection on the ground, leading to a change in elevation angle, a Freya antenna array was installed on a wooden support so it could slide up and down. This enabled Freya to detect the target's altitude without the aid of other devices (such as Würzburg radars). FuMG 41: In an attempt to increase the range without changing the transmitter, several Freya antenna arrays