[Bruce Dennis]

"A. D. I. (K) Report No. 365/1945
THE FOLLOWING INFORMATION HAS BEEN OBTAINED FROM P/W
AS THE STATEMENTS HAVE NOT AS YET BEEN VERIFIED, NO
MENTION OF THEM SHOULD BE MADE IN INTELLIGENCE
SUMMARIES OF COMMANDS OR LOWER FORMATIONS, NOR SHOULD
THEY BE ACCEPTED UNTIL COMMENTED ON AIR MINISTRY
INTELLIGENCE SUMMARIES OR SPECIAL COMMUNICATIONS.
RADIO AND RADAR EQUIPMENT IN THE LUFTWAFFE – V.

I.F.F.
1. This report is the fifth of the series dealing with radio
and radar equipment in the Luftwaffe. As in the case of the
previous four reports (A.D.I.(K) 343, 357, 362 and 363/1945), it
is based on interrogation of General Nachrichtenführer MARTINI,
Director General of G.A.F. Signals, and some members of his
staff, and has been supported by a number of relevant documents
of recent date which were in the possession of the General's
Chief of Staff.
2. For convenience in reading, the report is divided into
three main sections covering the following applications of
I.F.F.
A - Flak.
B - Early Warning Radar.
C - Air-to-Air Recognition.

GERMAN VIEWS ON THE IMPORTANCE OF I.F.F.
3. Before the outbreak of war the practical application of
radar was mainly concentrated upon its use as an aid to Flak,
and the Germans did not realise the importance of an early
warning service until after hostilities had begun. In
consequence the need for the identification of friendly aircraft
picked up by radar sets arose initially mainly in conjunction
with Flak requirements.
4. Owing possibly to the fact that the first solution to the
Flak problem was not as successful as that achieved in
connection with early-warning radar, while at the time great
importance was attached to Flak as a defensive weapon, much
effort was devoted in the first few years to producing a
suitable identification system for that arm.
5. By 1944, the American bomber force was able to bomb Berlin
by daylight in clear weather with insignificant losses by Flak,
despite the application of various radar and optical aids. As a
result, Flak had fallen into disrepute as to defensive weapon.
6. This opinion of the ineffectiveness of Flak was very
pronounced among the members of the Signals Staff of the G.A.F.
and it is hoped to give their reasons for this view in a later
report in this series on Ground Radar. Flak was regarded by them
as an out of date weapon and for this reason as well as on
account of the inherent difficulties of the problem, practically
no effort was made to provide a means of identification of
friendly aircraft for Flak purposes during the later stages of
the war.
7. On the other hand the position of early warning radar was
exactly the reverse and as early warning became vital so the
G.A.F. attached ever greater importance to I.F.F. for early
Warning radars, particularly in connection with bad weather and
night raids involving feints and spoof sorties. It was
considered essential to obtain a clear picture of the tracks of
Allied raiding aircraft and to avoid any confusion with German
fighter aircraft which also flew in group. The latest endeavour
in this field went so far as to provide separate identification
for different G.A.F. night fighter units in order to assist
ground control still further.
8. Increasing attention was also being paid to air-to-air
recognition between German aircraft but this, though considered
desirable, was not thought to be nearly so important as the
essential requirement that friendly aircraft should be
recognisable to the early warning service In order that a true
picture of the progress of Allied bomber attacks could be
obtained and fighter aircraft vectored to the attack by ground
control.

A – I.F.F. FOR FLAK
9. At the beginning of 1939 it was intended to equip all
Flak sites with a radar fire control apparatus then being
developed by the firm of Lorenz. The Lorenz apparatus proved
unsatisfactory and in the course of 1939 the Würzburg, produced
by Telefunken, was adopted for Flak fire control.
10. The need for the recognition of friendly aircraft for Flak
purposes had been apparent to the Germans and an I.F.F. set for
use with the Würzburg was produced, and was available at the
outbreak of war. It was known as the Zwilling and was given the
designation FuGe 25.
11. The 50 cm. Würzburg transmission received by the FuGe 25
caused the FuGe 25 transmitter to broadcast an audible morse
signal on the same frequency, which could then be heard by the
Würzburg operator in his headphones. As this was not a retransmission
of the Würzburg radar pulses, it gave no indication
of range and might equally well have originated from an aircraft
in the vicinity beamed by some other Würzburg. General MARTINI
was fully aware at this stage that the FuGe 25 was no real
solution to the identification problem.
12. Meanwhile, the firm of Gema had produced the Freya for
the Navy, and in October 1939 eight of these had been taken over
by the G.A.F. and were stationed on the East end North Frisian
Islands, where they proved their value la combatting early
British bombing attacks directed against the North Sea ports.
13. In November 1939 Gema demonstrated an I.F.F. set for use
with Freya which proved to be forerunner of the FuGe 25A
Erstling. General MARTINI recognised immediately that this Freya
I.F.F. which operated on the principle of returning the Freya
pulses and which provided range measurement, was a far superior
solution and he endeavoured to use his authority to have the new
principle universally adopted for Flak.
14. At that time many different departments were involved and
partly on this account and partly because FuGe 25 Zwilling was
sponsored by the Technisches Amt, MARTINI failed to gain his
point. (see also A.D.I.(K) 334/1945),paras. 95-96). It was only
with great difficulty that he was able to persuade the R.L.M. in
early 1940 that an order for 3,000 FuGe 25A for Freyas should
be placed and even then considerable delay occurred before they
were supplied.
15. During 1940 and 1941 the shortcomings of the Zwilling
gradually became apparent, particularly in connection with night
fighter operations, but by the time the advantages of the
FuGe 25A Erstling had been recognised and the set had been
finally adopted, over 10,000 Zwilling sets had already been
manufactured.
16. The original Freya used a wavelength of 2.40 metres to
which the FuGe 25A Erstling responded on 1.90 metres. To permit
of the universal application of the FuGe 25A a small transmitter
called the Kuh had to be built into other types of early warning
radar in order to interrogate the airborne set. The response was
picked up by the Gemse receiver unit. In this manner a simple
pair of wavelengths were, in effect, set aside for aircraft
I.F.F. purposes.
17. In spite of these achievements the Flak problem had not
been completely solved, as it was not possible to produce a
sufficiently beamed transmission on 2.40 metres to be certain
that a response seen on the recognition tube was that of the
aircraft in the Würzburg beam. Various expedients were tried out
but as mentioned above, the problem still remained unsolved up
to the end of the war and was finally abandoned.

FUGE 25 - ZWILLING.
18. The FuGe 25 Zwilling (= twin) received the Würzburg pulses
on a 50 cm. carrier wave and re-transmitted a morse recognition
signal on the same frequency, but as it did not re-transmit the
pulses received, no range could be obtained by the ground set.
The shortcomings of this system were recognised in 1941 end a
series of attempts was made to overcome them by using responder
technique as detailed below.

HÄUPTLING.
19. In order that the Flak Würzburgs should get a range with
their recognition signal, the FuGe 25 Zwilling was converted so
that the pulses received on 50 cm. were re-transmitted on the
FuGe 25a wavelength of 1.90 m. This conversion was complete by
the autumn of 1942. The new set was called Häuptling. About this
time, however, Allied jamming of Würzburgs commenced and the
Germans were forced to produce Würzburgs on various wavelengths
known as Insel A 53.0-54.2 cm. and Insel B 56.7-58.0 cm. As a
result, the FuGe 25 no longer responded to all Würzburgs and so
the Häuptling did not fulfil its task.

KUCKUCK.
20. When Würzburgs on various frequencies were introduced, the
basic FuGe 25 set was finally given up and the FuGe 25A Erstling
became the standard airborne set.
21. All Würzburg radars were provided with a Kuh type
transmitter on 2.40 m. called the Kuckuck, the aerials for which
were placed in the Würzburg paraboloid. The resultant polar
diagram was extremely wide-angled as compared with the Würzburg
beam and a response at the same range as that of the aircraft
held in the beam was not positive proof of identity if many
aircraft were about.
22. In addition the 2.40 metre transmission spilled over and
could be picked up at distances of as much as 10/15 km. behind
the Würzburg set, so that even aircraft behind the Würzburg were
triggered off. As Flak was only interested in aircraft within
firing range of the Würzburg, this procedure was at its weakest
with the very type of recognition for which it had been
expressly designed, and in consequence Kuckuck was finally
condemned as a failure in 1943.

WOBBELBIENE.
23. With the failure of Kuckuck the Germans decided that the
only solution to the Flak problem was to apply British I.F.F.
technic. The FuGe 225, known as Wobbelbiene, which was designed
to sweep through (Wobbeln) the 50 cm. to 60 cm. band and act as
responder (Biene), was developed for this purpose and was to
have been introduced in the winter of 1943/1944. This, however,
still did not cover the new Würzburg Insel C of 62.3 - 63.8 cm.,
which was being introduced in 1944.
24. Further modifications to Wobbelbiene were considered in
order to cover this waveband but many difficulties, both
mechanical and electrical, had been encountered with the design
despite the fact that the set was said to be a direct copy of
British R.F.3090.
25. The project was ultimately given up before being used
operationally and up to the end of the war no satisfactory
solution to the Flak problem had been found.

B - I.F.F. FOR EARLY-WARNING RADAR.
26. The Kuh and Gemse arrangement in conjunction with the
FuGe 25A whereby two wavelengths, 2.40 m. for interrogation and
1.90 for response, had been set aside for I.F.F., worked
satisfactorily for early-warning radars. MARTINI's staff
considered that this system was superior to the then current
British principle of an I.F.F. set sweeping through the various
early-warning wave bands and responding only intermittently to a
particular frequency.
27. When Allied jamming became serious, it was fully realised
that the use of one special wavelength for recognition purposes,
rendered recognition very vulnerable to Allied countermeasures
although it was considered difficult from a technical point of
view to carry out effective jamming.
28. To anticipate this eventuality, a tactical requirement was
formulated in 1943/1944 calling for a FuGe 25A working on a new
frequency and the Erstling-Grün was designed and manufactured,
but never put into use as the Allies did not employ the expected
counter-measures.
29. With the advent, during the course of 1944, of automatic
sweeping ground radar with a P.P.I. presentation like
Jagdschloss, a new problem for I.F.F. arose. So long as the
recognition signal emitted by the aircraft in responding was a
morse letter there was no guarantee that it would come up
effectively as the beam swept over the target aircraft.
30. The Germans' first solution of this problem, was the
Erstling-Rot, a form of FuGe 25A, which responded with the morse
identifications separated by a six-second dash, thereby ensuring
that response was sufficiently continuous for the ground set to
sweep the aircraft at least once whilst the airborne set was
transmitting the long dash.
31. In 1944 Allied radio countermeasures became more intense
and it was realised by the Germans that the general principle
applied in all their anti-jamming countermeasures must also hold
for I.F.F. This principle was to have a number of alternative
frequencies available for every type of set and a new tactical
requirement embodying this facility was, therefore, formulated.
32. It was decided that the new I.F.F. set must also give
continuous presentation of the recognition response so that
ground controllers could immediately identify friendly aircraft.
At the same time this would solve the identification problem for
panoramic ground equipment of the Jagdschloss type.
33. This requirement led to the development of the Neuling
which, however, had not been used operationally up to the end of
the war. It appears to have been a set with a number of novel
features which are discussed below at some length.
34. In complete contradiction of the principles used in all
earlier sets, the problem of I.F.F. for centimetre radar was to
be solved by using the searching beam to trigger off the I.F.F.
set. A small unit called the Frischling was to convert the
centimetre wavelength to a frequency which would be accepted by
the standard Erstling receiver.

FuGe 25A – ERSTLING.
35. The FuGe 25A is the well known set which has been installed
in every German aircraft since about the beginning of 1942 and
which had also been used for Egon control(see A.D.I.(K)
357/1945). It was often referred to as the Erstling.
36. It was a responding transmitter receiving on 2.40 m.(the
original Freya frequency) and re-transmitting the pulses
received on 1.90 metre. A morse signal repeated roughly every
two seconds was superimposed on the re-transmission, six
alternative codes being available. It was claimed that an
advantage of using a morse letter as recognition was that it
could easily be read by ear through the operator's headphones
and that this was easier than following the recognition C.R.
tube by eye.

ERSTLING-ROT.
37. In the early part of 1945 the G.A.F. began to introduce a
form of FuGe 25A known as the Erstling-Rot. It was designed to
deal with recognition difficulties occurring with ground radar
of the automatic sweeping type such as Jagdschloss.
38. The Erstling-Rot separated the morse signals by a dash of
six seconds duration - a period sufficient to ensure that the
ground radar swept the target. It embodied an improvement in
that it permitted of 18 different morse recognition signals
being superimposed on the re-transmission instead of only six as
in the case of the original Erstling.
39. Originally it was to have a more powerful transmitter but
this requirement was allowed to lapse when it was realised that
the factors limiting range were the sensitivity of the FuGe 25A
receiver or the power of the ground interrogator. A project for
a more powerful ground interrogator called the Gross Kuh was
considered, but as the ranges obtained with the normal Kuh were
thought to be adequate this idea was abandoned.

ERSTLING GRÜN
40. The G.A.F. signals staff realised that the use of a single
frequency for recognition purposes exposed them to the danger of
Allied R.C.M. In view of this a version of the FuGe 25A
operating on a new frequency was produced which was known as the
Erstling-Grün. The wavelengths used were to be 2.52 metres for
interrogation and about 2.00 metres for the response.
41. The small shift in wavelength was dictated by the need to
avoid new ground equipment. With the relatively small frequency
change the Kuh and Gemse were capable of being adjusted to the
new frequencies by the field "S. und I" (maintenance and repair)
teams, and so an economy was effected.
42. Erstling-Grün was never used operationally as Allied
jamming of the original Erstling frequency was never
experienced.

NEULING.
43. The FuGe 226, usually referred to as the Neuling, was to
have been available for installation in operational aircraft by
December 1944, but owing to difficulties encountered during the
trials carried out at Rechlin it was not yet ready at the time
of the capitulation. Lorenz were responsible for its production
and Dr. KRAMAR of that firm was considered the expert on its
technical aspects.
44. The Neuling, which was considered to be a good solution to
the identification problem, was designed to overcome previous
difficulties and to provide new facilities. The tactical
requirement originally called for were:-
(a) Continuous presentation of I.F.F. signals on all types
of early-warning radar including panoramic radars
such as Jagdschloss.
(b) Twelve alternative pairs of frequencies for I.F.F.
(later reduced to six pairs) - each pair to consist
of an interrogating and response frequency.
(c) Air to air recognition between German aircraft.
45. This ambitious programme was not fulfilled when the
FuGe 226 was tried out in the later part of 1944 at Rechlin, but
the experts who carried out the trials believed that the main
requirements could be met by sacrificing half the pairs of
frequencies, thereby limiting the set to six frequency pairs.
46. To meet requirement (a) and provide continuous presentation
of the recognition signal on the ground radar, and at the same
time permit the simultaneous use of a number of different
frequencies, the responder and transmitter were to sweep very
rapidly through the selected band which was believed to be 125 -
167 mc/s.
47. The receiver and transmitter sweeps were synchronised a few
megacycles apart, so that the response was always on a slightly
different frequency to that of the interrogation. This sweep was
to be carried out sufficiently rapidly for the blip on the
recognition tube of a ground set interrogating on one of the
frequencies to appear continuous to the eye of the operator.
48. Presumably, even allowing for after-glow effects, the
frequency of sweep must have been extremely high. It was not
known exactly what repetition rate was used, nor what technical
method was employed to obtain such a high rate of sweep through
the band.
49. Great importance was attached to requirement (a) as it
allowed ground controllers to obtain continuous recognition on
Jagdschloss type P.P.I. tubes and so distinguish between
friendlies and hostiles. It also greatly assisted the control of
friendly fighters by Freya stations.
50. The requirement (a) for continuous presentation of the
recognition signal appeared to have precedence over the
requirement (b) for twelve alternative pairs of frequencies
since in order to meet (a) Rechlin decided that the number of
channels available would have to be cut from twelve to six
pairs. It was found by Rechlin that squeezing twelve separate
frequencies for response (which could not overlap with
interrogation frequencies) into the swept band caused the band
width of the individual responder frequencies to be so narrow
that the recognition blip became too thin and indistinct on the
ground radar recognition tube.
51. It was hoped that requirement (b) - the provision of
alternative frequencies - would prove a safeguard against
possible Allied R.C.M. It was also believed that it would aid in
mitigating clutter on the recognition tubes of Freya etc., since
interrogation would be spread over a number of frequencies.
Wandering blips resulting from neighbouring ground radars which
triggered off other aircraft obscured the tube and caused this
clutter.
52. Efforts had also been made to overcome this trouble by
building an arrangement into ground radars which prevented
interrogation being carried out continuously as was often the
undesirable habit of operators. By means of this arrangement
power was cut off from the Kuh aerials about a minute after the
interrogation switch had been depressed and this device also
prevented interrogation until a further short period had
elapsed.
53. Requirement (c) was only third in importance. The air to
air I.P.F. facility, however, entailed a disadvantage which was
regarded as a serious one, namely that when being used for that
purpose the Neuling was no longer capable of responding to
interrogating by other radar apparatus on the ground.
54. The tactical application of the Neuling must be considered
in relation to the defence problems which the Allied bomber
forces set Germany in 1944. The Germans regarded it as essential
for the defence and more particularly for night defence that
they should be able to obtain an absolutely clear picture of the
air situation and identify Allied bomber streams unequivocally
and at a glance. The P.P.I. presentation of ground radar like
Jagdschloss and Forsthaus was beginning to be appreciated and
attempts were being made to control directly from these
panoramic displays.
55. It will be remembered too that German night fighter Gruppen
operating under the Verbandsflug system flew together in loose
groups or patrolled in the area of a selected beacon. It was,
therefor, considered essential that these aircraft should be
immediately identified as friendly on the P.P.I. tube and not
confused with a bomber stream. It was also held to be of great
value to ground controllers to have a means available for
identifying one Gruppe from another with equal immediacy.
56. To attain these requirement one of the six interrogating
frequencies available was allotted to early-warning radar and
the remaining five were to be given to different night fighter
Gruppen or Geschwader. The Neuling in each aircraft was then so
switched that it could receive and respond to two of the six
Neuling frequency pairs, viz. the early-warning frequency and
the frequency allotted to the Gruppe to which the aircraft
belonged.
57. For Jagdschloss panoramic ground equipment a complete
continuous identification picture divided into friendlies and
hostiles could be obtained by interrogating on the earlywarning
frequency.
58. By simply training a knob the transmitter and receiver
could be switched to the frequency pair of a particular
operational Gruppe and this presented no technical difficulty
with Breitband aerials. Aircraft of that Gruppe could then be
identified immediately in the over-all picture on the P.P.I.
tube. This facility was considered a great advance both from
the point of view of I.F.F. and of ease of ground control of
night fighters.
59. P/W who claimed to have seen a ground P.P.I. display
during the Neuling trials stated that the recognition blip came
up on the tube as an extension of the reflected blip at
slightly greater range and that it subtended a greater angle in
the display. He described it as a "sausage rather longer than
the aircraft blip and sitting on it".
60. Reference has been found in a document to a Neuling
covering the band 1,000-1,500 mc/s. The P/W who was responsible
for the formulation of radar requirements stated that he had
never heard of a Neuling on this frequency, but suggested that
it might be for use in responding directly to the beam of 25
cm. ground radars such as Forsthaus F. This suggestion appears
unlikely, however, as it seems to involve a departure from the
Neuling principle.

FRISCHLING.
61. In view of the introduction of highly beamed 9 cm. ground
radar such as Forsthaus Z and Jagdschloss Z, it had been decided
to depart from the principle of using a separate interrogation
frequency and to employ the search beam to trigger off I.F.F.
For this purpose a special attachment to the airborne Erstling
called Frischling had been planned. This was a receiver on 9 cm.
which converted the frequency to that of the Erstling so that it
responded on 1.90 metres.
62. With the planned introduction of the Neuling, consideration
was given to a modification of the Neuling whereby a Frischling
attachment would be built in for the purpose of converting the
frequency and so trigger off the Neuling in the same manner. P/W
was not clear whether this would only apply to one frequency of
the Neuling, nor did he know what technical method would be used
to accomplish it.
63. Frischling was to be produced by Telefunken but was still
in course of development when the ear came to an end.

AIR-TO-AIR I.F.F.
64. For some reason as yet unexplained, the German interest in
air-to-air recognition only became great enough for suitable
equipment to be designed during the last stages of the war.
During the year preceding the termination of hostilities,
captured night fighter crews consistently maintained that
some improved form of I.F.F. which would allow recognition
of friendly aircraft was expected but no attempt appears to
have been made to adapt the FuGe 25A for this purpose.
65. The present P/W assert that so long as I.F.F. and airto-
air search operated on metric wavelengths and could not
be sharply beamed, the problem of air-to-air recognition
could only be half solved, as a range identification only
was obtained. It was realised that with a dense bomber
stream there would be so many aircraft comparatively near
to the fighter that recognition by range only was not very
valuable. Nevertheless the Neuling FuGe 226, which was
shortly to be introduced, was to have provided air-to-air
I.F.F. facilities.
66. In the beginning of 1945 the Germans tackled this
problem for centimetric search gear and proposed to depart
from their original principle of separating search and I.F.F.
interrogation. The search beam of centimetre equipment was to
be received by the Frischling attachment to the airborne
I.F.F. set and the centimetre frequency so converted that the
I.F.F. net was triggered off.

NEULING.
67. In the Neuling, which has been described above in
detail, it was planned to provide air-to-air I.F.F.
facilities by the use of a special switch which, when
depressed, reversed the roles of the receiver and responding
transmitter. The interrogating aircraft could then trigger
off the I.F.F. set of neighbouring aircraft and receive its
response on the receiver portion of the Neuling.
68. The response was to be fed through to the SN 2 or other
set in use and the presentation of this recognition signal
was to take the same form as in the Freya, i.e. a second
time base carrying the I.F.F. signal was to appear to one
side of the main time base.
69. During this operation no I.F.F. response could be made
to interrogating ground stations, and this caused some
apprehension. To discourage excessive use of air-to-air
interrogation, the switch in the aircraft was to be awkwardly
placed and inconvenient to operate – a typically German solution
to a problem of aircrew training.

FRISCHLING.
70. The night fighter search apparatus – the Berlin N.1.A.
and the Bremen 0 on 9 cm, and later probably the München on
3 cm were to go into service some time in 1945 as will be
discussed in a future report in this series.
71. To provide air-to-air I.F.F. facilities the
Frischling, mentioned in paras. 61-63 above, was to be
attached to the FuGe 25A and later built into the Neuling
as a modification. It was to convert the centimetric beam
transmission of air-to-air search apparatus to the
frequency of the Erstling FuGe 25A so that the latter was
triggered off directly by the searching beam.

FALTER WITH GÄNSEBRUST.
72. As early as 1940, experiments had been made with infrared
homing on to aircraft exhausts using an infra-red
telescope of the Bildwandler type called Spanner. This met
with only limited success on account of restricted range and
the dependency of infrared on clear weather conditions but the
idea was never completely dropped.
73. With the introduction of night fighter commentary and the
Verbandsflug tactics in 1944, it was required that night fighter
units should fly in groups and keep as close together as
possible.
74. It was, therefore, proposed to introduce an aid in the
shape of some form of infra-red navigation lights to be viewed
through an infra-red telescope. The latter, which was a form of
Spanner, was named Falter. As, however, the field of view of
this telescope was confined to about 15° it soon transpired
during trials at Werneuchen that it was not a practical
proposition.
75. In 1944 the idea of using infra-red for recognition
which had long lain dormant was once more evoked by the
discovery that British bombers were carrying en infra-red
recognition light. It was, therefore, proposed that German
night fighters should home on to the infra-red lights by means
of the Falter.
76. For mutual recognition between night fighters an infrared
lamp termed "Gänsebrust" was also planned. It was hoped that
Gänsebrust might not only allow recognition between German
aircraft but possibly afford some protection from British night
fighters which might become uncertain in their recognition of a
German night fighter if the Gänsebrust was flashed intermittently
even though the British code in use for the night were
not known.
A.D.I.(K)and S.D. Felkin
U.S. Air Interrogation. Group Captain
2nd August 1945"