Editing Chain Home (section)

==Detection, jamming and counter-jamming==

===Early detection===
From May to August 1939 the [[LZ 130 Graf Zeppelin|LZ130 ''Graf Zeppelin II'']] made flights along Britain's North Sea coast to investigate the 100-metre-high radio towers that were being erected from [[Portsmouth]] to [[Scapa Flow]]. LZ130 performed a series of radiometric tests and took photographs. German sources report the 12&nbsp;m Chain Home signals were detected and suspected to be radar; however, the chief investigator was not able to prove his suspicions.<ref>{{harvnb|Pritchard|1989|p=55}}. Many of the German experts believed radar at 12&nbsp;m wavelengths was not likely, being well behind the current state of the art in Germany.</ref> Other sources are said to report different results.{{efn|Claims have been made that the LZ130 missions (1) failed to detect any radio emissions of interest at all; (2) failed to identify the true purpose of the new British stations, concluding the towers were for long-range naval radio communication, not radio location; and (3) failed to identify the origin of the signals as the towers that had aroused the interest in the first place. It is agreed that German scientists were not certain of British radar defences, and these claims may reflect the debate among those scientists.}}

During the Battle of France, the Germans observed 12&nbsp;m pulse signals on the western front without being able to recognize their origin and purpose. In mid-June 1940, the ''[[Deutsche Versuchsanstalt für Luftfahrt]]'' (DVL, German Aeronautic Research Institute) set up a special group under the direction of Professor von Handel and found out that the signals originated from the installations on the coast of the English Channel.<ref>Gerhard Hepcke, [http://www.radarworld.org/radarwar.pdf "The Radar War"]</ref>

Their suspicions were finally proven in the aftermath of the [[Battle of Dunkirk]], when the British were forced to abandon a mobile [[GL Mk. I radar|gun-laying radar]] (GL Mk. I) station in Normandy. [[Wolfgang Martini]]'s team of specialists was able to determine the operation of the system. GL was a rather crude system of limited effectiveness, and this led the Germans to have a dim view of British radar systems. However, an effective system requires more than just the radar; plotting and reporting are equally important, and this part of the system was fully developed in Chain Home. The Germans' failure to realize the value of the system as a whole has been pointed to as one of their great failings during the war.

===Anti-jamming technologies===

The British had been aware that the Germans would determine the purpose of the system and attempt to interfere with it, and had designed in a variety of features and methods in order to address some of these issues even as the first stations were being built. The most obvious of these was CH's ability to operate on different frequencies, which was added to allow the stations to avoid any sort of continuous-broadcast interference on their operating frequency. Additionally, the Interference Rejection Unit, or IFRU, allowed the output of the intermediate stages of the amplifiers to be clipped in an attempt to finely tune the receiver to the station's own signals and help reject broadband signals.

More complex was a system built into the CH displays, implemented in order to remove spurious signals from unsynchronized jamming pulses. It consisted of two layers of phosphor in the CRT screen, a quick-reacting layer of zinc sulphide below, and a slower "afterglow" layer of [[zinc cadmium sulphide]] on top. During normal operation the bright blue signal from the zinc sulphide was visible, and its signal would activate the yellow zinc cadmium sulphide layer, causing an "averaged" signal to be displayed in yellow. To filter out jamming pulses, a [[Color gel|yellow plastic sheet]] was placed in front of the display, rendering the blue display invisible and revealing the dimmer yellow averaged signal. This is the reason many radars from the War through to the 1960s have yellow displays.

Another method was to use range-only measurements from multiple CH stations to produce fixes on individual targets, the "Chapman method". To aid this task, a second display would be installed that would be fed the Y-axis signal from a distant CH station over telephone lines. This way the operator could directly compare the two signals, eliminating the delays if this information was transmitted by voice. This system was never required.

===First attempts, halting follow-up===
When jamming was first attempted by the Germans it was in a much more clever fashion than had been anticipated. The observation that the transmissions of the individual stations were spread out in time, in order to avoid mutual interference, was exploited.<ref name=war>{{cite web|url=http://www.radarworld.org/radarwar.pdf |title=The Radar War by Gerhard Hepcke Translated into English by Hannah Liebmann page 8-9 |access-date=10 February 2013}}</ref> A system was designed to send back spurious broadband pulses on a chosen CH station's time slot. The CH operator could avoid this signal simply by changing their time slot slightly, so the jamming was not received. This caused the station's signals to start overlapping another's time slot, so that station would attempt the same cure, affecting another station in the network, and so forth.

A series of such jammers were set up in France starting in July 1940, and soon concentrated into a single station in Calais that affected CH for some time. However, the timing of these attempts was extremely ill-considered. The British quickly developed operational methods to counteract this jamming, and these had effectively eliminated the effect of the jamming by the opening of the [[Battle of Britain]] on 10 July. The Germans were well on their way to develop more sophisticated jamming systems, but these were not ready for operation until September. This meant that the CH system was able to operate unmolested throughout the Battle, and led to its well-publicized successes.<ref name=war/>

By the opening of the Battle in July the German ''Luftwaffe'' operational units were well aware of CH, and had been informed by the DVL that they could not expect to remain undetected, even in clouds. In spite of these warnings, the ''Luftwaffe'' did little to address this and treated the entire topic with some level of disdain. Their own radars were superior to CH in many ways, but had proven to be only marginally useful. During the [[Battle of the Heligoland Bight (1939)|Air Battle of the Heligoland Bight]] in 1939, a German [[Freya radar]] detected the raid while it was still an hour away from its target, yet had no way to report this to any of the fighter units that could intercept it. Getting the information from the radar to the pilots in a useful form appeared to be a difficult problem, and the Germans believed the British would have the same difficulty and thus radar would have little real effect.

Some desultory effort was put into attacking the CH stations, especially during the opening stages of the Battle. British engineers were able to quickly return these units to service, or in some cases simply pretend to do so in order to fool the Germans into thinking the attacks failed. As the pattern of these attacks became clear, the RAF began to counter them with increasing effectiveness. The [[Junkers Ju 87]] [[dive bomber]]s were subjected to catastrophic losses and had to be withdrawn from battle. The Germans gave up trying to attack CH directly on any reasonable scale.<ref name=war/>

Thus, CH was allowed to operate throughout the Battle largely unhindered. Although communications were indeed a serious problem, it was precisely this problem that the Dowding system had been set up to address, at great expense. The result was that every British fighter was roughly twice or perhaps more as effective than its German counterpart. Some raids were met with 100% of the fighters dispatched successfully engaging their targets, while German aircraft returned home over half the time never having seen the enemy. It is for this reason that Churchill credits Chain Home with winning the Battle.

===Spoofing jammers, jitter===

A second jamming system was eventually activated at [[Cap Gris Nez]] in September, using a system that triggered its signal in response to the reception of a pulse from CH. This meant that the system responded to the CH station even if it moved its time slot. These systems, known as ''Garmisch-Partenkirchen'', were used during [[Operation Donnerkeil]] in 1941. Further improvements to the basic concept allowed multiple returns to be generated, appearing like multiple aircraft on the CH display.

Although these new jammers were relatively sophisticated, CH operators quickly adapted to them by periodically changing the pulse repetition frequency (PRF) of their station's transmitter. This caused the synchronized jamming signals to briefly go out of synch with the station, and the blips from the jammers would "jitter" on the screen, allowing them to be visually distinguished. The "Intentional Jitter Anti-Jamming Unit", IJAJ, performed this automatically and randomly, making it impossible for the German jammers to match the changes.

Another upgrade helped reject unsynchronized pulses, supplanting the two-layer display. This device, the "Anti-Jamming Black-Out" unit, AJBO, fed the Y-axis signal into a delay and then into the brightness control of the CRT. Short pulses that appeared and disappeared were muted, disappearing from the display. Similar techniques using [[acoustic delay line]]s, both for jamming reduction and filtering out noise, became common on many radar units during the war.

===Klein Heidelberg===
{{main| Klein Heidelberg}}
The Germans also made use of CH for their own passive radar system, known as ''Klein Heidelberg''. This used CH's transmissions as their source, and a series of antennas along the Channel coast as the receiver. By comparing the time of arrival of the signals from a selected aircraft, its range and direction could be determined with some accuracy. Since the system sent out no signals of its own, the allies were not aware of it until they overran the stations in 1944. Most of the stations had only just been built when they were overrun.<ref>{{cite tech report |first1=Nicholas |last1=Willis |first2=Hugh |last2=Griffiths |title=Klein Heidelberg – a WW2 bistatic radar system that was decades ahead of its time |url=https://www.researchgate.net/publication/229030676_Klein_Heidelberg-a_WW2_bistatic_radar_system_that_was_decades_ahead_of_its_time }}</ref>