How could the Nachtfee data have been conveyed towards an aircraft under guidance?


In progress

Page has been started on 17 February 2012

Status: 11 April 2012


Please notice also, that on 10 April 2012 the entire simulated loop chain have been closed. (Bladwijzer20) The Nachtfee signal transmitted by HF means towards FuG25a - its receiver output providing the Nachtfee orders onto the simulated aircraft display - and the combined aircraft deflection phase, together with the actual received order signal,  retransmitted by HF means towards the Gemse receiver. Its video output fed finally onto the Nachtfee controls screens (LB2 and HRP2/100/1,5A)


In our previous report:

Nachtfee evaluation and conclusion page

It is already pointed that even with modern means it is merely impossible to maintain full synchronism between the Nachtfee console and the independent FuG 136 system in the aircraft. In my perception it is even today hardly possible to maintain full synchronism between two entirely independently operating systems which is lacking any means of synchronisation.     The only likely method possible might have been the human brain, by which means temporary synchronism had to be maintained manually from time to time. Although, to my knowledge this nowhere is mentioned, a tuning-fork-oscillator may have constituted a means of quite stable frequency or timing source. Its advantage is that some types are more or less electrically tuneable over a some limited range.

Our experiments have so far proved that the frequency deviation is < 0.7 Hz (at 500 Hz), between the starting up of Nachtfee and after a few hours operation. During the course of its operation, deviation is reducing to about 0.05 Hz. For practical reason, we have to switch off the experiments every time completely, whereas the wartime Nachtfee thermostatic quartz oven was likely running continuously; even when the mains is being switched off (having a separate 220 V a.c. input).     However, seemingly a negligible figure, but even this tiny value is making Nachtfee a worthless system. It is very unlikely that aircraft systems operated under such favourable operational conditions than we can accomplish in our museum.


A down side in respect to our current experiments is that we would need a tuning fork oscillator at say 500 Hz. When a provision for frequency fine adjustment is lacking, an experimental modification could be implemented. This based on a technique accomplished in the 1937 Rohde & Schwarz portable CFQ quartz clock.



The basic principle on which a tuneable fork oscillator is relying ( drawing reproduced from the above source)


As we in Holland say: a knife cuts on both sides - first I would like to counter objections that what I am estimating is not realistic. Though, also that it is a quite well possible means as to  maintain some kind of synchronism manually, between the actual Nachtfee data vector and the displayed 'order' on the aircraft display unit.

As long as we do not have such an apparatus, our experiments hardly will generate - the ultimate conclusion!

A likely problem may be, that there are not so many around who is having an idea how such a device may look like.


 Is there someone around who can help us with finding the missing links, like a tuning fork oscillator (at 500 Hz or near by) eventually having a different frequency?

Please contact us at:

And type in what you read

Those participating will be recognised and fully referred on to.



Let us first consider what the basic principle of the Nachtfee 'order' or command system was



The basic flow of Nachtfee data towards the aircraft under guidance

This drawing equals the one of Radar News 19 fig. 6

Please double click on this drawing as to open it in pdf



The only touchable device that we have is the Nachtfee console. This have been described extensively, so that no details will be, in this context, dealt with. Please consider the various links very down this page.


From which point exactly in the Freya-EGON system the Nachtfee feedback signal was derived we don't know. Though, it was most likely derived from somewhere at the end of signal chain; and this implies near to the transmitter/antenna coupling.




On 21 February 2012


Left us consider two purely hypothetical conditions

Whether we like it or not, we have to deal with it as there must have been a way by which means the downside of the Nachtfee system could have been countered. All in respect to what we have discovered until today.


The following two drawings and considerations are fully hypothetical!



Let us, hypothetically, consider that the ultimate shown order pointer (signal phase) is  returning to the control CRT (LB2) in the Nachtfee 'order' or command console.

Please double click on this drawing as to open it in pdf



Is this a realistic option? Why not?


An advantage is that the actual Nachtfee pointer is returning and displayed. Though, is this really true? Honestly speaking: no - as the trajectory has to be passed twice.



A third option would have been to modulate the returning EGON signal in some way with the time base frequency

Please double click on this drawing as to open it in pdf



An advantage would have been, that it was possible from the ground to control the 'order' or command presentation vector in the aircraft. In some respect it may reveal why some of the controls of the Nachtfee apparatus are existing. For instance, the 'Phase' control at the front panel. It is only influencing the phase of the data (output) signal. Thus doing what is also meant for the actual 'order' pointer. Also the curious number scale which signal phase is influencing the feedback signal phase on the control screen (LB2), but not having an impact on the data signal. Why is it having a number scale, and the 'Phase' control not?


Maybe this is the solution in respect to the application of the dual beam CRT, which was designated with: Range adjustment tube? I can not judge it yet, though hoping now for a next fruitful brainwave.


There is also another phenomenon which we have not yet dealt with: The aircraft is constituting a body having a velocity of say 300 km/h = 83.3 m/s. What is its implication on the phase of the Nachtfee signal? Each Nachtfee signal period is (for Q5) 2 ms. The aircraft is displacing with roughly 0.16 m within a single Nachtfee cycle of 2 ms. A frequency of 500 Hz is constituting a λ of 600 km; 16 cm versus 600 km is a negligible figure. (Bladwijzer2)




On 22 February 2012 (Bladwijzer3)

We probably may say Eureka!


Please bear in mind, the previous paragraph where we noticed that λ of a 500 Hz wave is 600 km. But the Nachtfee data, by whatever means, is having to pass the trajectory twice. Thus distance has to be divided by a factor of 2; making it 300 km.


I was merrily paralysed by realising that this maybe correlate with the until now called 'number scale'. I rushed to the museum as to verify whether my assessment is valid. And it is! Hence, the scale is indicating distance, as to which the Nachtfee data signal should be compensated for.



This discovery is changing a lot!


I just wonder where this tiny cylindrical hole is being meant for? Is it hypothetically meant for unlocking the distance scale drive, as to allow new distance calibrations? Maybe it is only is a tiny hole without further meaning. This point has, nevertheless, to be investigated in due course.


Again hypothetically, but definitely every time coming nearer to the ultimate truth.



For simplicity I have drawn the selector switch being inside the FuG25a transponder, but it is likely that it should have been outside the crammed Ersting chassis frame

I trust, that for most of you, this drawing is self explaining.


The returning time base data towards the Nachtfee apparatus is having a time delay in respect to their mutual distance. When a Nachtfee aircraft is surpassing the equivalent of a distance of 300 km, the returning sine wave is starting at 0 km again and ambiguities is caused.


What I have already mentioned in previous findings, is that the what I then called 'number scale', is found necessary to control correctness of the returning time base feedback signal.


Although, I cannot yet judge all the implications, the difficulties encountered by the impossibility of keeping mutual system synchronism is by this means fully countered. When the aircraft time base is reaching too much out off synchronism, this can be countered by means of the 'Phase' and 'distance' controls. The latter is only influencing the feedback vector on the LB2 control CRT. This is also implying that the 'order' or command blip on the aircraft display is depending upon distance from the ground station.



We have decided to implement our FuG25a and Gemse system into the Nachtfee system experiments



The full schematic of the FuG25a IFF transponder Erstling, including some minor modifications as to adapt it for FuG136 operations (yet hypothetically)


Please notice the implementation of a selector switch by which means it is possible to operate the system in regular mode ('E' = EGON), as well as in FuG136 mode designated by me with 'N' + Nachtfee. Of course, all is still hypothetical. But this is our first trial as to implement my Nachtfee understanding making it a system.

Please do not think that the sine wave, which is constituting the deflection signal of the aircraft CRT, is now being linked directly onto the grid circuit of Rö8. What we only need is a first order differentiated pulse, marking just the few µs after the sine wave has started a new cycle (just having crossed 0° of the axis nucleus) again. Consequently, providing an EGON like response signal towards the ground station. To my understanding, it is lacking range information, but it is the appropriate reference by which means Nachtfee was becoming a mature system. (all still hypothetical)


Our Nachtfee test will be commenced about the grid 1 (G1) of valve Rö 8. The vertical brown pvc wire (just behind resistor W42) is the only connection that is more or less easily accessible. Hence, hypothetically favourable that it was here interrupted as to adapt it for FuG136 operation as well. The brown wire is coming straight from the anode circuit of valve Rö5


It is quite understandable, that there is hardly space left for modifications, but we will have to find a way to implement the (screened) wires. Although, not yet checked it, but I would prefer connecting the three screened cables onto free pins of 'Bu1' (lower side of the FuG25a schematic) or eventually connector 'Bu3' (just visible on the next photo) 

After giving it a new thought, the Nachtfee 'order' or command signal may also be derived from the anode of Rö4, which is in a galvanic way connected straight onto pin 7 of Bu3. I cannot, however, judge whether it is practically possible getting access as to disconnect about C32 (G1 of Rö5). I am afraid that this will be impossible due to insufficient access.  


The Nachtfee signal will be derived from the anode of Rö5 which is feeding the input circuit of Rö8. On the left hand side we see the transmitter valve type LS50. In this configuration providing 300 - 400 W pulse peak power at a PRF of 500 Hz

 The compactness of the FuG25a wiring is well visible. Please notice that valve Rö5 had been accepted in week 6 of 1945 (mid February 1945).


Down on the left we see the 'Phase' control, which is having influence on the data signal phase (virtually moving the 'order' or command blip where it should actually pointing at), whereas the distance scale is having to compensate the implications of distance of the 'feedback' signal; and, maybe, the additional time delay in the EGON - FuG25a - FuG136 unit as well


We have discovered now an explanation for the until now rather mysterious controls.


However, still some unsolved operational and technical aspects may be left. But this is making our Nachtfee project often so thrilling! Reconstructing operational and technical queries, by judging given facts a thorough thought. Implementing the experiments as to how an aircraft CRT presentation should have been operated and drawing conclusions of the impossibilities of keeping system synchronism practically, led me finally to consider a way countering the faced downsides. The many experiments done also pointed that some of the Nachtfee controls did not make sense, as, for instance, the 'Phase' control without feedback (knowing what is happening on the aircraft display). Also, why is there a control having a scale covering 0 - 280 (0 = then equal to 300)? It was my query - does the aircraft speed conflict with the phase of the data signal? After I found that this did not, because during every Nachtfee cycle of 2 ms the aircraft is roughly displacing 16 cm versus λ of the 500 Hz data being 600 km. It is also realised, that the operational range of Nachtfee is consequently 300 km. In a flash I realised that this may be what is given on the 'number scale' control. I rushed to our museum as to check whether this is true. And, it is! Since, most remaining system queries are falling in their holes. Of course, there are still some left unsolved. I hope, nevertheless, that later experiments can bring most answers. A problem might be encountered as to simulate distance. Here we may have to implement modern digital techniques, for example, as to variably delaying the Nachtfee data signal (simulating changing distance).


New additional hypotheses


What should be controlled?

A disadvantage is, that I have never watched seriously the according numbers on the 'distance' scale, as these didn't make sense to me. I knew that using this control I was able bringing both the 'order' or command pointer in line with what appeared on the control screen (LB2) (thus rotating at will the feedback blip). But it was possible to set it everywhere else as well.

According the Radar News 19 drawing fig. 6 the dual beam CRT was designated being a Range adjustment tube. I have no idea yet how this has to fit in the Nachtfee system. I have to commence some new trials as to determine how the various parameters may fit together. Why were they adopting a dual beam CRT type (in those days certainly a very expensive device)? To my current understanding their time base is equal, albeit, that their horizontal as well as vertical deflection systems are all wired in counter phase. Thus say system one is painting a spot from left to right, the second system is doing it in the opposite way thus from the right towards the left hand CRT side. The same is occurring to the vertical deflections. This might making sense, when each CRT system is getting signals from two different channels. However, apparently this isn't the case. Or, a deliberate kind of sabotage must be dealt with. Though, I have in this respect not yet found an indication. Although, we have encountered some very strange wiring faults. However, a dual signal chain necessitates two vertical amplifiers but, only a single one is existing (Rö7).      




On 24 February 2012


Today I would like to look closer to what the implication of the 'distance' scale-control on the control screen (LB2) is.

Our pulse- time delay module proved to be defect. Thus simulation on changing distance has to wait for a better occasion.

It, nevertheless, was a fruitful day again.


First I tried by which distance (scale) number, or km setting, the 'order' blip was also pointing North. This was at about 50 km (scale reading)


However, another new aspect was also discovered. It might be possible, that the two (back-to-back) pulses shown at the dual beam CRT (HRP2/100/1,5) are introduced as to determine what the time base centre or null is. The feedback blip on the right hand screen is pointing equally as does the 'order' pointer. Now both are brought in line, but this needs a pre-distance setting of say 50 km. My guess is, that by means of the dual presentation it is guaranteed that the pulses are adjusted just where the centre of the time base is. Thus where say 0° is situated.


Distance control is set at 0 km

It is clearly visible that although the 'order' pointer is set at N that the (feedback) control blip is pointing at about 335°. This difference is clearly recognisable on the left hand screen. It is however, possible to compensate this negative deviation by adding the same angle value, but now in a positive manor, thus setting the 'order' pointer at 25°.


This time the Freya-Polwender selector has been activated (distance scale still set at 0 km)

The same about 25° phase lacking (negative sense) is also occurring. Please notice also, that the signal phase and their pulse shape have interchanged on the dual beam CRT.


We may estimate, more then simply hypothetical, that the purpose of the dual trace CRT is most likely to adjust both the signal phase of the 'order' pointer as well as the phase of the feedback blip. In practice this should then being the time-base-null-phase-blip arriving via the EGON system. Which is to be compensated for by means of setting the 'distance' control appropriately. Whether this is in accordance to the designation Range adjustment tube? Or, with range adjustment is being meant the setting of, what we call, the distance scale. Using the expression range may be originating from a translation of the word distance? Although, in German language "distance (Entfernung)" and "range" may have caused confusion.


In the meantime we are also preparing the FuG25a in combination with the Gemse receiver to become operational again. For this purpose, Dick Zijlmans is preparing wooden mounting frames. In the meantime I am preparing the essential wiring of the FuG25a system. Dick may be wiring it, but is is necessary to show him what it is about prematurely.

As to ease wiring for Dick Zijlmans, I would like to start with experimentally wiring the FuG25a system. Dick is then later replacing the experimental wires by modern means.

The reproduction quality of the FuG25a schematic is a bit fuzzy, and it needs some amount of system understanding as to reconstruct its surrounding wiring. It needs acquaintance to understand the sometimes rather confusing wiring.

Investigating the schematic of FuG25a more in details, I discovered that it is possible to derive the Nachtfee data signal directly from the anode of the 'audion' or detector valve (Rö4). However, getting full access to its circuitry, I would think that the previous shown solution is more likely. Maybe they have commenced a combination of both. It is also found, that additional wiring is simply possible, without having to irreversibly change the FuG25a chassis; by simply using some open pins of the connector Bu1.    For full schematic details: FuG25a manual



On 25 February 2012

My thought went again on the pro and cons of the application of a time base feedback signal from the aircraft display. Then it came up in my mind, would it make sense when we can mix up both the Nachtfee 'order' signal with the actual time base phase? An appropriate circuit is a kind of coincidence stage. My first thought was, did this technique exist in other German wartime circuits? Yes, it did. The Nürenberg ECCM stage belonging to Würzburg radar was having such a circuitry. Where the audible propeller modulation had to coincide with the gating pulse originating from fine range blanking marker of EAG62/65.


Thus why not giving it a chance of investigating its implication, hypothetically?


Hypothetical set up of a Nachtfee system, where the feedback (blue line) signal is indicating the coincidence of both the Nachtfee ground signal as well as the actual time base phase of the aircraft 'order' display in respect to distance. For this occasion I have shown the returning feedback signal being out off phase. Whether because the distance compensation setting was not adjusted appropriately or that the 'Phase' control on the front panel of Nachtfee wasn't adjust correctly; or, that so far no action had been undertaken. Please notice (Bladwijzer19) for a changed concept that works rather well since 3 April 2012!


It is necessary that both the actual setting of the Nachtfee 'order' pointer on the ground stays in concert with the displayed 'order' in the guided aircraft. Nachtfee made only sense when mutual synchronism could be maintained. We have already proved, that it is inevitable that two independent time base systems have to be employed.


It is evident, that the 'coincidence' technique should be experimented as well. However, my first concern is to delay the existing Nachtfee feedback pulse, as to look what its impact on the Nachtfee controls is (simulating a returning signal from distance). Sadly, our special generator is defect. Lacking documentation we have to wait for another solution.

 Please consider also my Blocknote survey report



On 27 February 2012

I did some experiments with watching what an additional phase shift is having on the function of the 'distance' correction control


How would it be possible to simulate phase shift as which is caused by operational distance?   Lacking yet a delayed pulse repeating generator, I took the same phase shifting network as had been used during the X-Y Lissajous experiments. By this means it is possible to simulate phase shift at any value, as long as the 'order number' of the network involved is adequate. A downside encountered was that the R/C ratio was not optimal. Causing sometimes a too much signal strength reduction.     We are making an external amplifying unit, which is to be designed that between the two amplifying stages we create a means which will allow us to implement any kind of phase- or pulse delaying circuitry. I have decided to change, nevertheless, the R/C ratio as to match better onto the higher impedances in valve circuits. The phase shifting principle stays nevertheless equally.

Encountered was that a tiny bit of shift was introduced between the dual beam CRT (Range adjustment tube) pulses and the blip painted on the control screen (LB2). I don't know yet its origin. Maybe it is the consequence of our experimental set up.     I also get the feeling that there is something wrong about the 'Phase' controlling goniometer (servo- or Drehfeldgeber); as its operation is a bit strange (Please see my: Nortebook pages XXXV -XXXVI). What might be the case, is that one out of the three-phase signal lines is mall functioning. I guess, that when it would operate as I expect the tiny phase shift should have been corrected (compensated for). 

It was also looked closer to the tiny hole in the 'distance' control knob. There is no hole at all; only a very small dot (dip) of a few tenth of a mm.


The phase shift circuit used is the one for the Y-channel (the called external 500 Hz generator received now the Nachtfee data output signal instead)


It worked as was expected, and we have to wait till a controlled delaying test rig is available. Be it made by means of a modern IC or an existing set. A good opportunity is, looking for it at the huge rally to be held at Rosmalen on 10 March 2012. For practical reasons, I have this time omitted an additional photo series, as these are only in very details conclusive. Please trust me that it works, and regard my Notebook for the pages LXI - LXIV




On 3 March 2012

Some new experiments were commenced


Until now the Nachtfee feedback signal was being amplified by means of a flying wire circuit hanging open in the air. Only being hold in a stationary position by means of a test clip connecting it onto the 450 V HT supply. Leaving too little room for experiments. It was decided to build a self supplied two stage amplifier module. As to stay in the time era of Nachtfee, we adopted a pair of EF14s.



As to keep its construction simple, Dick Zijlmans provided an Amroh "Uniframe" chassis. By the way very versatile chassis parts from the 1950s. Where a quite wide variety of frame parts is available

Dick Zijlmans also provided the HT transformer.

The two stages are of equal design. Each one operating without automatic grid bias, though with quite high anode resistors (100 kΩ). Its purpose is mainly to achieve a square wave signal. Between both stages a provision is made which allows implementing a coupling circuit at will, a direct ac coupling or one from outside. It is, of course, also possible to pick up the signal directly from  this circuit point, thus using a single amplification stage instead. The output signal is being differentiated as to derive sharp, relatively short, pulses (an additional diode is neutralising or blocking the remaining inverse pulse content). My estimation is, that these were likewise handled in the EGON system. It proves, that with minimal means short pulses can be generated from a sinewave signal.




Shown is an already less sharp feedback pulse

Please compare the other pulses shown in the Nachtfee project. It is clear that also the blip shown at the control screen (LB2) is having a different shape.

The two back-to-back pulses are being adjusted against each other by means of the 'distance' control which has to be set at '230 km'.


This aspect is also having interesting implications. As is described in the Nachtfee Inbetriebnahme (Bladwijzer9) webpage, I encountered also a problem in respect to the operating voltage of the LB2. When the CRT voltage is being increased above about 1 kV the LB2 brightness potentiometer is starting to deliver smoke and is failing since. What is causing this I don't know yet. But, it forced me to look closer into details how the circuit is designed. For this webpage it is significant noticing that the way the blip is being presented onto the CRT screen, is depending upon the way the signal is having its reference level. Thus when no pulse is available is the quiet-level high or low (thus: is the signal going up- or downwards). Another difficult aspect, is that when you use a differentiated network that the input signal may have been changed in its signal phase, the form of the circuit output may not change. Why not? Quite simple, because such a circuit needs the edge of a sharp pulse; and when signal phase may have been changed, there always is another sharp up-going pulse edge to respond at. This is what we notice on the dual beam CRT designated as: Range adjustment tube. Although we changed during the experiments the signal phase the two pulses were still pained as usually.


Though, how would it be possible to manipulate this aspect in respect to the EGON/Nachtfee data-output of the FuG25a receiver? I thus strongly believe that this is having a consequence to what some P/W (PoW) has stated. (Bladwijzer8) To my understanding, taking all parameters encountered so far, this is simply nonsense. I know that Fritz Trenkle relied heavily on Felkin reports, which I guess, once R.V. Jones may have provided to him (the ominous reference [101] is already used by Fritz Trenkle in 1966, without giving proper reference data. My, hypothesis: for understandable reasons he wasn't yet allowed to possess it. Only after Winterbotham's publication in 1974 the ban on these kinds of wartime materials were step by step lifted. Since he openly gave for the [101] reference Felkin. However, no one could trace it yet, as the about 1000 Felkin reports is being stored at PRO/NA Kew, under a total different file reference).

The Felkin papers are normally of rather good quality and by no means ignoring German achievements. But, they relied heavily on P/W interrogations, and only when a number of referees have been interrogated it is possible to draw more reliable conclusions. But, Nachtfee was secret and had never matured, that only a few were actually knowing the truth. 


I strongly believe, that the P/W interrogated may not having themselves seen the Nachtfee system operationally in an aircraft. My hypothesis, the inner and outer signal (blip) may have originate from the two available pointers at the 'order' or command compass.



On 1 March 2012

I received a kind message by Jørgen Fastner from Norway, in which he quotes from a just received source of information  (Bladwijzer5)



On 5 March 2012



JØrgen Fastner copied very kindly, certainly with great dedication, his newly obtained rare paper copies for us. He, like nearly all who are doing so, is unaware of the downside of copying in jpg as well as in full colour format. These files are becoming excessively large, he managed to copy 14 pages and making it > 42 MB; which converted in pdf still is holding about 37 MB!

Please notice: a message to all copying documents, when you are doing so, please use bmp format instead - as programs often will internally convert this content into 'gif or png'. Also try scanning it in black-and-white line drawing format and not as a photo image. JPG is good for reproducing photos, that is where its algorithm is being meant for. Is is not capable handling text like images, a text will always be more or less spongy (aliasing)!!!  

This is reducing the file content tremendously!! As to make the content of this document realistically accessible for you, I had to print it firstly with our mono colour HP LaserJet and then scanning it again. Our Agfa Arcus 1200 scanner is luckily having extremely good threshold provisions. Thus it is possible ignoring unwanted background signals as, for instance, being caused by the yellow flavour of old paper. We nevertheless, have to find an intermediate compromise between background noise and the line quality of the information. However, handling as described, the file content have been reduced to about 1.5 MB only!


Please notice the reference onto FuG136, which is the German military designation of Nachtfee; I believe is also the type reference for the Nachtfee display in the aircraft

Please also notice the information on the same line: 'neue Anlage, keine Unterlagen'. Translated freely: 'new system, information not available'


Also interesting to us: 'Gewichte noch nicht vorhanden, Grössenmäßig wie etwa FuG25a. Es besteht u.a. aus einem Sichtgerät, welches beim Flugzeugführer angeordnet werden muß'.

Briefly, the display is about the size of FuG25a and is to be mounted near to the chief pilot

For those interested in the full document please double click at:

 FT-Anlage Besprechung am 16.5.44 über P194

Also of historical significance is, that this document has been the product of a meeting held at Rechlin, the GAF research Establishment, on 16 May 1944. We have already noticed, that Nachtfee was developed at Rechlin by T. v. Hauteville  (Bladwijzer8). My guess, that this circumstance is likely the cause that Nachtfee is considered.



On 7 March 2012

We investigated the interaction between the sharp feedback pulses and distance correction


The photo caption is explaining how we have to adjust the back-to-back pulses versus the 'range scale' (Entfernung-Skala). As I wondered in a previous trial, why there exists a phase difference when seemingly the both pulses are covering?


Taking more notice that we have to adjust the steep slope of the first pulse edge against each other one. Though, because both time base are writing in opposite direction, their mutual pulse edge is what counts only


It is quite good visible that the 'distance adjustment' versus the alignment of the back-to-back pulse edges is the solution



On 31 March 2012


During my preparation for my talk on Nachtfee for the Dutch OTC on 1 April 2012, my attention was watching for undisclosed information on the following line drawing


We have dealt with aspects of this drawing for several occasions now, but my attention has not been drawn especially to the painted aircraft display


The information, however, 'Adjustable Airborne Presentation unit' may well have an implication upon our current survey. It has been found, that nearly all information in this drawing is having a relevant meaning. Does 'Adjustable' mean that the airborne operator had to keep synchronism between the Nachtfee on the ground and the local display in the aircraft manually? Also mentioning 'Switch for Receipting Messages' may tell us something. This may well indicate that the system was only activated after some flying time. In some respect I doubt this, because the 'Pathfinder' aircraft has to follow an exact trajectory, as to ascertain that a target is being approached in the shortest time possible. That is, in my perception, the most likely tactical meaning of a combined Freya-EGON station.

FuG25a and EGON both imply transmitting IFF replyin signals. We may assume nevertheless, that Nachtfee is operating with about the same prf frequency as did Nachtfee, thus with 500 Hz.


I am not yet grasping the full implications of what this British report is trying to tell us

The text above may indicate that two signals at once were being transmitted. Where the British Y-Service really listening into the direct signal from a Freya-EGON ground stations, or have they monitored the aircraft replying signals?

Noticing the quite many references in British and German reports, the idea is coming up that Nachtfee was considered a full operational system. Quite some new aircraft types were under consideration and many of them should be fit with Nachtfee (FuG136) as well. Please notice the Blohm&Voss P194 project above; also on internet Arado 234P was among them. We may however, consider that neither of those projects have matured. Quite understandable, as Nachtfee was tactically an offensive system, and the German capability to become offensive was vanishing rapidly. In this respect it is very worthwhile studying the war diary about minutes of meeting of KTB-Chef-TLR over the period from 12 December 1944 up to 4 April 1945!

On the other hand, our Nachtfee apparatus is fit with a control called 'Phase' have no scale and the controlling CRT (LB2) is only showing the actual signal phase of a 'feedback' signal! We have also proved, that synchronism between the Nachtfee apparatus on the ground and the aircraft display is a most delicate matter. Might the meaning of 'Adjustable' imply that the dedicated operator had to watching both the actual time-base phase versus the waiting position due South or Pauke? This would make the application of the Nachtfee system un reliable. Because this conflict I still believe that at least the time-base-phase had to be returned towards the control display (LB2) in the Freya-EGON controlling station.



Please take also notice of the new page: Nachtfee FuG25a test setup (simulation of the Nachtfee data chain towards the aircraft under guidance)  On 31 March the Nachtfee order system worked in conjunction with a simulated aircraft display successfully!

To be continued in due course


By Arthur O. Bauer


Please consider also our addional page on the implication of Freya-EGON and Nachtfee


Since August 2012

Please don't forget to use the handsome: Nachtfee Chronology page


And, the PowerPoint progress page (converted into PDF)





Please go back to, or proceed with: FuG136-Nachtfee starting page! (Status: 5 March 2012)

Please go back to, or proceed with: Nachtfee survey page 2 (status: 8 December 2011)

Please return to, or proceed with: Nachtfee survey page 3 (status: 21 December 2011)

Please return to, or proceed with for the survey pre-phase to: Nachtfee 3a (status 8/1/2012)

Please go back to, or proceed with: Nachtfee MLK Lab. Survey (status: 13 December 2011)

Please go back to, or proceed with: Nachtfee-Inbetriebnahme (status: 5 March 2012)

Please go back to, or proceed with: Nachtfee evaluation and conclusion page (status: 1 March 2012)

Please continue or proceed with: Nachtfee FuG25a test setup and data transfer simulation (status: 29 March 2012)

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