www.duxford-update.info     

  The first of a series of descriptions of

  early examples of electronics warfare

 

German Bomber navigation beams used over the U.K. in the early part of WWII.

During the early stages of WWII, both the RAF and the German Luftwaffe,  discovered that accurate night navigation over hostile blacked-out territory, was an extremely difficult skill to master.

In 1940, the German air force introduced a novel radio beam bombing system. The concept was developed into three varieties of beam types, each a refinement on the previous approach. All used a technique based upon the Lorenz blind landing approach system, already in use in Germany before the war.

 

Surfed in? Click here to display the nav. buttons

Heinkel 111 of KGr 100 with X-Gerat receiver

Sources

 

 

Thanks to early enigma decrypts from the code breakers at Bletchley Park, British Technical Intelligence began to receive clues that a radio based night navigation system was being used to guide the early Luftwaffe bombing raids over central Britain. The fascinating story of how the secrets of the beam system were gradually pieced together, and how effective counter measures were conceived and put into action, is told in the book “Most Secret War” by R.V. Jones. My account concentrates on the technical aspects of the story. I recommend R.V. Jones book for those also interested in the subtleties of the discovery of the beams and the politics and scientific dogma that nearly got in the way of the efforts to understand and counter this very real threat to British industry in the darkest days of WWII. My other highly recommended source is Alfred Price's book " Instruments Of Darkness". This contains more technical details of the beam performance and gives a realistic view on the effectiveness of the countermeasures. It also outlines the development of the beam systems by Lorenz and Telefunken before the war began. 

 
 
 1   Knickebein

We begin with an overview of the radio beam technique used to get a useful degree of accuracy over distances in excess of a hundred miles from the transmitting station:

Even with a massive antenna, it is very difficult to obtain a radio beam width much narrower than 5 degrees, with the radio frequencies in use at that time. At a distance of 150 miles, a 5 degree wide beam would be 13 miles wide .............. not really good enough to find a town let-alone a specific factory!  So how did they solve the problem?

 
The solution was to create two identical radio beams, side by side, with a small angular offset. This gave a narrow overlap region midway between the radio beams that a bomber could be directed along.

 

 
 
This technique was in use before the war, but at much shorter range, to provide a blind landing system at major airports. In Germany it was known as the Lorenz system and most German medium bombers were equipped with Lorenz receivers and antennas to assist in night and bad weather landings. The system operated on a radio frequency of around 30 MHz.  A simple amplitude modulated dots and dashes system was used to let the pilot know if he was drifting to either the right or left of the beam centre. One beam was modulated with dots (at 1150Hz audio frequency). The other beam was modulated with dashes, synchronised to appear in the gaps between the dots. If the pilot was midway between beams, he received a continuous 1150Hz tone in his head-phones. If he drifted one way, dots began to become apparent above the tone. If he drifted the other way, dashes became apparent. With very little practice, it was possible to maintain a course midway between the beams. However, to extend the range over hundreds of miles, a massive beam antenna array with a high power transmitter was needed. In the aircraft, the Lorenz radio receiver also needed modifications to increase its sensitivity. This, in outline, was the first of the bomber beams. Developed by the Telefunken company, the Germans gave it the codename "Knickebein". This roughly translates as the "crooked leg" and is thought to refer to the plan view of the enormous rotatable antenna array used at the beam stations.
 

He 111 at the RAF Museum Hendon

  

Duxford's He 111 awaiting restoration
 
The maximum range at which an aircraft could receive the beam is limited by the curvature of the earth, however the 30MHz radio signal experiences some diffraction and can just be detected by an aircraft flying at 20,000 ft at approximately 270miles from the beam antenna. The Heinkels and Dorniers operated by the Luftwaffe at the beginning of the war could not fly much higher than this when still carrying their bombload. The first Knickebein antennas were erected in Germany at Kleve, near the Dutch border and at Stollberg just South of the Danish border.

 

  

 

 

 

The original Knickebein antennas at Kleve and Stollberg (Bredstedt) were huge structures 100ft high and 300ft wide, mounted on bogies on a circular railway track

  
To create a a pair of narrow radio beams at 30MHz takes a very large antenna array. The first Knickebein antennas were enormous structures mounted upon a girder framework supported on railway bogies using a circular track to permit the whole structure to be rotated, to direct the beam towards the bomber's target. The antennas were over 100ft in height and in excess of 300ft wide. A phased array of multiple vertical dipoles was used to create twin beams with a narrow overlap zone of around one third of a degree in width. After the fall of France, beam stations with somewhat smaller antennas were also set up much closer to Britain on the Cherbourg peninsular. Because of the shorter distances involved, the effective beam widths could be reduced to much less than a kilometre over the South of England.

 

 

 2  Finding the Beams

The British scientific intelligence service was actively looking for any information that might reveal the progress made by German forces in the use of radar systems. Inputs came from the code-breakers at Bletchley Park, from interrogation of captured Luftwaffe aircrew and from analysis of the electrical equipment found in crashed German bombers.  From these inputs RV Jones and his colleagues began to pick up hints that another form of radio based system was being used to guide bomber aircraft over the U.K. Enigma decrypts and aircrew notes drew attention to the huge antenna system at Kleve in Germany.

Analysis at Farnborough of the blind landing system recovered from a crashed Heinkel, revealed a high sensitivity receiver add-on to the basic Lorenz instrument. With 5 pentode valves, the extra sensitivity was well beyond anything required for local airfield use. The pieces were coming together....at last an aerial search was organised to hunt for beams with a Lorenz signature. An RAF Anson equipped with a Hallicrafters S-27 radio receiver covering the 30MHz band used by the German Lorenz receivers, began to fly up and down the East coast of England in search of the beams. On the third day of the search, the crew successfully located not just one, but two Lorenz type beams intersecting over Derby (home of Rolls Royce Aero Engines). One beam was from a source in the direction of Kleve and the other from Stollberg. The cat was out of the bag!

 

(Click on the Anson photos to display the full size pic)

 

 

Duxford's  Anson

Inside the Anson: Pilot

 

The Hallicrafters  S-27 radio receiver

would have been in this racking

Navigator behind the pilot

 

3 Jamming the Knickebein beams

As soon as the existence of the beams had been confirmed, steps were taken to set up an organisation with the responsibility of neutralising their effect via electronic counter measures. This unit was known as 80 Wing. However, the expertise to design the necessary jamming equipment was found at the Telecommunications Research Establishment, where a small team was under the direction of a brilliant young physicist, Dr Robert Cockburn.

Work was immediately started on purpose built mobile jamming transmitters, but these would take a few weeks to develop and produce (even in wartime). An interim solution was found in the form of adapted medical diathermy units. These were modified to generate radio noise on the 30MHz Knickebein frequency band.

The German beams were given the code name "headache", so it was no surprise to find that when the purpose built jamming transmitters were introduced, they were given the code name "aspirin"!

The Aspirin units were powerful transmitters amplitude modulated with dashes on the same audio frequency and at the same pulse repetition frequency as the Knickebein beam system. The radio frequency could be adjusted to any of the frequencies used on the Knickebein transmitters. In the presence of the jamming signal, the bombers tended to head away from the dashes beam in search of the dots transition region and soon lost the original beam. These began to have a significant impact on the effectiveness of the beams.

The Ansons (now attached to 80 Wing) would locate the beams and measure their operating frequency, in the early evening of a raid. The jamming units would then be moved into position and set to the appropriate frequency. With the target for the night raids established, it was also sometimes possible to bolster local defence by moving in additional Anti Aircraft guns. Although the early night fighters (Blenheim 1F and Defiants) had yet to become effective, they could also be made aware of the target area.

(Note: As AI MkIV equipped Beaufighters and effective ground controlled interception radar and control centres were introduced in early 1941, things became very different!)

 

 

Straight from the head-phones!

On the right you will find link buttons that run a series of Javascript animations with sound files.  These are intended to give an impression of the sound the bomber pilot might have heard in his headphones as he followed the beam.

The first is with no jamming signals. Next the impact of a local noise jammer is shown and finally the effect of a high power "Aspirin" jamming transmitter which is much more effective at hiding the real beam signals.

 

Beam riding He 111

 

 

 
 

Click to hear He111 pilot head phones signals

 

Click to hear the effect of noise jamming

 

Click to hear the the effect of an aspirin jamming transmitter

 

The Ansons of 80 Wing would have intercepted the beams by flying across them. The last button indicates the sound the radio operator would be looking for.

Click to hear Anson radio op head phone signals

Beam hunting RAF Anson

Some Limitations:   (aka my excuses)

Unfortunately the aircraft speed in the animations varies depending on what else your computer is up to, but the sound file plays at a consistent speed.  This means you may find the sound is not well synchronised with the aircraft location.  I can see I'm going to have to learn how to programme in "Flash"!

If you would like to minimise these effects: you could turn off any other programmes you have running and pause any background music you may be playing.

The sound files are MP3, so you need a player that can handle these. I find it works best with Internet Explorer and any player such as Windows Media Player, Real Player or Quicktime. On the Firefox browser, I can only get it to work using the Quicktime Player and the animation is not as smooth!

If you use a browser with a magnification setting...please set it to 100%. The windows should display as 1000x600 pixels and should close automatically when the aircraft reaches the window edge.

 

 
4 X-Gerate

Developed in parallel to Knickbein by a team lead by Dr Hans Plendl working with the Lorenz company, the X-Gerat system used a similar overlapping beam concept, but made several refinements, making the system somewhat more accurate than Knickebein.  Beam stations for X-Gerate were set up in occupied Europe, as close to the U.K. as the geography permitted. A  higher radio frequency band in the region of 70MHz was used. This enabled smaller rotatable antenna arrays to be utilised.

There were 3 director beams, all using different radio frequencies. First, a short range broad beam to ease the initial beam acquisition. Next were two narrow beams, configured to provide minimum beam widths at medium and longer ranges from the beam station. The effective beam width over the target was significantly narrower than Knickebein.

Three cross beams were also utilized, to provide a semi-automated bomb release capability. The first cross beam was fairly broad and provided a wake-up-call, warning of the proximity of the second narrower beam. Upon crossing the second beam, the bomber crew started a clockwork timer mechanism. Upon crossing the third cross-beam, another lever on the timer was set. The timer used the elapsed time between the second and third cross beams to  measure the aircraft's ground speed and this determined the time period after the third beam crossing, before the timer mechanism automatically released the aircraft's bomb load. (Flying altitude was pre-programmed into the device.) 

The beams were modulated in a similar way to Knickebein, but with a subtle difference which was initially missed. Very narrow audio filters were used to minimise noise distraction. In addition to headphones, the pilot was provided with a"kick" meter in which an arrow pointer kicked to the right or left to indicate in which direction course corrections should be taken.

 
The X-Gerate equipment was installed exclusively in the Heinkel 111s of  KGr 100.  These aircraft were intended to act as pathfinders for the main bomber force.

KGr 100 aircraft could be identified by the 2 large X-Gerate antennas on the rear fuselage and by the unit identity code of "6N". (The forward antenna was a standard fit on the He111)

 

5 Jamming of the X-Beams

The similarity of the X-Gerate director beams and cross beams, to those used by Knickebein meant that a similar jamming philosophy could be used.  New high power jammer transmitters were quickly designed and built, to operate on the higher frequency 70MHz band used by X-Gerate. These were modulated as before, with dashes only. The new jammers were code-named "Bromide" and soon reasonable numbers of the units were in service.  However, analysis of KGr 100 raids revealed that the new jammers were not having a significant impact on the accuracy of their bombing, which for the time was still remarkably good. After the notorious raid on Coventry, X-Gerate units were recovered from a crashed KGr 100 Heinkel. To their horror, the analysts discovered that the modulation frequency used on X-Gerate was 2kHz. This new modulation frequency had prieviously been incorrectly measured as 1500Hz. The narrow band audio filters fitted to the receivers meant that the 1500Hz modulation of the bromide jammers went largely un-noticed by both crew and the kick meter. The bromide transmitters were hastily modified to generate the correct tone and the impact on KGr 100 bombing accuracy was soon apparent. However, the frequency calibration of the receivers used to monitor the beam radio frequencies in the beam spotting Ansons, provided an ongoing problem. Deficiencies in calibration often resulted in the bromide transmitters being tuned on the adjacent radio channel to the actual beams, due to inaccuracies in the frequency measurements. This required considerable work before a reliable pattern of jamming was achieved.

 

6 Y-Gerate

A further variation on the bomber beams was largely anticipated by the British Scientific intelligence team.  The new Y-Gerate beam system was again master minded by Dr Hans Plendl.  This system used only director beams. The distance travelled along the director beam was established via radio signals sent out to the bomber from the beam station. These ranging signals were received and then automatically transmitted back (i.e. transponded) by the bomber, to the beam station, on a slightly different radio frequency. The time taken for the signal to make the return trip was measured very accurately and gave the precise position of the aircraft along the beam.  However, a direct measurement of time to the required accuracy proved problematic....... so:

My thanks go to Onno van Gent for advising me on the actual approach used in the Y-Gerat system:

The technique used to establish the bomber's distance from the beam station involved applying a frequency modulated tone to the transmission from the beam station. The transponder in the aircraft retransmitted the same modulating signal back to the beam station (on a slightly different carrier frequency). By comparing the phase of the modulating signal returned from the aircraft to the phase of the transmitted modulating signal at the beam station, it was possible to accurately deduce the total round trip distance travelled by the radio signal.

Initially the modulating frequency was set to 300Hz. 

Radio pulses travel at the speed of light (approx 3 x 105 km/second).

A complete 360 degree phase shift between the outgoing signal and the returning signal, would correspond to a total distance of one wavelength.

Velocity of propagation = frequency x wavelength   ....... so at 300Hz, the wavelength   =  (3 x 105 ) / 300    km   = 1000km. 

The total distance travelled by the signal is of course twice the distance of the aircraft from the beam station. So a phase shift of 360 degrees at 300Hz modulating frequency corresponds to an aircraft location 500km from the beam station. So using a 300Hz signal, one degree of phase shift corresponds to a distance of 1.388 km.

By increasing the modulation frequency, the distance corresponding to a one degree phase shift reduces (e.g. at 3000Hz, one degree of phase shift corresponds to 138.8 m.) However, it becomes possible for the phase shift at 3000Hz to exceed 360 degrees, at realistic aircraft distances. The system was therefore developed to initially use 300Hz to give an approximate location, then the modulating frequency was increased to 3000Hz, to provide a more accurate position fix (in the knowledge of how many multiple wavelengths were involved at 3000Hz as a result of the initial 300Hz measurement.)

With this information, an accurate "bomb release" instruction could be provided to the bomber.  A single Luftwaffe bomber group was equipped with Y-Gerate. This time it was III Group of KG 26, (III/KG 26). A single large antenna mast, just behind the cockpit on the top of the forward fuselage, identified a Y-Gerate Heinkel 111. The KG 26 unit marking was 1H.

 

He111 of KG26 showing the location of the Y-Gerate antenna

(between the cockpit and the dorsal gun position)

 

In November 1940, British radio listening stations began to pick up unusual signals in the 42 to 48MHz frequency band. These were test transmissions of the new Y-Gerate beam system. Analysis of the signals confirmed the concept described above. An additional feature of the director beam was a move away from the use of two mechanically angled antenna arrays, each with audio presentation of dots and dashes to guide the pilot along the beam. Instead a single "phased array" antenna was used. The beam direction could be shifted by small changes in the phase shift applied to the adjacent dipole elements. The beam was thus electronically switched between the two phase settings, to provide a very precise equi-signal path that lead directly over the target. The aircraft receiver detected alternate signal pulses from each beam angle, and an analyser on board the aircraft established any drift away from the equi-signal line between the beam centres and provided a visual indication to the pilot, to enable him to bring the aircraft back on course.

 

7  Y-Gerate Jamming Techniques

During the work-up phase, an effective radio countermeasure was established by Dr. Cockburn's team to disrupt the bomb release instruction. My two reference books report variations on the jamming technique applied:

Both agree that a sensitive ground based receiver was used to pick up the ranging signals on 46.9MHz from the bomber. These signals were then routed to the high power early television transmitter at Alexandra Palace, which according to Instruments of Darkness, re-transmitted the signals back to the beam station (on the same frequency) causing an error in the range estimations.

The description in Most Secret War indicates that the Alexandra Palace transmissions were sent, instead, on the bomber's receiving frequency of 42.5MHz, causing the bomber to immediately re-transpond the signal back on 46.9MHz.  Which would be picked up by the beam station and again by the ground based receiver, resulting in a further 46.9MHz signal being transmitted by Alexandra Palace and so on...... The result was a string of unsolicited returns to the beam station in addition to an occasional "wanted signal". The net result was to break the ranging link and remove the ability of the beam station to give an accurate bomb release message. 

(I find the R.V. Jones version more plausible as otherwise the ground station would be trying to transmit several killowatts on the same radio frequency and at the same time as its driving [sensitive] receiver, which sounds like a very bad idea (feedback would be very difficult to avoid).  Also the omni-directional antenna of the aircraft made it more vulnerable to jamming signals than the very directional beam station antennas.)

This jamming system was code named "Domino" and first became active in February 1941. A second Domino system soon followed, covering the Cherbourg beam stations. This was located on Beacon hill near Salisbury.

On 3rd May 1941, three III/KG 26 aircraft equipped with Y-Gerate were shot down. The Y-Gerate units were salvaged and sent to Farnborough for analysis. The details of the beam steering system were discovered. This opened up a second jamming opportunity, as it was found that simply by transmitting a continuous modulating tone on the beam frequency, the automatic analyzer on board the aircraft lost synchronisation and gave completely arbitrary steering instructions. Jammers using this technique were quickly produced and code named "Benjamin". 

 

8  The Return Of KGr 100 With An Upgraded X-Gerate System.

During April and May 1942, KGr 100 was redeployed from the Russian front back to France.  The Germans planned to take reprisals for allied bombing of the town of Lubeck and initiated what became known as the Baedeker raids, with bombing of Exeter, Bath, Norwich, York and Cowes. Since their departure for Russia, the KGr 100 aircraft X-Gerate system had been modified to include an additional modulation signal at 15kHz, which is above the normal range of frequencies that can be heard by the human ear.

Enigma decripts from Bletchley Park had already alerted British scientific intelligence to the new 15kHz threat and the Bromide jamming equipment was appropriately modified to deal with any future use of the new system over the U.K.   However, the jammer modifications were not planned to be activated until the new X-Gerate system was actually observed in action, for fear of giving away the Enigma code breaking activities.

When KGr 100 returned, acting as a pathfinder force on the Exeter raid, the very good bombing accuracy was a powerful indication that a new bombing aid was in action, unimpeded by the British jamming transmitters..... but the British monitoring receivers did not pick up the expected 15kHz modulation.  R.V. Jones himself became involved in the search for an explanation. It was soon discovered that the audio bandwidth of the British monitoring receivers was too low for a 15kHz modulating signal to be detected. Once this was remedied, the situation was immediately clear. The 15kHz modulation was activated on the Bromide jamming transmitters and KGr 100 bombing accuracy returned to its previous much less spectacular levels.

 

9 The Night Fighters

While researching the saga of the beams, a thought kept coming to me: If you know the exact track that the enemy bomber force is going to follow, within the accuracy of a Knickebein beam, why would you want to jam those beams when they could lead the bombers right into the guns of your night fighters?

It wasn't long before I found at least some of the answers to this question: The chances of finding individual bombers in wartime winter night time conditions, without the aid of radar are evidently not high.  Even if British nightfighters had been equipped with beam guidance units, up to the Autumn of 1940, they would have been primarily relying on number one eyeball to find the foe. The available aircraft could barely catch their targets even if they managed to catch sight of them. The development of effective airborne interception radar, purpose built nightfighters and ground control with appropriate radar (GCI) had not advanced far enough for a nightfighter battle to be a viable first option. The jamming strategy was realistically the only sensible route to follow......  but within months of the initial beam operation the nightfighter situation began to progress very quickly:

8.1 The aircraft:

During mid 1940, the nearest thing Britain had to a night fighter was the Blenheim 1F. (A short nose Blenheim with a quad 303 gun pack fixed to the fuselage underside.) These carried the very first airborne interception radar units, but it was a painful learning exercise with very unreliable radar and a top speed somewhat slower than most of the enemy bombers. Boulton Paul Defiants were also fitted with AI radar and joined the night fighters from the Autumn of 1940. However, it was late in 1940 that the first Beaufighter night fighter units formed up. This was exactly the machine the squadrons needed. Although not as fast as had been hoped, it was fast enough... and the punch packed by 4 x 20mm cannon was completely lethal in comparison with the Blenheim and Defiant .303s. The large airframe could easily support the radar equipment and by early 1941, the Beaufighters were ready to demonstrate what they could do.

Blenheim 1F

Defiant

Beaufighter

8.2 AI Radar

I've not found much detail on Google so far on the early development of AI radar. Mark III sets appear to have been the first to go into the Blenheim 1Fs, but it was a Mark IV set in Blenheim 1F that achieved the first radar directed shoot-down on the night of 2/3July 1940. The Mark IV AI unit went into full scale production and was fitted to Defiants and Beaufighters. The antennas were a combination of arrow shaped and straight vertical dipoles. Mark IV range was a maximum of around 4 miles and a minimum of circa 600yds. (More on this subject when I find some good sources).

8.3 GCI Stations

The Chain Home Radar network around the coast of the UK was an extremely effective outward looking early warning radar system for use over the sea, but provided no inland cover.  There was virtually no effective ground control interception (GCI) capability in mid 1940. However, suitable radars operating at around 200MHz, using Plan Position Indicator (PPI) displays were in development. By the end of 1940, the system was being rolled out and by spring 1941, an extremely effective organisation had taken shape. 

8.4 Ongoing Developments

By May 1941, the night skies over the U.K. had become a much more hostile area for a German Bomber. The interception control techniques were improving, the nightfighter crews were learning fast and the radar was becoming more reliable. During the month of May, the Beaufighter squadrons alone accounted for over 100 enemy aircraft. The tide was turning. Then, the bombing abruptly stopped. Hitler had turned his attention (and his bomber force) towards Russia. In 1942/43, the Mosquito largely replaced the Beaufighter as the primary UK home defence nightfighter. These aircraft were ultimately fitted with centimetric AI radar and presented a formidable foe to any unwanted nocturnal visitors.  It was really only at this point in time that enemy navigation beams could begin to be regarded as more of an opportunity than a threat!

Mosquito

 

References and links for further reading:

Books:  (NB These books cover many additional WWII topics beyond the battle of the beams)

Most Secret War by Dr. R.V. Jones 

          (My copy dates from 1979 then published by Coronet Books/ Hodder & Stoughton) ISBN 0 340 24169 1

Instruments of Darkness by Dr. Alfred Price

          (My copy is the revised 2005 edition published by Greenhill Books) ISBN 1-85367-616-0

Beaufighter Aces of World war 2 Andrew Thomas 

          Osprey 2005  ISBN 1-84176-846-4

Mosquito Fighter/Fighter-Bomber Units of World War 2  Martin Bowman 

          Osprey 2005  ISBN1-85532-731-7 (later period than the beams)

 

Web sites found on Google: (There are many more)

http://www.vectorsite.net/ttwiz7.html#m3

http://www.geocities.com/pentagon/2833/general/tactics/knickebein/knickebein.html

http://explanation-guide.info/meaning/Battle-of-the-beams.html

http://www.atlantikwall.net/eov_knickebein-stellung_k6.htm

http://users.pandora.be/bunkers/knick.htm