Klein-Heidelberg

a quite mysterious bi-static radar system

A thrilling survey

Klein-Heidelberg (K-H) was a kind of parasitic radar system. The first of its kind in the world

My attention was triggered after a query put forward by Hugh Griffiths and Phil Judkins onto me last year. One point concerned was - how would the Klein-Heidelberg display have been accomplished?

In July and August 2009 the facts were quite astonishing falling all at once in the right positions. Hugh Griffiths had traced a document in the German 'Bundesarchiv', which was a recollection of a GAF officer who did put his briefed expertise on paper on 30 May 1945. He had, however, to rely on a translation. Hugh also forwarded a translation of a paragraph of Karl-Otto Hoffmann's famous book on the GAF signal services (Ln-Truppe). Hugh did put some of the facts together and concluded how the CRT might have looked like. In this phase my experience with Würzburg radar helped me to put things straight and I could deduce the actual block diagram of K-H.

It soon resulted in the next block diagram drawing which later proved being fully in accordance with the facts afterwards discovered.

Block diagram of the Klein-Heidelberg system. For practical reasons, I have implemented the power supply of the two receivers being inside the display unit, but it actually was outside (next to) its frame.

Please notice the Klein-Heidelberg PDF document below, where both is shown. The Waechter-Geraet was drawn after a PoW interrogation who had served at the Vaudricourt Klein-Heidelberg site in North-West France. This person also told his interrogators, that he had been technically instructed by a 'Flieger ingeneur', which may well have been the same 'Flieger Ober Ingeneur' who has put his general K-H knowledge on paper on 30 May 1945. This latter person was brought to a PoW camp near to the German-Danish border. It is also likely, that he was kept there as to be around during the 'Post Mortem exercises' held during late June and early July 1945. Post Mortem was an interesting operation, as its aim was to investigate the effectiveness of the German electronic air defence. They could accomplish it because most of the German radar and control-sites fell un-damaged in British hands (North-East Germany and Denmark). They returned most of the German crews back to their former wartime posts. Behind every German operator stood British watching personnel. One of the interesting results was that by far not all claimed British electronic countermeasures were fully effective. It was revealed that tracking of British aircraft approaching, even when they were fully employing their EW means, that in one way or another some of the German services were still be able to provide an overall air picture, be it sometimes later than was without EW jamming. Given the circumstances, Bomber Command mobilised a quite impressive armada of about 200 bomber aircraft!

How did it work?

I have modified this quite often reproduced map

Dover is constituting the Chain Home radar site. Its transmitter signal is transmitted Omni-directional and is arriving after a travelling distance 'L' at the Klein-Heidelberg station, and is fed onto the main-range receiver shown on the block diagram on the left-hand side (display A). Let us consider now that a target at the outside ellipsoid (+) is also being illuminated by the same radar pulse that has arrived earlier in the radar receiver at Oostvoorne. The radar signal arriving at the skin of the target, say an aircraft, will be scattered in many directions, of which only a small part is returning towards the radar receiver located at the Dover site. Another small portion of the bounced radar-pulse-energy will follow the route Rs towards the parasitic Klein-Heidelberg site at Oostvoorne.  It is quite understandable that the arriving signal has travelled over the distance of Rt+Rs. The clever idea was materialised by Dr. Wächter maybe in accordance with the Naval and GAF research site at or near Mont Couple in North-Western France (project research started about 1942).

Guess, for this occasion, the target is actually measured being at ellipsoid number 26

Let us first look at the kind of main signals, like the one constituted by 'L'

Please regard the block diagram. On top left, we see an antenna which only purpose is to pick up from the air the direct CH signal which travelled distance 'L'. In the following figure down this is called the Main pulse (master reference). The signals may originate from targets or other sources. This main pulse is constituting the synchronisation reference of the Klein-Heidelberg system. Chain Home signals employed a PRF of 25 Hz and its transmitted pulse power was rather high (>500 kW).

This screen picture is similar to the CRT shown on the left-hand side of the block diagram (display A)

The basic idea was, that when we move the virtual target on the perimeter of the dotted line that Rt+Rs will having a constant value, hence, it is thus acting as being at an ellipsoid! What they did was, they fabricated a map  having their loci both at Dover and Oostvoorne and showing a total number of 40 ellipsoids.*  For each other CH-site they produced an additional map. In practice, each K-H site necessitated, say, three or four map series. For the case a British CH-site was off the air or being jammed (they sometimes encountered signal problems, and it was then difficult to synchronise their K-H system). It is quite clear, that this technique only worked as long as Rt+Rs is > L. However, this limitations is not very significant as in most circumstances enough ellipsoid perimeter was useable. When really necessary they had to select (switching over to) another appropriate CH-station frequency. The only thing to do was, to tune the two receivers at the appropriate frequency and off they went again! (maybe re-tuning the synchronisation module a bit and selecting the appropriate map)  *The actual CRT was taken (adopted) from the Würzburg system, as its course range crt (type LB13/40) has an engraved scale of 0-40 km, which were for this occasion constituting the ellipsoid numbers.   

This CRT screen picture is showing the corresponding target-ellipsoid-number (display B)

Let us look closer into the display section on the right-hand side of the block diagram. Its signal was received by means of an equal type of receiver. Though, please remember that a target can be at any point of a given ellipsoid thus for a given condition of Rt+Rs. How could they determine at what spot a target actually is? Here a high gain directional antenna is becoming involved. They used the huge back plane section of a Wassermann S antenna. A shortwave array of 18 x 1/2 λ dipoles was staged in three rows (3 x 6)(CH operated between, say, 21 - 30 MHz). The K-H operator had to scan the sector concerned only in azimuth. When a signal appeared at the screen he only had to read off the actual screen number (= ellipsoid number) and the azimuth bearing of the DF antenna. The position of a target is herewith determined! Of course, this system never reached the accuracy of a regular radar system. But, it was merely immune against window (chaff) or other kind of interference. The actual system range (Rt+Rs) was up to 600 km. A PoW interrogation stated that the Germans measured regularly at ranges of say 390 km and sometimes beyond. It has to be noticed though, that in late 1944 the British intelligence knew about Klein-Heidelberg and reacted internally like they had been stitched by an insect! The best remedy would have been to switch-off most CH stations. The resulting disadvantage was not accepted and they finally started to let their PRFs jittering (maybe about December 1944). This certainly has made the operation of Klein-Heidelberg a bit more difficult, as it might have inflicted to some extent their own system accuracy. However, the Germans still managed to obtain parasitic bearings.

As we have already seen, the Chain Home radar PRF was 25 Hz which constitute 40 ms intervals, this equals several thousand km distance. To keep it practical, the Germans skipped the long distance range and watched only the first 2 ms for scattered target signals.

About early September 2009 this was what I knew about K-H.

A few days later Phil Judkins visited me in Holland, as we both had to prepare our DEHS AS09 presentation. Among the documents he brought along was a British TRE file on "Operation Post Mortem" in particular dealing with the See-Elefant station at the Danish isle of RØMØ. Phil was reading the text and I was studying the schematics. I was becoming more and more excited as I realised that the electrical schematics were very well fitting with the block diagrams made some weeks before. We went back to the text and found: that the See-Elefant display system was equal to Klein-Heidelberg! Why not calling it then Klein-Heidelberg? The reason is obvious, as RØMØ is about 800 km away from the nearest British CH station. They had brought-in their own illuminating radar transmitter code-named Kopfe, the receiving site was called Rüssel (still employing a Wassermann S antenna mounting frame).

The station at RØMØ was manned by personnel coming from Holland, where they had been engaged at station "Max". This station was also tracking beyond the regular radar ranges (thus See-Elefant like). Where it (sometimes) watched very long distance targets. Trenkle (as well as KOH) even mentioned that a convoy near Spitsbergen had been watched (located). After the battle around Arnhem (September 1944), Max was abandoned and its personnel moved to RØMØ. I guess, that they took along the 'Wächter Gerät' and other small devices to their new site at RØMØ.

This parasitic radar technology was not existing on the Allied side during World War Two. However, in the 1950s it gained significance as technology allowed since more accurate tracking of OTH (Over The Horizon) targets. In this respect the wartime K-H like apparatus was only the very beginning of an important aspect of long range radar technology of today, sometimes called: Longwave radar.     My operational account is a somewhat simplified version of the actual state of affairs, as the German operators encountered all sorts of difficulties, sometimes of technical - but also of tactical nature. It must be noticed, however, that most Klein-Heidelberg sites were actually only operational for some months before they had been knocked-out, and that it takes time to learn how to operate a system appropriately. 

Klein-Heidelberg system, including the schematics of all modules of the Wächter apparatus

PDF 1.4 MB

It took me quite some time to clean the schematics, but, also correcting the sometimes very faulty schematics. Whether this was done deliberately is not known to me. One fact, however, is that it might well prove that hardly any British engineer or technician has looked into it as to approve the drawings. As some wirings were so obviously faulty, that it everyone understanding very simple electronics or electrics must have strongly complained the subject. That it was approved shows that no one has bothered with it in detail!     However, that we know now the inter-wirings between the modules concerned is only due to the fact that the TRE report, mentioned previously, was explaining what kind of screen picture is to be seen by means of the control-selector-switch (its position number) in the centre of the Wächter-Display apparatus. Now I knew which line-number function was going to the central interconnecting-board. When this information had not been provided, we most likely would not have been able to reconstruct all the wiring interactions. 

My final remarks: It is astonishing that it took 64 years before these technical details saw daylight again! Even well known Fritz Trenkle was kept in dark as far as it concerned the technical details of Klein-Heidelberg. It was the chain of information: - the post war description of a German Flieger Ober Ingeneur kept in the Bundesarchive - the query put onto me by Hugh Griffiths on the K-H screen display - Karl-Otto Hoffmann's explanations. And, not at least - my personal involvement in Würzburg circuitry as a spin-off of my book on Würzburg radar combined with the general understanding of German wartime apparatus concepts. Which allowed me to draw the block diagram of Klein-Heidelberg. Finally, it was Phil Judkins who traced the crucial TRE file on See-Elefant at RØMØ.

Maybe is Hugh Griffiths paper of 2008 of interest, as it scientifically explains the bi-static principle of Klein-Heidelberg and related systems (please be patient, as it might take a minute downloading its content):

http://ftp.rta.nato.int/public//PubFullText/RTO/EN%5CRTO-EN-SET-133///EN-SET-133-01.pdf

April 2010

Arthur O. Bauer

Go back to, or proceed with: Handbooks papers and product information