bits and pieces

Initiated on 17 December 2012

Status: 6/8 January 2013


1 - 5, 6a, 7 

Our Nachtfee survey has reached a semi vacuum, where in the meantime all possible aspects have been dealt with; my imagination is showing currently signs of emptiness. My main thought is, nevertheless, bothered with the query 'how going on further'?

Forcing my mind in a different direction, I decided to clear the deadlock in respect to British official files, like R.V. Jones' A.D.I (Sc) and the Felkin (A.D.I.(K) reports, tracing the latter being a direct spin-off of our Nachtfee project. It has to be said though, R.V. Jones' technical understanding matched far more the truth than did Felkin's office.   

I have therefore transcribed sections of the Felkin files and will handle them in regard to what we have discovered how Nachtfee really works.

On the forehand, Felkin and his dedicated staff may have lacked the slightest imagination on which basic techniques Nachtfee truly relied! We will see that the same incorrect story will be told time and again, without any new imagination. Apparently, they interrogated some PoWs and built their entire understanding upon these limited interviews. Maybe a bit mixed up with their own fantasies.  


Nevertheless, viewing the principle drawing below, at least someone must have investigated Nachtfee more thoroughly and likely having grasped its principles!


This drawing forms the nucleus of the entire Nachtfee project

Let us first consider: Range Adjustment Tube

No one could have designated its application without a more or less comprehensive understanding of the Nachtfee technique.

Also: Control Tube

Although, using an oscilloscope may point into the direction of controlling some system parameter, this would also have been the case in respect to the Range Adjustment Tube. This (LB2) CRT is controlling whether the Nachtfee 'order' signal has arrived correctly in respect to the Nachtfee ground console 'Range off-set' compensation; as it shows what returned from the aircraft IFF transponder.

In non of the Felkin's files is even a single hint pointing into the direction of the fact that Nachtfee relied upon feedback loops. When they could have grasped only a single aspect, name it a single loop; although, effectively it are two loops to be dealt with.

Quoting (Felkin Report No. 357/1945, for its full transcript):

107. This system was used operationally, but it was found that, apart from the susceptibility to jamming, other pulse transmission could interfere by unlocking the system* and thereby cause the message to be false value. It was not therefore possible to use it in single-seater aircraft.*

They mention in (107.): that Nachtfee was vulnerable of jamming, with the danger that the 'order' blip would start moving uncontrolled. This is proving that they had not the slightest idea upon which technique Nachtfee relied!    Please bear in in mind: the Nachtfee data signal consists of a sine-wave (however first order pulses were and are transmitted), which occasionally changes its signal phase in the domain of time once on demand, just when a new 'order' is constituted. The next cycle however, no one can know what previously may have been manipulated. Be it, someone is operating an independent time-base source having exactly the same frequency as well as a controlled phase relation, like was and is done in our simulated aircraft display system. Only such a system is capable of noticing that some phase change in the domain of time once occurred. (Please consider Bladwijzer199 for a detailed explication). The difficulty understanding Nachtfee is that space is an essential domain of time parameter either way. Someone without this essential information cannot get knowledge of the content of a message. The distance or range between both the Nachtfee ground station and the airborne platform is playing a most significant role.

Let us imagine, that a Nachtfee ground station transmits without changing its data content (Nachtfee data consists of a continuous sine-wave of constant frequency). Presuming that both time bases run exactly in accordance synchronously; and that both Nachtfee and the aircraft 'order' pointers are aligned at, say, due North. After an aircraft took-off the space between the ground and airborne system is widening; the order blip or signal will start moving anti-clockwise along the time-base line constantly and reached its starting position again when 300 km is been bridged! (flying in a straight line from the controlling Freya-EGON station) This system aspect should, of course, be controlled; and the only means doing this is controlling the Nachtfee 'Range off-set' manually. The way doing this is monitoring the returning transponder signal blip on the Nachtfee control screen (LB2) (shown next) (neglecting fine tuning by means of the dual trace CRT); we know already that both the upwards and downwards signal-path is equal and that 500 Hz equals a λ of 600 km, just where a new circle is due to start. Upwards is maximally 300 km and downwards is 300 km as well, hence, making together 600 km. It is thus evident, that 'Range off-set' between 0 and 300 km has become a very significant system parameter.

However, 'Rang off-set' is only compensating for the influence distance or time is having on what is where painted on the Nachtfee time-base traces. We have not yet dealt with the incorrect 'order' blip vector painted at the aircraft time-base line owing to the actual phase difference between time-bases involved and distance. The only means provided within the Nachtfee concept is the 'Phase' control (A) available on the Nachtfee front panel. Please consider the second photo just on the lower left-hand side of the 'order' compass. Which is lacking a scale and, more significantly, its behaviour cannot be monitored by whatever means on the Nachtfee controlling screens directly! This aspect bothered me for a long period of the passed year.     What is the main system bottleneck?     Previously, we considered that both the Nachtfee as well as the aircraft time-bases are running exactly in 'phase'. Though, how do we know that this actually is true? Extensive experiments proved, that we have to count with mutual accuracies > 2 x 10-7, implementing a security factor even  10-8. There is no way around that in some-way-or-another, a time-base-reference should be fed back towards the Nachtfee controlling system. I call it the TB signal. By whatever means, it has to be included within the transponder signal (Nachtfee + EGON + TB). This latter signal will then also be made visible on the (LB2) controlling CRT screen (please notice the blip at about 8 minutes past the hour on the next photo). Since we are able to manipulate the TB signal against the internal Nachtfee time reference. For strangers among us maybe curious - operating the 'Phase' control will only having influence on the TB reference blip which will move along the time-base-line, whereas the returning original Nachtfee blip will not rotate against it at all! (neglecting small system errors) 


Because the actual Nachtfee data signal in respect to its internal time-base system constitute a coherent system loop, whereas the not connected aircraft time-base TB is not. 'Phase' control (A) is manipulating the overall Nachtfee phase timing in such a manner - that effectively the data signal leaving the Nachtfee system is arriving virtually just where it should. In other words - 'Phase' control quasi rotates the to be transmitted Nachtfee data in a way that it counters the airborne time-base-phase-off-set in the domain of time. Please refresh your understanding by considering (Bladwijzer200) again.



 The blip due North shown at the control screen (LB2) is the Nachtfee data signal, that returned downwards from the airborne FuG25a IFF transponder. This pulse vector should equal the Nachtfee 'order' (pointer) setting. This only works correctly as long as the 'Range off-set' parameter is adjusted accordingly. The blip at about eight minutes past the hour constitutes the TB phase reference signal, which adjusted at this current vector aligns both time-base references accurately against each other (this vector is only valid in our simulated system setup and will differ in other configurations)

 For those interested in a more technical explication, please consider:

Nachtfee explaining its principles


Nachtfee in the domain of time



Was jamming a major factor in respect to data reliability?

Only to a temporarily extent.

It cannot corrupt the content of Nachtfee data whatsoever. It can, of course, temporarily making data reception impossible, but instantly jamming is reduced in signal level the full trustworthy 'order' data is available again. When jamming lasted long, it might taking a few seconds to readjust 'Range off-set' and eventually the TB phase-reference again. The received Nachtfee 'order' signal is returning to the Nachtfee ground console via the combined IFF signal feedback; on the ground they could monitor when jamming takes place.     To what I know, EGON was left generally un-jammed, maybe occasionally though certainly not being hampered systematically; like was Benito.


A next example of misunderstanding or misinterpreting

Please consider at the same time the next photograph!

105. The presentation screen in the aircraft was inscribed with various commands both inside and outside of a circular time trace. There was a stationary zero blip in the 12 o’clock position, and when an instruction was radiated the pulse received caused a second blip to emerge from the first and travel around the C.T. tube in a clock-wise direction, presumably by a slight change in prf.* This blip came to a stop and the command corresponding to its position could be read off on the inside of the time trace where a total of about eight  different instructions were inscribed.* It took from one to two seconds for the blip to travel round the tube.


106. It was thought that the further eight commands inscribed on the outside of the trace were denoted by the blip making a complete circle and starting round a second time before coming to rest at any one of the eight sectors.* In this way a total of sixteen different orders could be given. For night fighters such instructions are “Turn left/right”, climb, dive, etc. appeared on the inner side and figures for transmitting bearing on the outer side of the trace.*



When we compare the previous text with the actual Nachtfee 'order' compass, one may not understand where they are talking about fully.  It is most likely that the division of 'order' sectors should be equal in both the ground apparatus as well as what is presented at the (FuG136) aircraft display

When we compare the text worded in 106 with the photo above, some confusion may arise. There does not exist in- and outside 'text-orders'. It even is not possible to arrange it, as there is simply no technical provision for it!

Nachtfee used only the visible text 'orders'. Where started the confusion likely? The interrogated PoW may well have seen the Nachtfee ground apparatus, where we see two plexiglas (perspex) pointers. The (bigger) outer one is to be used only as being a temporarily memory. You may ask, how do I know this so definitely? As inside it is only provided (linked) with a spring loaded click mechanism (Bladwijzer206 the multi click disc is shown, whereas the 'order' pointer had only a single click-stop at due North ). Only the inner- or smaller pointer is directly linked onto phase-shifting goniometer (B). Please fresh-up your memory with considering (Bladwijzer191) again. Though, why have they implemented a semi useless pointer?

My hypothetical estimation: during operation the Freya-EGON system is guiding a Pathfinder aircraft. At a certain instant it is decided that a particular 'order' or command should be conveyed towards the aircraft. The operator in-charge announced, say, 1 Rolf; which stood for change your actual flying course 5 degrees to the right (they might have drifted too much to the left). The first thing the Nachtfee operator had to do was rotating the big pointer such that it points at the just given 'order' vector.     We have to take another aspect into account first. The Nachtfee system employed also 'waiting for new orders'. This was accomplished by operating the Freya-Polwender selector (please consider the switch just on the right of the big CRT screen at Bladwijzer198). Regularly the Nachtfee 'orders' start moving clock-wise from due North, the British called it 12 o'clock. However, the Freya-Polwender switch only interchanges the polarity of the two Nachtfee data (output) wires, causing a 180° signal-phase change. Although, the Nachtfee 'order' pointer is still facing at due North, its data blip at the other-end of the line, thus on the aircraft 'order' display, will point now due South, as it will do on its own (ground) control screen. To my understanding this was the Nachtfee signal-waiting-position. As long as it pointed due South, no action was due to come.     Thus after the Nachtfee operator got the instruction 'eins Rolf', his first task was setting the memory pointer accordingly and then operating the Freya-Polwender and setting it in its regular position (handle facing now upwards). With a delay of less than 1 ms the 'order' blip painted at the aircraft order display jumps also at due north, like the blip is doing at the Nachtfee ground control screen after not yet 2 ms (for it bridging twice the aircraft range).

Then the operator continues after a certain interval with slowly rotating the 'order' pointer clockwise until it is facing equally as does the bigger memory pointer. I will not bring the proof, the ground operator will know that the order is received correctly, as he receives the responding IFF signal; to my concept understanding, together with the also arriving TB pulse, which represents the actual time-base-phase of the aircraft system, the system loop is closed and under normal circumstances at both screens, on the ground as well as in the aircraft, giving equal vector blips! Please refresh your concept understanding (Bladwijzer189)


The 'order' blip pointing at 'eins Rolf' (turn 5 degrees to the right)


Giving you a second example



This photo shows what might have been displayed upon the aircraft display waiting for new orders (Freya-Polwender being operated). On the other-hand it may otherwise showing the actual 'order' Pauke (open your bomb doors). The difference being: the Freya-Polwender signal blip jumps to up or down, whereas the 'order' data signal rotates clockwise slowly towards a vector where it holds for a while

I will not discuss whether the text is on the outside or inner side of the screen or 'dartboard' like division. The only aspect that counts, is the fact that both the instructions provided on the Nachtfee ground system should equal the vector divisions of our simulated aircraft display.


Up to present we have not yet been able to trace any kind of document on Nachtfee techniques. Although, some non technical rubbish is around!


Quoting from: Air-40-2232    626/1/a.i.4.(a).

EGON-Gerät FuG25a attachment

The Egon Procedure, at present being used relying as it does on course and bomb-release signals passed by W/T and R/T, is as yet only in a extemporised form. A new attachment to the FuG25a in the aircraft will shortly provide for visual signals appearing on a Cathode Ray Tube and will eliminate all W/T and R/T, course and bombing signals without basically altering the present procedure. This attachment consists of a unit placed in the fuselage of the aircraft and a presentation unit, placed between the pilot and observer., embodying a Cathode Ray Tube of about the same diameter as the Lichtenstein (Liechtenstein, AOB). The scale on the Cathode ray Tube consists of a circle divided into segments (??) of which represents, and is marked with one of the code words used in the EGON Procedure. The circle is divided surrounded by a outer circle which is marked clock-wise from 0 to 9, the 0 being at 12 o’clock in the circle, so that the whole circle has much the appearance of a dartboard. In operation,  therefore, visual signals on the tube will replace the aural signal at present used. A short blip, radiating from the centre of the tube, first appears in the segment representing the appropriate signal, such as “Autobahn”. Bearings are given in the same way by a long blip, also from the centre, appearing opposite a succession of figures in the outer circle, i.c. 3 followed by 5 and 5 = 355°.  Course corrections and height can be given in the same way.

When we compare the previous ground 'order' compass scale we may have some difficulties understanding how things may have been matched.


Continuing with: Felkin Report No. 357/1945 (the latter being a transcript of the entire file)

94. A further line of development was the provision of a new type of control table giving the exactitude of plotting necessary for pinpoint bombing. This control table, which was under development at Rechlin, and use of complicated mechanical apparatus for accurate projection of the position of the bomber. One of these tables was destroyed in the course of a daylight air attack on Rechlin in February 1945; two other tables were in existence at the time of the capitulation, and were located somewhere in Western Germany. 

What kind of tables they speak about? We may never know it exactly. Was such a table already subject of a wider configuration where Nachtfee was only a part of?    The only thing we know, is that we may assume that 5 Nachtfee units have been hand-built altogether. Ours carrying on quite some places the 'Roman V' number, we may assume that our set may have been the last hand-built of the series. Which is in accordance with the given fact that some of the Jahre tar-roll-capacitors carry a date of April 1944 (the way this was done points that the devices were tested during their manufacturing process, thus not after it had been delivered), and that quite some RV12P2000 valves have been accepted in autumn 1944. It is therefore plausible, that our set had become ready for operation in late 1944. Taking into account, that some urgency pressure existed - but also the sluggish way military organisation often act, leaving apart the chaotic circumstances of those days; thus some goes fast and other aspects are progressing quite slowly.      

Where it once had been captured we don't know. Viewing the practice, that every belligerent was keeping what they have captured for themselves (often refusing sharing it with their Allies), our unit must thus have been within an American occupied area. Prof. Helwig Schmied once mentioned in a phone conversation early 2012, that one or two sets have been captured in the vicinity of Vienna. Rechlin was captured directly by the Russians; our device hardly may have come from there. We opt for Bavaria or Austria as the most likely areas.     We may likely never know the real truth!



On 25 December 2012


We have often considered Nachtfee with some pessimism, but was it a full failing system? I don't think so, as, according the lower Felkin 357/1945 text:

103. The original purpose of Nachtfee was to provide a solution to the jamming of R/T control systems, and it had been used operationally Pathfinders of K.G. 66 for laying mines in the Scheldt estuary. It was to be adapted for the night fighters to overcome our jamming of the night fighter commentary. This mine-laying operation must have been in late 1944.


Generally speaking the A.D.I.(Sc) reports originating from R.V. Jones' office are of a far more elaborate quality than were the ones provided by A.D.I.(K);  it has to be said though, we have only knowledge of what came out under Felkin's name.

A.D.I.(Sc) Report 33 (I am not entirely certain whether it actually is number 33, which latter figure does not fit well in the other report numbers; though this is what Michael Gyges once told me)

 Page 13

 EGON Control

 We first encountered the EGON procedure as a method of control for pathfinders of K.G. 66 and probably other units in to February 1944 raids on London.         KG 66 used it  when they dropped Schwan and Lux buoys to give the He 111s their launching point for the V1 attacks on the Midlands of 24/12/1944. The method is now extensively used for the control of jet propelled aircraft of Battle Unit Hallensleben in this attacks an airfield and in support of the army; the ground control in this connection is carried out by certain companies of I/Ln.Regt. (Mot.) 54 and these companies received instructions in February to retain Benito personnel on EGON procedure. Appendix II contains the R/T code for communication between one ground station and another. Some of Hallensleben’s aircraft were earlier controlled by Benito (FuG16ZY, AOB) and some by EGON, but EGON may now be the sole method used by this unit. EGON is also widely used for the control of day and night fighters; in both cases its tactical employment is probably very similar to that of Benito. 

My (AOB's) comment, in this paragraph they deal on the one hand with what we understand being Nachtfee and on the other hand with EGON generally, thus directive transmissions towards aircraft and eventually keying the EGON range-measuring pulses with some Morse code messages; also some Morse replies coming from the IFF transponder. EGON originally constituted IFF which was becoming more or less separated from its secondary radar function becoming a means also capable of passing on messages. Nowadays, secondary radar plays an important role in conveying a wide range of information, like the flight-number, the number of passengers as well as destination, though indirectly also technical data like oil consumption of the jet engines etc.

When Jones discovered this revolutionary application he fairly stated:


Quoting: 11.1     Recognition is traditionally a difficult matter; the Germans have surpassed tradition   (A.D.I. (Sc) Report 101 page 5)


The A.D.I.(Sc) Reports generally deserve more attention and should, Deo volente, being one of my future commitments.



On 30/1 December/January 2012/2013


New thoughts came up

Would it have been possible to implement into the FuG136 aircraft equipment a similarly built quartz system as was done in the Nachtfee ground console? Please notice (Bladwijzer61)

The advantage would have been, that the warming-up behaviour may be responding in a more equal manner than does our simulated reconstruction. I myself doubt strongly that they were capable of constructing two entire equally behaving quartz vibrators. Quartz crystals in those days were made of natural quartz, which quality (also purity) varied quite; often visible in having a different R1 parameter whilst it originate from a single batch although, frequency specification is equal.

We also have to consider, that it were not two quartz crystals to be matched upon each other, but a series; as there was a single Nachtfee consol which should control several aircraft systems. Fitting all airborne apparatus with equally responding quartz parameters is totally beyond from what was and is makeable.

The advantages would have been tremendous though, could such a construction withstanding the harsh avionic environment? There speaks much for replying, no! The thermostatic controlled oven should have been of a rigid construction, as it should be capable of coping with, say, + 40° to -  40° C, as well as a high degree of humidity. Not discussing the tremendous fuselage vibrations and shocks. The quartz bars shown in the next photo does not directly convincing me that it could. The technical requirements being in the scale of modern military requirements were every device should be shock-proof.

On the other hand, I cannot proof that it was not!



Viewing the opened Nachtfee quartz compartment

Already from a technical point of view, the controlling device is a mercury-thermometer just right of the most right-hand quartz housings. This device operates, to my understanding, constantly within a region where the mercury level is just-, or just not, touching the two Pt contacts. What happens when vibrations, thus gravity, forces Hg to just going upwards for, say, a hundreds of a mm?

On the other hand, it would have been possible to operate instead a Pt thermometer-bridge; although I doubt that it could respond so accurately as does a Hg thermometer. We may estimate, that the size of the quartz bar inside the metal container is not much shorter in length. Thus constituting a quite long device, where shocks (g) may well stressing the quartz-bar mounting. Whether the electrodes are at the far ends of the quartz bar, or, the electrodes have been deposited onto two (opposite) rectangular long-sides of the quartz bar. The latter situation would make the use of quite long contact-wires necessary, with all the downsides.

On the other hand, the Nachtfee console is having a quartz-driving-level potentiometer which adjustment can be controlled on the dual beam CRT optimally (please notice Bladwijzer45). I regard this being un-realistic in an aircraft. According foregoing information, most of the electronics was mounted in the fuselage. An extra dimension would have been, that the best way coping with environment is keeping all critical sections (circuitries) being  build-in a hermetically sealed-off housing. A technique not uncommonly in Germany. 

Please do not forget, they should have dealt with mutual frequency or phase stabilities of at least 10-7. A dimension that (our) Nachtfee by its own means never fully reaches.


I therefore still believe strongly, that mutual time-base-stabilities may well have been 'the' crucial technical bottleneck! 

This may likely being the reason why it never matured.


As long as we do not have access to genuine wartime, or just post war, technical evaluation reports or documentation, all my considerations will having a great deal of speculation.

The advantage of this fact, is, that our imagination and grey cells being sparked time and again. Doubtless, without this aspect we never would have reached the state of affairs where we are today!   



On 6 January 2013



Time today is right to focus onto what R.V. Jones explained in the quite unique A.D.I. (Sc) Report No. 101, title: German Radar Development. This file was first addressed to the most important Office in Britain being the War Cabinet; especially directed to Lord Cherwell Churchill's Scientific Advisor (formerly name Professor Lindemann)

Reflected is Jones' state of knowledge on 6 March 1944

Only a couple of weeks after Nachtfee is experimentally and tactically operated for the first time.



Churchill's office received apparently file copy number 18. Whereas copy number 1 was addressed to the head of his own Air Ministry (Secretary of State)


Reflecting some lines on page 6


Page 7 is showing a rather good understanding of what goes on. Please remember that this report is issued on 6 March 1944!

When you compare this phrase with what Felkin's office published after the war came to a conclusion, like Felkin 357/1945, there can be hardly a doubt what the difference of intelligence content between both offices was - but quality!

Quoting from 12.5

Confirmation has come from prisoners from I/K.G.66, who say that they have to fly straight and level from some distance through the A.A. defences, and receive course corrections from the ground station (EGON-B in combination with Nachtfee, AOB), which give them the order to drop flares, and claims to tell them (when they arrive home) where they released their flares to within 300 metres. The orders are transmitted by R/T either on the FuGe 16 (FuG 16, AOB), or on the FuGe 10 (FuG 10, AOB), with R/T attachment; from the language used it seems likely that the bombing method was evolved by a controller with experience of Freya-controlled fighting (the bomb dropping order, for example, is "Pauke, Pauke", which is the standard for nightfighter order to attack.

This may provide a clue to the tactical method used.    The Freya-AN control of fighters consists of turning the split beam on to the target, and telling from the size of the right or left responses of the fighter's echo whether he is too much to the right or or left in his approach.    The controller then orders "Rolf" or "Lisa" (10° Right or Left) as required, and it is significant that similar instructions are used in the FuGe 25A (FuG 25a, AOB) bombing.    The system as at present practised is probably therefore to take a Freya with split, sight it over the target, and fly the bomber down the line of sight, giving course corrections as required.    When the correct range is reached, the controller orders the bomber to drop his flares.     The FuGe 25A is only essential to the system at long ranges or to identify the bomber. The beam of the Freya is so wide that even when the bomber is off course considerably, he is still within the field of view;    there would be no need to swing the Freya once it is aligned. This method is much more probable at present than the dual range cut system of 'Oboe' proper, and it implies that each of the three Freya transmissions heard (Freya-EGON or EGON-B, AOB) could handle aircraft separately.    The accuracy is unlikely to be much better than a mile except with very well trained crews.

It is evident that Jones possessed a quite good picture of what we call 'Nachtfee' and the associated aircraft (FuG136) apparatus. That tiny errors of understanding existed is logical, had Jones' Office to rely upon both scientific intelligence and to some extend on preliminary PoW interrogation. Like, Rolf stood for 10° (or 5°, AOB) to the right is for understanding what is going on, for us, not relevant. (should have been successively 2 Rolf and 1 Rolf)

For me intriguing, is the fact that from his expressed description he might have seen, or was reported on, evidence of the real technology in some of the crashed aircraft. Jumping to the first drawing of this current webpage, there can be no doubt that someone in Britain or in the US must have had (for us currently crucial) technical knowledge of what Nachtfee is about; at least after medio 1945; especially in regard to FuG 136, being the system carried in some of the I/K.G.66 aircrafts. The first drawing was published in a translation of a GAF radar periodical date 25 February 1945; this translation is known as Radar News 19.

Doubtless, Fig. 6 is not of a German origin. For at least two reasons: firstly, the German reference expressed that owing to the current state of the war, it cannot be predicted when Nachtfee could be produced (expressing it in a different way). Why giving then a drawing containing so many details? Secondly, it is hardly plausible that a detailed drawing appears in a German magazine without further explanation!    What also underlines my assessment, is the way it has been drawn - typically non German wartime like; dealing with pointers needs always to be reflected upon somewhere in the according text paragraph. (please notice the next few lines)


1) “Nachtfee” (FuG 136).

Airborne attachment to the “Erstling” receiver (weight 12 kg.), ground

attachment to the “Egon” transmitter, needing two operators. Transmission of 16 visual

signals which are read off on a cathode ray tube in the aircraft. The apparatus is only suitable

for two-seater aircraft, as a continuous intensive watch has to be kept on the visual indicator.

Transmission of commands is only possible to an aircraft which flies in the lobe of the ground

apparatus [EGON-B station, AOB]. Owing to interruptions in production, it is not possible to

give a date for delivery.

Transcript* of the British translation of the GAF periodical Funkmessnachrichten (19) regarding Nachtfee

* I transcribed it as to make this document fully 'searchable' on the web. It may well be regarded being an exceptional rare and significant time document.

The actual way this text phrase is being translated passes on to me a 'smell of non German wording'. Why should they put in such a paragraph an apparent down-side of the Nachtfee system?

This latter statement is my assessment and responsibility

On the other hand, it might have been regarded necessary for their future (Anglo-Saxon) readers, as to get a minimal feeling for what it was about. Nevertheless, relying (most likely) on current technical understanding on Nachtfee, just the kind of details we so desperately would like to seek!


According to what an academic told some weeks ago, is - the fact that the Allies captured huge quantities of military documents and papers. In the course of the 1950s and later - the US returned most of these captured files to the Germans; in contrast Britain did not. Noticing the materials we recently got from England (from private hand), a lot (maybe far lot) of the captured materials have been disposed off instead. And is lost for ever. Sad, but this is the way it went.     In contrast, the Americans microfilmed the documents before they returned it; it has to be said though, that not all returned. Another sad point, is, that the young West-German Republic did not know what to do with it. Fritz Trenkle told me once in the 1980s, that German archives returned, for instance, a document, say, a 'Würzburg radar file' to Telefunken. What should they do with it, as it was completely obsolete material with which they did not want to be connected with any longer? Resulting in permanent loss as well! That still quite some is around, is mainly due to the fact that there once have been a lot of copies around.    

Though, sadly not on Nachtfee,  likely because it never matured!



On 24 January 2013


The aim of this current webpage, is to underlining what we have found in respect to British wartime intelligence reports. It is already pointed, that their quality varied quite much. The best one were those on behalf of A.D.I. (Sc) de facto those from R.V. Jones' Office.


A.D.I. (Sc) Report 33? date 11.4.1945

Quoting from page 17 and 18

Methods of Ground-to-Air Communication


The main point is quite clear, although, not mentioning Nachtfee by its name, what is described here actually is Nachtfee

Although, R.V. Jones normally is rather accurate in providing information on German technology, here he apparently is falling into a pothole; simply (partly) incorrect facts is dealt with. Nix 10 sectors, but 16 sectors, which is in full accordance to the Nachtfee Command or 'order' compass. Please notice also the examples more up and down this page.

However, very significant on the other hand is quoting: The blip normally shows opposite 0 (= due North, AOB), but when a signal is being sent it moves to a position opposite one of the other numbers ... Focussing first on this particular phrase: What they are expressing, is, that the Nachtfee 'order' blip waiting position is due South (= 180°) and when an 'order' is due to come it jumps to 0°, which equals due North. Strange! This fact unravels why the Nachtfee ground console is being fit with a switch having in one position the text: Freya-Polwender*. What does it implicate? The only thing happening, is, that the two Nachtfee data-output-wire connections are interchanging. Call those a and b. When the Freya-Polwender switch is being operated a continues on line b  and b on line a. This is curious, how can this work? Please remember what have been described extensively, that the Nachtfee data signal system and the internal control-time-base-signals are interacting in coherence! The only purpose of the Freya-Polwender selector is: to set the Nachtfee data signal in a 180° rotated position. Thus, the blip facing at due North jumps to 180°, call it due South. Which latter position indicating that Nachtfee is not yet transmitting command- or 'order' signals. Only, after the blip or pulse jumps to 0° (due North), the 'observer' in the aircraft is informed (knows) that he should be ready for new 'orders' soon.      * Polwender may also be briefly translated: connection interchanging  


The Freya-Polewender switch is the one just right of the dual-trace CRT (the big one); the Freya-Polwender plate being about right of its painted time-base trace. This plate is mounted below the switch, which means that its handle should point downwards when Freya-Polwender being operated; hence, during action this handle should face upwards


Quoting further from page 17 and 18

We have listening evidence on two occasions of a Freya on about 125 Mc/s giving a second pulse. In one case the Freya was to be an EGON Freya; the second pulse was phased at a constant interval from the normal pulse and the two blips were of equal size. The latter point excludes the possibility of the second blip being a permanent echo. In the second case the phase difference of the two pulses varied rapidly. It appears highly probable that the second pulse in each case was in fact giving a signal of the type described above.

What is described are two situations. The first one handling the Freya-EGON signals where the Nachtfee signal most likely operated at channel Q5, which is 500 Hz and thus equals the regular EGON PRF. It is evident, that this will causing two signals having a more or less constant phase difference. The second case, equals the situation we currently are operating, where Nachtfee operates, for example, channel Q8 (506 Hz) and the EGON PRF still being 500 Hz. That those signal pulses are of an equal amplitude is owing to the fact that these both are using the same transmitter. Please consider the next photo.    As to simulate as near as possible the wartime situation, we also transmit pulses having a PRF of 500 Hz (without further using them), equalling the EGON signal pulses.  It is extensively dealt with the fact, that signals having a mutual phase difference of at least n · 2 Hz are not hampering (interfering) one-another much. (Bladwijzer210)


The smaller pulses constitute the Nachtfee 'order' signal. The seemingly a bit broader pulses representing the simulated EGON pulses. Their pulse-width look broader than they actually are, because they move over the CRT screen because the scope triggering captures the Nachtfee pulses of 506 Hz (say 2 ms), whereas the EGON pulses move a bit during the camera exposure time. It has to be noticed, that both signal pulse-width were set for this occasion broader than is necessary, only for viewing convenience

Hence, the current scope time-base is set at 500 µs/cm (4 x 500 µs = 2 ms)

What is found being a bit strange, is the fact that this report is carrying a date of 11 April 1945. Why are they telling us this, as the Baby-Blitz occurred a year before?

Quoting from page 13:

Maybe lifting some of the secrets, is the reference to the German tactical application of Nachtfee when they dropped radio-buoys (types Swan and Lux) in the North Sea accurately, as to navigate (and bearing) upon so that He 111 aircraft could launch V1s towards the British Midlands on 24 December 1944


Not particularly referred onto is the fact that the Germans dropped mines as well in the Scheldt Estuary as to hamper Allied ship movements to and from Antwerp Harbour; here also Nachtfee was used operationally.


On 28/29 January 2013   


We went through Nachtfee aspects which was dealt with in British wartime documents. However, all our efforts did not bring to daylight 'technical documents' from the era 1943 - 1944 or 1945. With this fact we have to live, though, what is ultimately reconstructed is, in my perception, quite much; albeit, one should never say this about his own commitment



On 6/8 February 2013


In the above overview of the current state of affairs of our Nachtfee subject, I mentioned that a 'Brainwave' might appear once

Yesterday I got one!


Jaap Keijzer visiting me in the museum was, of course, shown the latest state of affairs on Nachtfee. I first was knocked down (depressed) by the fact the the LB2 circular CRT only painted a very small horizontal ellipsoid. Was the EL11 output transformer failing? The first thing I did was opening the rear cover plate, though measured at its anode > 200  V, which indicate that the defect has to be found elsewhere. Luckily quite soon I tried to shorten a circular shape forming potentiometer and Jaap shouted it works again - no it does not! It was recognised that a wire of a potentiometer was broken off. I believe that this has been caused a year ago when the LB2 brightness potentiometer was replaced three times, owing to a short circuit in the LB2 filament transformer (- 2kV against ground).


However, I explained him why it is inevitable to feedback towards the ground-system the actual time-base-signal phase of the simulated aircraft display ('TB'). I doubted first but after a few seconds I realised that I did not explained it incorrectly. Though, that the two vital controls Range off-set + system 'Phase' might have been controlled by a single operator.

For instance, the Würzburg operators were designated B1 - B2 - B3 ...; B likely standing for Beobachter 1 - 2 .... I trust that similarly was accomplished in the Freya-EGON site.


On the left-hand CRT screen the correctness of the Range offset being indicated (the small vertical illuminated scale window upper left of the 'order' or command compass. On the smaller LB2 CRT screen we notice two blips. The one due North is the returning Nachtfee 'order' pulse which is aligned upon correctly versus Range off-set. The blip pointing at about 8 minutes passed the hour is the 'TB' reference pulse originating from the simulated aircraft time-base. From experience, this vector being set by means of the 'Phase' control correctly. The 'Phase' control is just left of the Nachtfee 'order' compass (down the black knob). It is clearly shown, that it does not have a scale nor any other means of watching what it is doing, but the blip at 8 minutes passed the hour is the only to be adjusted reference!

Please view my older YouTube film which shows how the 'Phase' control interacts with the 'TB' reference pulse originating from the simulated aircraft system

Nachtfee testfilm 11

This film was taken before the full understanding of Nachtfee matured.

The second film shows two aspects of the Nachtfee control screen, namely viewing the returning Nachtfee 'order' signal which is being used as 'to adjust range offset'.

Nachtfee Testfilm 36

One important aspect should not be forgotten:

The 'TB' reference (in the previous photo the blip at 8 minutes passed the hour) is being adjusted upon. So far in our experimental setup especially the first three hours of operation the 'TB' reference is 'walking' constantly, and can be corrected by means of the 'Phase' control. It actually should be rather stable, but it is the internal Nachtfee time-base reference that moves for hours! Please remember, the Nachtfee system relies on the 'domain of time'. We have also dealt with the necessity of aligning the offset of range. However, the 'TB' reference signal originating from the aircraft is also inflicted by range, hence a 'domain of time'. The regular Nachtfee operator should thus having controlled both the Nachtfee range offset as well as the 'TB' phase shift caused by the time the 'TB' signal is underway from the IFF transponder and the instant when it is being made visible on the Nachtfee LB2 control CRT. (the blip pointing 8 minutes passed the hour, shown in the previous photo) Of course, neglecting the initial mutual time base drifts, which are additionally to take care of. Doppler is not playing a role, it is only the mutual distance, thus time, that counts!

A second Nachtfee operator was most likely taking care of the 'order' or commands sent towards an aircraft under guidance. We have already been informed that the Nachtfee signal blip in its waiting position is pointing due South. This is accomplished by simply operating the Freya-Polwender switch just right of the dual trace CRT (its two output wires being interchanged); the firstly due North facing blip jumps then to due South. As long as this was the case the aircraft operator (Beobachter) knew that the ground console is in the waiting mode. When now someone of the Freya-EGON crew gave a particular 'order' is first due North to be sent, the console operator switched over to normal operation firstly. The according signal (with and without Freya-Polwender) is also visible on the LB2 control screen. What is shown there is what returned from the FuG25a IFF transponder. Please consider for more details the 'Explaing it a bit differently'. This same signal is used as to adjust the range off-set on the dual trace CRT exactly. Its operator may also have been responsible for the correct adjustment of the 'Phase' control. This control is manipulating the overall Nachtfee signal phase. It is only inflicting the TB reference signal and in no way the Nachtfee 'order' signal (viewing it on the LB2 control screen), as these signals act in coherence in conjunction to the internal Nachtfee time base system, like the Nachtfee 'order' data, it is derived from a mutual source. Though, what it does, is manipulating the Nachtfee data signal 'Phase' in such a manner, that an 'order' will arrive (phase shifted) just on time where an 'order' should be painted on the aircraft CRT screen. What it however does, is changing the Nachtfee data signal-phase in such a manner, that a Nachtfee 'order' blip is being painted just at the correct CRT screen vector! Nevertheless, this action cannot be monitored from Nachtfee control CRT screens!

I have to make soon a special video film in which is to be demonstrated how the various signals and controls are interacting.


I have, nevertheless, to admit that this operational reconstruction may be regarded hypothetical and arbitrary.

On  the other hand, viewing the experience which has been cumulating over the period we deal with controlling the Nachtfee system parameters, it is hardly thinkable that all could have been accomplished by a single Nachtfee operator. This is why, in my perception, there is no way around splitting the work-load and letting the two entirely different tasks doing by two (well) trained specialists. My guess, the one controlling the aspects of the 'domains of time' is accomplishing a more demanding job. 

Temporarily conclusion: it is very likely that the Nachtfee console was being managed by two operators. Whether these were called B x? I don't know. My guess - the two operators were sitting next to one another in front of the Nachtfee console.   The left-hand person controlling the returning signals in the 'domain of time' and the right-hand operator taking care of the Nachtfee 'order' system. We know, that synchronism between the Nachtfee ground console and the aircraft time-controlling system should have had a mutual stability of say 10-7 at least. It is, however, possible with quite some exhausting efforts to counter some of the system instability by means of frequent TB 'Phase' adjustment. The correctness of the TB 'Phase' is crucial and when an operator would make mistakes, an actual 'order' will appear in the wrong 'order' segment!  It is therefore likely, that two men were responsible for Nachtfee's well functioning.


It is evident that without meticulously correct controlling Range off-set and 'TB' signal 'Phase' - a conveyed 'order' can causing disasters! What to think of: open your bomb doors instead of fly a circle, or returning to base whereas he should initiating offensive action? 


To be continued in due course    


By Arthur O. Bauer



Please consider also, or proceed with the: Nachtfee YouTube demonstrations

Please return to, or proceed with: Nachtfee things done

Please continue with, or return to: Nachtfee in the domain of time