explaining its operation

by means of both drawings and YouTube films

Page initiated on 15 February 2013

Status: 18 February 2013


Nachtfee (FuG 136) have been explained in various ways, but it might make sense to combine textual explanation with dedicated YouTube film strips.


Let us first remember what the Nachtfee system is about

Objective: conveying 'orders' to an aircraft which is having an 'order' display onboard. The content of data, is, maybe was not intended to be, nevertheless, not detectable by enemy intercepts. Theoretically, its signal could have been blocked by interference, but its data content was never corrupted (when the Nachtfee data after all came through).  Only two fully synchronous running time-keeping systems can secure that uncorrupted data is being conveyed. The system is to be controlled accurately in the 'Domain of Time' by means of the feedback of the TB reference pulse; its actual signal phase is being adjusted upon as well. Constituting a dual closed loop system; where the human factor being decisive. 


Film 57: Viewing the current setup of our Nachtfee ground console. Operating the original Nachtfee apparatus, the transmission section relies on modern, partly digital, technology. On the right-hand side we notice the Gemse receiver, which 'video' output is being fed straight onto the Nachtfee video input (Impulsamplitude); as to simulate the wartime conditions EGON pulses are also being handled. EGON was originally capable of measuring distance as well (the signal meant for the FuG25a IFF system). Although, it is nowhere exploited in our experimental set up, its appearance at the 'order' screens proves that genuinely the EGON signals does not interfere with the presentation of the Nachtfee data signals at all . (0008) 


On the left-hand side we see the Nachtfee ground console, opposite the IFF transponder FuG25a with an attached aircraft display

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Adjacent to the Nachtfee console on the outgoing way we encounter the Freya-EGON transmitter; on the incoming side we have the Gemse receiver, providing signals for both the regular EGON as well as the Nachtfee feedback channel.


Why does need the principle of Nachtfee a feedback signal?

Please regard the previous drawing again.

The Nachtfee data signal is bridging the range between ground and airborne platform twice. The λ of 500 Hz is 600 km; as range has to be bridged twice the maximal operational range is 600 : 2 = 300 km; which is the theoretical Nachtfee system range. Where the 'order' signal phase has rotated, in the domain of time, from 0 to 360.     The FuG25a IFF transponder is not only 'transponding' the EGON signal but also the Nachtfee data signal content, which just is after the detector also being fed (split) onto the aircraft 'order' display.*  However, the originally two data signals will return towards the Gemse receiver. Where the video output takes two directions; one towards the Freya-EGON system, which is neglected further in our experimental considerations, and is secondly fed onto the Nachtfee control channel.     * The system relied basically upon a single upwards frequency and a single downwards channel; albeit, that the downwards signal is using a different frequency channel.

It have been proved previously, that the distance between the Nachtfee ground station and the flying platform is an essential system parameter. Distance in respect to the behaviour of EM waves is an aspect in the 'domain of time'. The Nachtfee console is fit with a special control, which I have designated 'Range offset' and which was in the post-war British drawing called 'Range Adjustment Tube' for which an extra control and CRT screen is provided. Range offset should be adjusted quite accurately and simply doing this on a 7 cm control CRT is not realistic. For it a very smart technique has been invented which allows fine adjustment within about a degree!     For better concept understanding consider also: Explaining its final principle a bit differently


Film 58: We are looking at the screen of the dual trace CRT HRP2/100/1,5(A). Bringing in line both the 'domain of time' of the signal that has bridged the range upwards and thereafter downwards to the Nachtfee console again. This, what I call 'Range offset', is being controlled in the next drawing by means of goniometer (phase-shifter) C. The way of aligning is quite simple. The HRP2 ... CRT is a fully dual beam cathode ray tube; its 8 deflection plates are, within their particular planes, cross interconnected. So that one trace is painting from left to the right and the other one doing so from right to the left. Also vertically one system painting the signal upwards the other one doing so downwards. This measurement is only valid as long as the Nachtfee 'order' vector being set at 0 or 180! The screen centre represents thus 0 and 180, consequently the base-line acts as being a 'cosine', the slope-steepness of the signal is just in its centre having its highest value.   (00028)

The purpose of all this is that the returning Nachtfee signal blip should getting exactly the same 'order' vector as it possessed when it left the Nachtfee data output. Considering an away moving platform, the only means possible obtaining this is to manipulate by means of 'Range offset goniometer C' until the Nachtfee data signal coincide at the centre of the HRP2 ... CRT screen shown in the previous film strip. Operating Range offset, thus goniometer C,  is having no effect on the actual Nachtfee signal phase; only the controlling time-bases being manipulated! (please notice the drawing below)   The result will be that also the LB2 control screen blip is staying fully in line (accordance) with the actual 'order' vector given at the Nachtfee 'order' compass. Please remember that the HRP2 ... screen presentation will only being valid as long as the Nachtfee 'order' pointer coincide with due North or South!


Viewing the three ways of manipulating Nachtfee signals

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The output signal of phase-shifter A (after amplification) is being split into two (blue) directions. One constituting the Nachtfee 'order' data chain, which signal-phase is acting as a 'current' data-phase; the other direction is constituting the deflection chain. It is clear, that the blue lines are actually interacting in full coherence to-one-another. When the output of goniometer A increases an amount say x than this is valid for both channels, whatever the settings of goniometers B and C actually are. This is even true when the Nachtfee data have bridged the range twice passing also through the aircraft transponder! In this respect we may even speak of symmetry. Thus the feedback data via the aircraft transponder stays still in full coherence with the ground system; albeit, that what is shown on the LB2 control screen might give a different impression. All will be looking fine, when distance or range have been taken into consideration. Hence, operating or controlling 'Range offset', is a major objective!


Film 59: Viewing currently the Nachtfee front panel with its 'Phase' and Range offset  control. Shown, for example, is that operating 'Phase' control thus goniometer A in the previous drawing, that the Nachtfee feedback blip or pulse is actually not bringing the Nachtfee closed-loop out of coherence; although, its overall signal phase is been manipulated freely at will! The small error is being caused in the electrical circuitry of Nachtfee though, essentially the system stays in full coherence.  (00027)

To understand the implications of what we just have discussed, the Nachtfee overall signal phase is changing when goniometer A being operated - thus does the Nachtfee data-signal-phase. This change in the 'domain of time' will cause that the Nachtfee pulse (on the screen regarded being a blip) is arriving 'manipulated in the domains of time' at a controlled instant upon the simulated aircraft display. The value of instant is also having its effect on the returning Nachtfee feedback signal. Only when Range offset being adjusted correctly we have full control on the system parameters in the 'domains of time'.

Not yet discussed, is the fact that we have no idea of what the actual time-base-signal-phase in the aircraft is. Nowhere mentioned, but there is no way around that its actual state of affairs should be feed back to the controlling Nachtfee console. For it only the 'Phase' control at the front panel does make sense. It has no scale, its actual state of 'Phase setting' can by no means be monitored by Nachtfee control screens. However, in film strip 59 we have seen what its impact is in respect to the arriving aircraft reference signal is. I have designated it the 'TB pulse' (time-base reference pulse). Virtually on the LB2 control screen the Nachtfee feedback blip is hardly effected by the setting of the 'Phase' control, whereas the TB pulse is responding directly onto the setting of this control.    The controlling time-base of the LB2 CRT has been already adjusted correctly against the offset of Range, otherwise the Nachtfee blip would not appear at the same vector as does the Nachtfee 'order' pointer.    Hence, distance is already compensated for, the TB reference pulse is presenting 0 phase of the aircraft time-base. It should therefore be adjusted or being brought in line with the Nachtfee control blip (aligned alreay by means of 'Range offset'); albeit, having an additional offset caused by extra (electronic) circuitry-time-delay. This latter system delay is to be regarded being constant and can thus be compensated for by means of a particular screen-vector-offset.    


The block diagram of the Nachtfee system, not shown is the 'Domains of Time' though, how the various modules (stages) are being linked together

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The TB pulse is injected at the output of the detector (available at test connector pin 9) and will be handled inside the FuG25a transponder as if it concerns a ground signal which should be 'transponded' towards the Freya-EGON ground station.    The TB pulses are being experimentally gated and during the TB-gating time the video content of our simulated aircraft display is being blocked (blanking) successfully. This works fine, please notice 23a .


Nachtfee's principle diagram

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The next film strip will deal with the implication of the Freya-Polwender switch; shown down on the far right-hand side of the drawing. It only interchanges the two Nachtfee data output lines; this only works as long we are dealing within a coherent system (loop). 


Film 60: Viewing the Nachtfee front panel. The LB2 control CRT is showing both what happens when the Freya-Polwender switch is being operated. The Nachtfee 'order' blip jumps to due South, whereas the Nachtfee 'order' pointer is still facing to due North.  (00013)

It has to be stressed though, that the shown system loop is a hypothetical reconstruction! Nonetheless, the only means possible, as it is inline to the drawing on top of this page and in full accordance to the available controls on the Nachtfee front panel. That we ultimately have been able to get where we are today, is only owing to the text-line in the British post war drawing! 'Range Adjustment Tube' proves wherefore the 'Range offset' control is being meant for. The LB2 control screen is after all a most essential instrument! It allows checking whether the Nachtfee control-loop works correctly in the 'Domain of time'. Phase control is also the only means by which the TB signal is adjusted upon correctly.   


Film 61: Shown is the simulated aircraft display together with the versatile synthesiser PM5193, which allows at 500 Hz to set its output signal within 0.0001 Hz (a tenth thousands of a Hz). However, even this value of the resolution steps is actually too coarse for us!  The TB pulse is after gating and pulse-shape-forming being send as a reference pulse towards the ground station. The Nachtfee order blip is adjusted as it is actually shown on the Nachtfee LB2 control screen. When the Freya-Polwender switch being activated, this causes a jump to due South on the simulated aircraft display. When a simulated 'order' is due to be expected the blip starts moving from due North in clockwise direction, and stops at some of the intended 'orders'.   (00037)

It may be noticed, that during the just not four minutes lasting film the blip drifted anti-clockwise for some degrees. This means, that the TB reference is actually having a tiny bit too low frequency (against the actual Nachtfee internal clock); we have to think of 0.00001 Hz, a value which we cannot interpolate. The advantage is, that it shows how delicate the mutual phase-stability in the Nachtfee concept is (at least: 2 x 10-7)! This might have been one of the reasons why the Nachtfee system concept never matured after the war.


We may also conclude, that Nachtfee was a system that heavily relied upon human judgements, because the 'domain of time' changed constantly and it therefore might have been rather difficult to operate.


To be continued in due course


By: Arthur O. Bauer




Please also, or proceed with: Nachtfee bits and pieces

Please return to, or proceed with: Nachtfee things done

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