[Bruce Dennis]

"SECRET A. D. I. (K) Report No. 369/1945
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 – VI.
Target Homing for Night Fighters.
German early warning Ground Radar.
1. This report is the sixth of the series dealing with radio
and radar equipment in the Luftwaffe. As in the case of the
previous five reports (A.D.I.(K) 343, 357, 362, 363 and
365/1945),it is based on interrogation of General
Nachrichtenführer MARTINI, Director General of 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.
POLICY AND REQUIREMENTS
2. Members of General MARTINI’s staff have often repeated
a catch phrase "Aller Funkverkehr ist Landesverrrat" - all
radio traffic is treasonable, and the G.A.F. were only too
well aware that a transmission of any type could be listened
in to by the Allies and then D/F'd. They were, therefore,
fully aware of the opportunities of homing on to
transmissions from Allied aircraft and so when airborne
countermeasures against the Freya ground installations were
first taken by the Allies, Köthen developed an apparatus
which would enable a German night fighter to home on to the
source of the jamming transmission.
3. This equipment was called Freya-Halbe (Halbe = half
signifying that it was a radar apparatus equipped with the
receiver half only and not the transmitter), and it was
tried out at Werneuchen in early 1943. The trials were
successfully completed by about June of that year and it was
then demonstrated to the authorities for use by the G.A.F.
night fighter units.
4. At that time, however, the German night fighter force was
commanded by General KAMMHUBER who was the creator of the
Kammhuber line and whose night fighter organisation relied
essentially on ground-controlled night fighter aircraft
operating in comparatively limited boxes. The suggestion of
homing on to jammers was turned down by KAMMHUBER out of hand
because he was a rabid opponent of any form of freelance
night fighting and insisted on strict adherence by his
aircraft to the limits of their box.
5. With the discovery of Monica and the development of the
Rosendahl and later the Flensburg homers on to Monica,
KAMMUBER still maintained his obstinate stand against any
departure from the box system of control. It was, therefore,
not until General SCHMIDT assumed the control of the night
fighting force in November 1943 and proceeded to introduce
freelance methods that D/F homers on to transmissions from
the bombers could be used operationally.
6. Although the technical experts were satisfied that the
homers on to transmissions of metric wavelengths were
successful, aircrews seemed to be unable to use them well and
results obtained were never wholly satisfactory. Exactly the
same applied to Naxos for homing on to H2S is its early days,
particularly when a number of minor troubles were still being
encountered with its electrical parts and, though it was
available in January 1944, for the first three or four months
comparatively little use was made of an excellent homing
device.
7. By about Easter 1941 the early troubles of Naxos were
overcome and crews began to gain confidence in its use; with
the success of Naxos reliance on all types of homing apparatus
increased. At this stage, however the R.A.F. had almost ceased
to use Monica and Naxos remained the only important set of its
type.
8. In view of the change of policy governing night fighter
operations experimental D/F receivers known as X-Halbe were
designed capable of adaptation to any metric wavelength which
might be used by the Allies. In addition one of the tactical
requirements laid down after 1943 for all future A.I. sets was
that provision should be made for the switching off of the
transmitter portion so that the receiver could act as a homer
on to any airborne jammers employed by the Allies. Naxos and
Korfu Z which covered the 1.5 cm. to 20 cm. band already
existed.
9. As R/T and W/T Jamming became more intense and even
ground control by commentary broke down, increasing use was
made of homing on to the bomber stream by means of receivers
of the Naxos type but there remained always the serious
disadvantage that these receivers did not supply range. It was
claimed that both Naxos and Korfu were so sensitive that the
bomber stream could be picked up at a range of 200 km. and
that in consequence useless chases often ensued. Estimates of
range had to be made by deduction and even in the case of
experienced crews the estimate of range by indirect means was
not always reliable.
10. Short mention is made below of another form of homing
device, the Kiel Z, which attempted to use the infra-red
radiations from the exhaust stubs of the bomber. A fuller
description of the Kiel Z was given in A.D.I.(K) 390/1945,
paras 41 to 48.
FREYA-HALBE.
11. The Freya-Halbe, officially known as the FuGe. 221, was
designed early in 1943 to home on to airborne Freya jammers
but owing to KAMMHUBER's opposition to freelance night
fighting was not adopted. Towards the end of 1943 when freelance
operations were introduced it was proposed to install
the twenty-five Freya-Halbe sets which had been manufactured
but, when they were indented for, it was found that the makers
had used various parts for manufacturing other apparatus and
that the sets had been virtually consumed as spares. Freya-
Halbe was, therefore, never used on operations.
ROSENDAHL-HALBE.
12. The first Monica set obtained by the Germans was
recovered from a British four-engined bomber which was shot
down over the town of Rosendahl in Holland and the name of
Rosendahl or FuGe. 221.A was then given to the D/F equipment
developed for homing on to Monica.
13. According to one of the P/W who had flown the trials with
Rosendahl, it was quite successful, and gave good D/F until
the night fighter came within 4 km. of its target, after which
the D/F became unreliable. For this reason the general
introduction of Rosendahl-Halbe was delayed.
14. It was ultimately discovered that the polarisation of
the receiver aerials was at 90° to that used by the bombers
and it was assumed that this was the cause of the poor D/F. For
some technical reason it was not found possible to twist the
aerial through 90° in order to obtain the right polarisation
and by the time that these difficulties had been overcome the
R.A.F. use of Monica had ceased.
15. An interesting experiment was carried out with Rosendahl-
Halbe when a set of Rosendahl aerials was mounted round a 150
cm. searchlight. The idea was to align the searchlight beam on
to an aircraft transmitting Monica. Considerable difficulty
was encountered in getting the searchlight beam and the axis
of the receiving lobe to coincide and by the time this had
being achieved R.A.F. bombers were no longer using Monica.
16. The Flensburg, officially known as FuGe.227, was another
attempt at solving the problem of producing a homer to D/F on
to Monica transmission. Difficulty was encountered with D/F
properties but the set was satisfactorily selective and could
discriminate between a large number of signals by tuning to
both the r.f. and p.r.f. It was used to a limited extent in
night fighter operations.
17. With the cessation of the use of Monica the original
Flensburg became known as Flensburg I and a series of other
F1ensburgs, numbered from II to VI, were manufactured to cover
the frequencies used by the mandrel screen and other Freya
jammers. The frequencies as given in documents were:-
Flensburg I 1.3 m. to 1.75 m. against Monica.
Flensburg II 1.7 m. to 2.6 m. against Freya A and B
band and Jagdschloss jammers.
Flensburg III
Flensburg IV
2.3 m. to 3.8 m.) against SN 2 and Freya
3.8 m. to 5.0 m.) C frequency jammers.
Flensburg V 25 cm, band against 25 cm. P.P.I. ground
radar jammers.
Flensburg VI 50 cm. band against Würzburg jammers.
18. It was not known to what extent these additional
Flensburgs had been used in operations. They were considered
to be a successful solution to the homing problem except for
the fact that the large aerials, particularly on the Freya
frequencies, reduced the speed of the aircraft considerably.
A.G.L.T.
19. The interrogation of British prisoners of war had
provided information with regard to Village Inn and some
details of it were known. It was thought to operate on a
centimetre wavelength and pieces of equipment had been found.
Nevertheless, P/W were convinced that though preparations for
using it had been made it had not yet been employed
operationally.
X-HALBE.
20. This was the designation of the airborne receiver which
could be adapted for D/F'ing any new metric radar that was
observed by the monitoring service.
NAXOS.
21. The Naxos, known as the FuGe.350, was a detector set
which received all transmissions on the 8 to 12 cm, band but
could not discriminate between different wavelengths in the
band.
22. The problem of producing a homer on to a beamed
transmission rotating at 60 r.p.m., as in the case of H2S, was
first tackled in March 1943, some two months after the
discovery of H2S. Little progress was made until an engineer
hit on the idea of getting continuous presentation of the
signals received by employing aerials rotating about twenty
times faster than those of the transmitter. The G.A.F. signals
staff were so impressed with the ease with which it was
possible to home on to a slowly rotating beam such as that of
the H2S that one of the requirements for the Berlin A was that
its rate of rotation in searching should be very high to
ensure that the Naxos solution to the homing problem could not
be employed against it.
23. The first trials with the Naxos were flown in December
1943 at Werneuchen and the first operational Gruppe to be
equipped with the set had it installed in all their aircraft
by the 25th January 1944.
24. A whole series of Naxos sub-types were produced and of
those the following were mentioned:-
Naxos Z. = (Zielanflug = Target Approach): was the original
homing device operating on the 8 to 12 cm. band; it could
not differentiate between frequencies in the band so that
if there was more than one H2S aircraft in the
neighbourhood, a confused picture was obtained.
Naxos ZR. (R Rückwärts = Backward): employed aerials placed
both above and below the after part of the fuselage of the
Ju.88 and served as a backward warning device for the
approach of British night fighters using Mark VIII or Mark
XI on the 9 cm. wavelength.
Naxos ZX. (X = X-band = 3 cm. band): was the 3 cm.
equivalent of the original Naxos Z. It operated on the 2.5
cm. - 4 cm. band.
Naxos RX. was the 3 cm. equivalent of the Naxos R and was
used as a backward warner against 3 cm. A.I.
Naxos ZD. was a combined homer for both the 9 cm. and the
3 cm. bands. The 3 cm. aerial rotated on the same axis but
above the 9 cm. aerials.
25. As stated, the value of Naxos was first appreciated by
crews in the early summer of 1944 when the increase in British
jamming had reached such a pitch that communications with the
ground were affected and it was difficult to find the bomber
stream. The picture obtained by Naxos, however, was nonselective
and it was not always possible to home on to a
single aircraft unless the aircraft in question was separated
from the others in the stream. On the other hand Naxos made it
easy to locate the bomber stream, which at that period was the
main preoccupation of the G.A.F.
26. Although estimate of range could be gained if the height
at which the bombers were flying was known, since, by climbing
and determining at what point the Rotterdam signals were no
longer picked up, the night fighter aircraft could judge the
distance of the transmitting aircraft. A full description of
the method of approach employed appeared in A.D.I.(K)
125/1945. paras. 93-98.
KORFU Z.
27. The original Korfu set, otherwise known as the FuGe.351,
was a development of a superhet receiver designed for
frequency modulated 9 cm. carrier communications purposes.
After the discovery of H2S it was adapted for use by the
German "Y" service and towards the end of the war was further
modified for use as an airborne set and then became known as
the Korfu Z or FuGe.351Z.
28. The aerials employed were of the Naxos type and gave the
relative bearing of the transmitter but the advantage of the
Korfu Z lay in the fact that it could be sharply tuned and
could, therefore, home on to individual aircraft.
29. It was also hoped that with the help of the Korfu Z night
fighters would be able to differentiate between H2S and 9 cm.
A.I. which the Germans presumed used different sections of the
9.0 to 9.3 cm. band. In this connection, as mentioned in
A.D.I.(K) 363/1945, it was hoped in due course to produce the
Berlin and other German "9 cm. " radar on the 8.6 to 8.9 cm.
band in order to aid German night fighters to differentiate
between British and German aircraft.
30. The Korfu Z was to have been ready by mid-summer of 1944
but its advent was delayed by the shortage of magnetrons, all
available specimens of which were required for the ground
Korfu used by the "Y" service. So far as was known the Korfu Z
was never used operationally.
Kiel Z.
31. The Kiel Z was manufactured by Zeiss and known officially
as the FuGe 280. Infra-red radiations from the exhaust stubs
of aircraft were picked up in a parabolic mirror and focused
on to an Elac lead sulphide cell. The field of view in a cone
of ± 10° was scanned. A wider field of view could be obtained
by moving the entire scanner by hand in the same manner as
employed with the Berlin M.1.A.
32. Shortly before the end of the war a number of Kiel Z sets
were tried out in operations but it was found that, although
they gave a range of about 4 km. on a four-engined bomber,
various difficulties arose. Infra-red radiations from the moon
and stars formed "permanent echoes" on the cathode ray tube
used as viewing screen, and were not always easily
distinguished from a moving aircraft. In addition, if the
target aircraft was between the fires caused by the raid and
the night fighter aircraft, the target was obviously quite
indistinguishable against the background of the fires.
33. Night fighters equipped with the Kiel Z were also to
carry the FuGe 218 Neptun R3 backward warning radar so that
they at least had warning of British night fighters
approaching from the rear.
FALTER
34. Falter was an infra-red telescope of the Bildwandler type
used by German night fighters for homing on to British infrared
recognition lamps. Reference to Falter appeared in
A.D.I.(K) 365/1945, paras. 72-76. It was not known if it had
been used operationally.
A.D.I.(K)and S.D. Felkin
U.S. Air Interrogation. Group Captain
2nd August 1945"