Every year since late 2011

I get in summertime the strong feeling that the Nachtfee Survey is not yet completed.


Enhanced on: 24 July 2014

Additional + 1 considerations



My wife Karin created this image, which symbolises: lifting Nachtfee's technical secrets


The small pointer is the one that actually provides a particular 'order' or command; the longer one is only constituting a mechanical memory. The latter having no internal link whatsoever.



What is missing?



It is most unlikely that we can find these materials in the British National Archive (formerly PRO/ Kew), as my friend Phil Judkins did a solid search there. He traced a lot, but all of a non technical nature.


But, where did our Nachtfee apparatus came from? Just from the US.


It is quite disappointing that no serious support is coming from that vast country.

I know, that it isn't easy.

But without support - we cannot come to a conclusion of this intriguing survey. We should not forget, that according to what we know, and the state of affairs to our apparatus, it was an experimental system. According Fritz Trenkle, five samples had been manufactured handmade. Nothing is build military alike.

But too many references from both German as well as Allied origin exist, that nothing was investigated upon. Two genuine German wartime papers, one dealing with Blohm&Voss project P194 and the other with a premature mentioning of it, and Radar News 19, the latter being an Allied post war issue (translation) date October 1945 (neglecting R.V. Jones'A.D.I (Sc) and Felkin A.D.I. (k) papers). The drawing put on the last page of Radar News 19 was very most likely not of a German origin, but a post war product. This brief drawing (Fig. 6) was for me 'the key' for eventually deducing how the Nachtfee system actually must have functioned. On the other hand, it is most un-German alike, giving details in a drawing without refereeing onto it in the accompanied text.  We therefore may assume, that technical and/or operational investigations once had been commenced.


It does not work, mentioning that we should approach the files of late Mr xx kept at the library Y.


What is foremost wanted, although, nowhere being mentioned: the Nachtfee system has to rely upon a two way 'signal phase' information exchange. I refer to what I call the 'TB' signal originating from the time-base source inside the aircraft 'payload', conveyed (additionally) by IFF means towards the Nachtfee console; which was integrated into the so-called Freya-EGON system (EGON-B). This reference signal has to be understand acting in the 'time domain'. The Nachtfee system simply could not have worked without this elementary phase information feedback. A very strong fact supporting my perception, is the existence of the so-called 'Phase' control at the Nachtfee console front panel (down on the left-hand side of the 'order scale'). Which setting cannot be controlled by what ever means build-in the Nachtfee console. It only changes the time-base signal phase (Phase control A). But, because the rest of the following system is being operated in coherence, it is impossible to know what it actually is causing. It determines, however, the instant at which the 'command blip' appears at the aircraft order screen, without any notice of the ground control. In my understanding, the additional TB signal should have a system-related off-set, which additionally is painted at the 'LB2 control screen' (ground console). And should be adjusted upon by means of the ominous 'Phase' control setting and adjusting it at a certain vector off-set against N (North being the starting point at the 'order' compass scale adjustable at the Nachtfee front panel). Interesting, is, that the returning 'order' signal is responding in coherence - and the TB reference is acting not accordingly. Please bear also in mind - that the distance covered between the Nachtfee ground console and the (moving) aircraft is, of course, equal for both directions of signal transfer (towards the aircraft and returning to the ground station).  For better understanding what the mutual implications are, please digest my PowerPoint presentation again (ppt converted into pdf)



On 24 July 2014

Additional + 1 thoughts engendered this early morning

Being quite well acquainted to German wartime practice, I considered - what the Germans might have called or designated the 'TB' signal-reference during wartime days?


What should it be about?

There is apparently no way around, that the ground station should have had knowledge of what the actual signal phase of the arriving - as well as the local time-base-phase is. The distance or range between the moving aircraft platform is being compensated for by means of the Range offset control setting (see photo down).  



Let us consider first what the routes of the engaged signals are

This quite basic drawing provides the Nachtfee principle. It is evident, that the distance from the upwards going and returning IFF and all other signals do bridge equal paths; which path should be controlled for within the Nachtfee console provisions.



For it they built in a range offset dial

Maybe not instantly clear, but this scale points to a range covering 0 up to 300 km. This control is, however, only linked onto a (servo like) goniometer (C) device acting as a phase shifter between 0 and 360°. This control is placed within the CRT time-base circuit only. Please bear in mind, that Nachtfee, as well as the linked Freya-EGON system, both operated at a PRF of 500 Hz. The Nachtfee could, at will, operate with a PRF differing in 2 Hz steps as to allow other systems being operated within the same frequency spectrum. These very small difference does no cause a system implication. 500 Hz equals λ = 600 km. However, distance has to be bridged twice (up- and downwards, thus like in radar technique, range should be divided by a factor 2. Hence, we get 300 km)



Viewing the Nachtfee console control screen (LB2)

The signal shown due North is the Nachtfee order signal returning from the IFF transponder and being 'range offset' corrected (this signal acts in coherence). The signal pointing at, say, 8 minutes passed the hour constitutes the arriving simulated aircraft 'TB' signal. By means of the 'Phase' control (goniometer A) its vector should be set at a specific vector (being non-coherent). In our case, and likely also during wartime, its vector constitute a constant system parameter; for which it should be corrected for. In our case being the actual displayed vector. This particular signal vector secures that the 'order' or command signal appears at the correct 'order' screen vector within an aircraft.

Only by this means is it possible to let the system operationally do where it is intended for.



a new dimension is coming into force.

Consider, without bringing for this occasion the proof, that the display system inside the aircraft cabin is out of order; would it be possible noticing this within the Nachtfee ground console? No (leaving out the option calling for changing IFF recognition channel one or two (nal 1 or nal 2).  Because the returning Nachtfee 'order' signal is passing through the I.F.F. transponder without further notice. Another aspect is also appearing, do we know what the sent 'order' signal phase will be at its instant arriving at the input of the Nachtfee aircraft system? No, we don't know either. However, this should be also considered for, as it is an essential parameter. Please remember, when both the Nachtfee system and a pathfinder aircraft are standing next to one other and the 'order' blips on both system CRTs are pointing equally; say, due North: what would happen with the 'order' blip in the aircraft when the latter takes off? The 'order' blip will start moving anti-clockwise on the circular time-base line; as its signal is steadily arriving with delay increasing. After passing a distance of 300 km it will be displayed correctly, but only at that short lasting instant! In the meantime, the blip has rotated over 360 degrees.

On the ground the blip position is being corrected upon by means of the above shown control knob C. But this control only corrects the CRT time-base-phase inside the Nachtfee console, and does no inflict the actual 'order' signal phase. It is evident that also this factor should be taken into account. But how?




The range offset control is designated C.

'Order' or command goniometer (phase-shifter) being designated B

Phase control A being the only existing provision inside the Nachtfee console - which act at both signal chains (channels), and lacks a controlling provision inside the Nachtfee console.

Please notice: that signals passing goniometers B and C act mutually in 'coherence'. Whatever the setting of goniometer A, the control CRTs will show no signal phase difference whatsoever, but it influences, doubtless, the signal phase send towards the aircraft under guidance (theoretically, in our apparatus depending upon temperature a slight influence does exist. In my perception, owing to the way the 3-phase signals being converted or derived from the single single-phase circuitry. The goniometer field-coils being supplied with a 3-phase signal, while the rotor or search coil only picks up a single phase signal).



Considering the foregoing explanation, I have decided, for an enhanced understanding, to implement the Nachtfee block diagram too

Please click at this drawing as to open it in PDF, getting a good copy

When you look closely, the three-phase feeding of the phase-shifting goniometers, will be understandable.

It is evident, that both controls B and C cannot be used for compensating both changing system range or distance, as it lacks an important information parameter - what is the signal phase when it arrives at the FuG 25a IFF antenna or receiver front-end; leaving aside a most significant parameter, what is the actual signal phase of the internal time-base generator inside the aircraft fuselage? These two additional system parameters are most important, as it determines at what CRT screen vector a given 'order' will be painted at the CRT screen in the aircraft cabin. Whatever one may think, there is no way provided knowing this, without an additional signal reference showing the actual time-base signal phase (compared to the one build-in the Nachtfee console). Regard, the aircraft time-base has a different phase, this is showing itself 'as if' distance or range is different. That information is being transformed by countering a particular (phase) deviation by means of Control A inside the Nachtfee console.     I came already quite early in our Nachtfee survey to the conclusion: that in some way or another a signal should be send downwards onto the Nachtfee ground console and being displayed at the LB2 control CRT screen too. I designated it 'TB'; which designation is, of course, arbitrary. As so often, before waking up some new thoughts engendered this morning; what designation would the Germans have given such a signal? Their abbreviations often are very clear.


My brief guess, in a non alphabetic order

The small characters might also have been written in capital (Bpr becomes BPR or BPr)*


* Leaving aside grammatical matters, whether it should be written Phase or Phasen etc. 


Providing only some examples - as it once might have been designated. This random list only is providing some typical German wartime abbreviations.

It might, nevertheless, helping US searchers, looking for additional keywords.

Consequently, it appeared in my thoughts also - that Freya-Polwender, might have stand for: Freya-Polaritätswender; a designation which certainly is also correct; this switch only interchanges the two signal output wires (consider the previous block diagram). This only is having an effect as long as the system operates coherent.



When life would be easy, its technique would have been common sense.



Repeating, what do we foremost would like to get knowledge of:

Post war papers concerning technical Nachtfee (FuG 136) matters (copies)

In particular, German wartime technical materials: like operational instruction papers, service manuals or preliminary instruction papers (schematics), Luftwaffe evaluation report(s). Another option, post war personal recollection, as was done by K. von Gregor in respect to Klein-Heidelberg on 31st May 1945 at Grauel.  (copies)

Allied (U.S. and/or U.K.) post war investigation or evaluation reports (copies)

There can hardly be a doubt, that some must have been investigated, be it briefly. Because, introducing a promising system, at least the subject must have been evaluated; especially regarding the quite many occasions where Nachtfee has been discussed. (copies)

Our main concern, is, the nowhere mentioned aspect of the above proven fact, that in some way or another the actual 'phase' relation of the 'flying platform' must have been feed back (additionally) towards the guiding Nachtfee ground console; call it the Freya-EGON station (EGON-B). This essential system parameter might also have been a crucial bottleneck. (copies)

Last, but not least: was it the necessary 'rather high time-base stability' that finally caused the abandoning of a Nachtfee like guiding system? In some respect, OBOE was operationally a more complicated path finding system than was Nachtfee; but might not have been so critical in respect to mutual 'long-term time-base stability' (2•10-7 - 10-8). (copies)



It was noticed from statistics, that many of you followed vividly the course of our intriguing Nachtfee Survey; lasting for almost two and a half years.


We therefore are calling desperately for your assistance.

You will be part of this survey and being fully recognised and credited for your contribution.


For the one who finally provides the crucial information, we invite him for a visit, on our expenditure.



Please contact us at:

(Please type in what you read)


For those really interested in this thrilling subject, go through the 32 dedicated webpages.

Starting first with FuG136-Nachtfee

Finally: Nachtfee seemingly a never ending Saga

You can access them all in succession from the Index page


By Arthur O. Bauer


Please consider also: our agenda including my reflections, of 1st September 2014