[Bruce Dennis]

"SECRET A.D.I.(K) Report No.525/1944
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.

FIGHTER DEFENCE OF GERMANY

CONTROL OF FIGHTERS BY THE "Y" PROCEDURE.
1. The Interrogation of G.A.F. fighter pilots in the past has
made it possible to form a picture of German tactics against
U.S.A.A.F. bomber formations from the point of view of fighter
interception force. Knowledge by P/W of the raid tracking
organisation on the ground, however, has up to the present
been lacking, and many gaps have remained in the picture,
particularly where the Fühlungshalter (shadowing aircraft) and
the "Y" controlled fighters are concerned.
2. Two G.A.F. Signals Officers, who had been directly
concerned with "Y" control sites - one man was a plotting
officer - have now described in some detail the method of
ground control at present being used in the operation of
fighter interception forces and of the shadowing aircraft;
their knowledge was chiefly confined to methods practised in
France, but they state that the same principles also apply to
operations in Germany.
3. Some of the information is of a semi-technical nature, and
the present report is therefore divided into two parts; the
first part concerns the operational aspect of fighter control,
and the second part deals with the equipment and method of
operation of the unit of control - the "Y" site.

I. CONTROL OF FIGHTER INTERCEPTION.
OPERATIONAL CONTROL.
4. In Allied attacks on Germany, the Jagd Division receives
and plots all Radar information on the movements of the bomber
formations, together with direct reports from the
Fühlungshalter aircraft shadowing those formations and the
plotting reports from the fighter "Y" control sites. It is
primarily on this information that the Divisional commentary
and the control of a whole interception action are based.
5. The sole medium for transmission of the Divisional
commentary is the "Y" site; the latter is connected by
landline to the Divisional plotting centre so that the
plotting of an operation is carried out at both those centres
simultaneously.
6. An interception can therefore be directed either from
Divisional Headquarters or from the "Y" site, but it is
normally the plotting officer at the "Y" site who puts out the
R/T commentary.
7. The area of control ("Führungsraum") of a "Y" site is
limited by the range of the transmitters used, and according
to P/W the average range may be taken as 250 to 300
kilometres; the Division therefore controls an operation over
its territory by making use of a chain of "Y" sites.
8. The area of R/T control of a "Y" site may be increased by
additional ground relay transmitters, known as "Brummer", so
that R/T communications with fighters can be continued outside
normal range. These stations and their operation are discussed
in Part II of this report.
9. The present P/W stated that the Central Operational
Headquarters ("Zentral Gefechtsstand"), situated in the Berlin
area, receives simultaneous information from each Divisional
plotting centre and makes its own plots on that basis; the
fighter commentary on the "Reichsjägerwelle" or such orders as
are put out on the broadcaster "Annemarie" emanate from that
centre, but according to the present P/W are only utilised by
fighter interception forces in case of failure of the
Divisional commentary, or the breaking up of an interception
formation.

"Y" CONTROL OF FIGHTERS.
General Principls.
10. The principles of "Y" fighter control, whereby the range
and bearing of friendly fighter are determined by a "Y" ground
station, are already well known.
11. In operations in Germany, the "Y" site, known as a
"Stellung", usually consists of five separate stations
("Stationen"), each of which comprises a transmitter hut and
mast, and a receiver pylon incorporating a D/F, and rangemeasuring
unit. All five stations are connected to a plotting
room situated on the site.
12. Each station of the site is allotted a separate W/T
channel, known as "Linie", consisting of a transmitter and a
receiver carrier frequency; thus a number of individual
aircraft can be controlled simultaneously within a given area.
13. In order to extend the control of a single aircraft, such
as a night-fighter, to that of a number of aircraft, such as a
day fighter interception force, it is only necessary to
include a "Y" controlled aircraft amongst the fighters of the
formation.
14. In this case the receivers of all aircraft in the
formation wi11 be tuned to the ground transmitter frequency to
receive R/T instructions from the plotting centre as well as
from the fighter formation leader.
15. In this manner up to five separate interception formations
can be controlled, each on a different frequency form a single
"Y" site. If necessary, all five formations can be brought
together to intercept a single bomber formation, or
alternatively, each of the five formations can be despatched
separately to meet the bombers or their fighter cover at
various points.
16. The ground transmitter carrier frequencies, known as the
"Gemeinschaftswelle", are distributed over the 40.4 to 42.3
mc/s. band at intervals of .05 mc/s. and the ground receiver
carrier frequencies, known as the "Messwelle", are distributed
over the 38.5 to 40.4 mc/s. band at the same intervals. A list
of such frequencies numbered 1 to 40, appeared in a recently
captured Signals Order (A.D.I.(K) 4.68/1944, paras.41-42).
17. In operation, the ground transmitter carrier frequency is
modulated by a continuous note of 300 or 3,000 cycles; the
receiver/transmitter in the "Y" aircraft (known in J.G.3 as
the "Lotse" aircraft) receives its modulation and
automatically re-transmits it on another frequency in the 38.4
to 40.4 mc/s. band; the aircraft transmitter frequency is
normally 1.9 mc/s. lower than the ground transmitter
frequency.
18. The re-transmission from the aircraft is picked up at the
"Y" station by the D/F receiver, by which the bearing of the
aircraft is than determined. The same transmission is also
received by the range-measuring unit, and the distance of the
aircraft from the station is determined by measuring the phase
difference of the modulation received.
19. The height of the aircraft under control is not measured
by the "Y" station, but is obtained over the R/T channel from
readings taken in the aircraft, thus all the data required for
the plotting of a controlled aircraft, viz. bearing, distance
and height, are obtained.
20. There are two methods of controlling aircraft formations
by means of the "Y" procedure, and these are illustrated
diagrammatically in Sketch I.
21. The first method, known as the "Begleiter", has now
dropped out of use, but is repeated here as a matter of
interest. By this method, the leading aircraft of a formation,
known as the "Führer" aircraft, also acted as the "Y"
aircraft, whilst the accompanying aircraft of the formation
were known as "Begleiter".
22. In the Führer aircraft the receiver was linked with the
transmitter and the receiver frequency was tuned to the ground
transmitter frequency, whilst the aircraft transmitter was set
to the frequency of the ground receiver.
23. In all other aircraft of the formation the receiver was
tuned to the transmitter frequency of the leader, and in none
of them was the receiver linked to the transmitter. The
formation was thus plotted solely by the position of the
Führer aircraft.
24. The Begleiter method has, according to P/W, now been
replaced by a newer method known as the "Gemeinschaftswelle",
in which the receivers of all aircraft of a fighter formation,
including that of the leader, are set to the ground
transmitter frequency ("Gemeinschaftswelle"). In this method
the aircraft is not the leader of the formation but he flies
as N°2 to the leader, with one or more similar aircraft as
reserves.
25. Upon referring to the Sketch, it will be seen that the
formation leader's instructions are not heard directly by the
"Y" station; they are picked up by the receiver of the "Y"
aircraft (in this case on 41 mc/s.) and automatically retransmitted
(on 39.1 mc/s.) to the ground station.
26. On the other hand, should the pilot of the "Y" aircraft
wish to speak to his own formation leader, he must pass his
message to the ground station on 39.1, mc/s., and the plotting
officer will repeat the message on the Gemeinschaftswelle.
Transmission of Commentary.
27. A plotting officer on the "Y" site, or his counterpart on
the extended line to the Divisions, is provided with
headphones connected to the receiver of the range-measuring
unit, and with a microphone which is connected to the
transmitter.
28. By depressing a key he can link the microphone with the
ground transmitter and can impose R/T speech on the already
modulated carrier frequency which it sends out. It is not
necessary to discontinue the modulating note, so that D/F’ing
and range measuring can therefore proceed at the same time as
the commentary.
29. In practice, the fighter interception force is led by the
"Y" control officer as far as within sight of the bomber
formation, at which point R/T control ceases to allow the
formation leader to direct the combat, and is only taken up
after combat when the fighters have re-formed for a second
interception, and the "Y" aircraft has made fresh contact with
ground control. During this time, however, the ground station
continues to plot the formation through the "Y" aircraft.

Fühlungshalter Aircraft.
30. It will be remembered that when a U.S. bomber force is
reported to be entering German territory a special shadowing
aircraft, the Fühlungshalter, wi11 be sent to meet and follow
that force and report its movements and position.
31. The Fühlungshalter aircraft operates under the control of
the "Y" plotting officer in the manner described above. In the
opening stages of an operation, the Fühlungshalter is directed
to the penetrating formation on the basis of Radar plots, and
upon making contacts it takes up a position usually above and
to the rear of the enemy formation.
32. The Fühlungshalter then reports strength, type of
formation, direction, height and fighter cover of the
penetrating force. After the initial report, further
information is only sent in the event of major changes in the
composition or movements of the U.S. formations.
33. As soon as the Fühlungshalter has made contact with the
bomber formation, all R/T control by the "Y" site ceases; the
Fühlungshalter aircraft communicates directly with Divisional
Headquarters through the receiver of the "Y" station and the
"Y" control officer stands by and listens to the R/T traffic
whilst maintaining a plotting control by the "Y" procedure.
31. According to P/W, the Divisional Plotting Centre at this
stage relies on Radar data for following the formations, and
only depends upon the Fühlungshalter aircraft to supply
immediate details of such circumstances as withdrawal of
fighter cover.

II. "Y" CONTROL SITE, AND IT’S OPERATION.
"Y" CONTROL SITE.
Manning.
35. A "Y" control station requires nine men per shift, or
27/30 men per 24 hour, for its operation and a site consisting
of nine stations is normally occupied by a company of 150/160
operators and 50/60 maintenance personnel. The personnel of
the "Y" control station is distributed as follows:
Transmitter hut: One operator.
D/F Cabin: D/F Operator and log-book
keeper.
Range-Measuring
Cabin:
A Supervisor, range-measurer
and log-book keeper.
Plotting Room: Plotter (friendly plots),
plotter (enemy-plots)
and plotting officer.
Communications.
36. Internal: The standard layout of the internal system of
commutations in a "Y" site is shown diagramatically in Sketch
II. In this Sketch "Station A" shows the general layout
applicable to all five stations, while "Station B" shows a
diagram of the internal communications which are in reality
repeated in all five stations.
37. The internal lines which are shown in the Sketch have the
following names and functions:
(a) Plotting line (Werteleitung): A telephone connecting
the log-book keeper in the D/F cabin to the range-measuring
cabin and "Y" plotting room for transmission of bearing
readings. (R in Sketch).
(b) Telephone (Ringleitung): Connects the D/F cabin, the
range measuring room and the transmitter hut for the passing
of internal instructions. (I in Sketch).
(c) Modulation line (Modulationsleitung): Connects the
range-measuring unit to the transmitter for carrying the
modulation note. (M in Sketch).
(d) Diode line (Diodenleitung): Connects the rangmeasuring
room to the transmitter hut; required only when a
Siemens range-measuring unit is used. (D in Sketch).
(e) Receiving line (Hörleitung): Open line which connects
the receiver and the range-measuring unit to the control desk
in the plotting room; a parallel line also runs to the
Division. (H in Sketch).
(f) Transmitter Circuit (Besprechungsleitung): Open line
which connects the Division and the plotting room to the
range-measuring unit. This can be used as a closed line for
internal speech or as an open line for transmission of R/T
instructions to the aircraft from the Division or the "Y"
plotting officer. (B in Sketch).
(g) Keying circuit (Tastleitung): Open line which
connects the Division and the "Y" plotting officer to the
range-measuring unit and transmitter. Depression of a key at
Divisional Headquarters or on the "Y" plotting officer's
control desk operates relays to the transmitter and modulation
line of the range-measuring unit, allowing R/T instructions
from the Division to be transmitted on the carrier wave to the
aircraft under control. (K in Sketch).
38. According to P/W, the twenty Channels connecting the five
stations of the site to the Division may be carried on two
single lines utilising a multi-channel carrier frequency
equipment.
39. External W/T Channels: The number of external W/T channels
varies, depending on the location of the control site, but
usually consists of the following:-
(a) Command network star (Befehlsstern d. Ln. Regt.) for
reception of tactical orders and general administrative
business from and to the headquarters of the Ln. Regt. to
which the company operating the "Y" site belongs - a Saram or
FuGe.3 set is used for this traffic.
(b) Divisional signals star (Divisionsbefehlsstern), used
for operational orders and transmissions of range values in
the event of failure of the ground lines to the Division. This
traffic is also usually conducted on Saram or FuGe.3 sets.
(c) Aircraft reporting frequency (Frontflugmeldewelle, or
more recently Gerätemeldewelle). On this channel W/T
transmissions are received from all Radar search sites giving
briefly the important data relating to aircraft activity
taking place within the area of the site. This transmission
takes place by day and night and supplements and confirms
information on enemy activity received from the Divisional
Headquarters; messages are written out and handed to the "Y"
plotting officer. A pack type W/T set is used for reception of
this traffic.

Brummer Relay Stations.
40. According to P/W, Brummer relay stations are placed
throughout German territory and are employed for relaying R/T
speech in cases where aircraft have flown beyond the normal
R/T range of the "Y" site.
41. The Brummer stations are connected to the Division H.Q. by
landline and are controlled by the Division; should a "Y"
plotting officer find that an aircraft under his control is
nearing the limit of R/T range, he will request the Division
H.Q., to connect with a Brummer station in the relevant area.
42. Whilst making this request, the "Y" plotting officer
states the frequency on which the "Y" station is operating,
and the Brummer will transmit at that frequency. According to
P/W, the R/T traffic on Brummer stations is usually confined
to directional and homing instructions, although "Y" control
can sometimes continue after the limit of R/T range of the "Y"
site has been reached.

Plotting Hut and Operational Procedure.
43. The plotting rooms of the five stations belonging to "Y"
sites are grouped, in a single hut known as the "Auswertung"
(plotting centre). According to P/W there are two types of hut
at present in use and a plan view of both these types is shown
in Sketch III.
44. The older type of hut contains a separate plotting room
and table for each of the five stations, but this type of
control contra is said to have offered no satisfactory method
of centralised supervision of plots and each plotter had to
act on his own responsibility.
45. The improved type of plotting hut has been developed by
the Flugmeldedienst (aircraft reporting service) and is
believed by P/W now to be widely in use in Germany. In this
type of hut a series of six tables is arranged on one side of
a rectangular room and each table is fitted with the normal
control point and is occupied by an M.C.O. plotter. The chief
plotting officer's table and control point are placed behind
the row of tables in such a position that he has a view of all
proceedings.
46. The reason, for the six plotting tables is that on some
"Y" sites the "Egon" method of fighter control is practised in
addition to the normal "Y" control and an extra plotting table
is set aside for this purpose.
47. According to P/W the Seeburg table has been withdrawn from
the majority of "Y" sites in favour of the present simple
method of plotting. In front of each group of three tables is
a vertical glass screen marked with the German fighter grid
and the main outlines of a 1:300,000 map of the area of
control.
48. Behind each screen are four plotters, three of whom
receive and plot friendly bearing and range values on the
reverse side of the glass screen. Plots are compiled from
range and bearing data in the range-measuring room of each
station and are passed through to these plotters in terms of
the fighter grid; the fourth plotter is responsible for
receiving and marking enemy plots.
49. All plots are numbered and in addition the courses drawn
on the glass screens during each operation are copied on to a
sheet of paper for record and reference purposes.
50. The extremely close and continuous contact with the "Y"
controlled aircraft enables the plotting control officer
immediately to detect any deviation of the aircraft from the
correct course and to rectify the error by ordering a slight
correction of course when necessary.
51. The plotting officer bases his instructions to the
controlled aircraft on the estimated position of the friendly
and enemy aircraft; in doing this an allowance is made for a
delay varying from five to ten seconds between the times of
the observations of enemy aircraft when originally made and
when finally received and plotted. By experience, specific
allowances are made for the extent of the delay from the
various stations supplying enemy plots.
52. When the fighters are nearing the limit of the range of
the "Y" site, the "Y" plotting officer is responsible for
handing the aircraft over to the next station; he must advise
that station, through Divisional Headquarters of the receiver
and transmitter frequencies in use, since the new station may
have been operating on other frequency channels. This step is
taken sufficiently early to allow the new station taking over
control to tune in on the R/T traffic of the controlling
station.
53. The "Y" plotting officer's task ceases for the time being
when the control aircraft or accompanying formation is brought
in sight of the enemy or, in the case of night-fighters, when
the aircraft is sufficiently close to its target to be able to
make an attack with the aid of search equipment.
54. Plotting by the "Y" station continues throughout the
sortie and combat of a fighter interception force, and does
not cease until the aircraft return to base after combat.

A.D.I.(K) S.D. Felkin
23 Sept.’44. Wing Commander"