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ELECTRONICS 
WARFARE 


A REPORT 
ON RADAR COUNTERMEASURES 


Released by the Noint Board on Scientific 


Information Policy for: 


OFFICE OF SCIENTIFIC RESEARCH 
"AND DEVELOPMENT 


WAR DEPARTMENT 
NAVY DEPARTMENT 


Distributed by The Office of The Publication Board 





ELECTRONICS 
WARFARE 


A REPORT 
ON RADAR COUNTERMEASURES 





» Released by the Joint Board on Scientific 


_ Information Policy for: 


OFFICE OF SCIENTIFIC RESEARCH 
AND DEVELOPMENT 


WAR DEPARTMENT 
NAVY DEPARTMENT 


Distributed by The Office of The Publication Board 





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Revised 10/19/45 


ELECTRONIC WARFARE: The Story of Radar 


Countermeasures 


CONTENTS: 

SINHOGUCHION wile. dvacsies sues. hooves) one nisi. 
2. The Nature of the Problem .................. 
3. The Achilles’ Heel of Radar................. 
A. December 1941.............. 0c 


5. Countermeasures in the War: 


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nga eax C6E53 Atk «ie — J Lee @. 


(a) Alaska and Jap Radar................ 
(b) We Prepare to Invade Sicily........... 
(c) Flak Evasion: How to Ruin a Billion 
Dollar Investment................05. 

(d) Breaching the Radar Ramparts of 
Fortress Europe..........0ceccceeeee 

(e) Electronic Ears to Go With Radar Eyes. 
(f) Fleet Radar Countermeasures.......... 
(g) Superforts and Porcupines............. 
(h) How to Heckle the Jap............... 
(i) “Black Cats,’’ Ferrets, and: Submarines: 

(j) Tuba and the German Night Fighters.. 
6. Epilogue: 

Special Problems pede ea es Oy ibis net ate 
Postwar Possibilities...........0.ccceceeees 


A Note to Editors and Readers: 


The development of radar countermeasures was a cooperative 
enterprise. Within the Federal Government this cooperation was 
among the civilian and military staffs of Army Air Forces, Army Signal 
Corps, and other War Department contributors, the Naval Research 
Laboratory and Bureau of Aeronautics of Navy Department, and 
through the work of Division 15 of the National Defense Research 
Committee of the Office of Scientific Research and Development. 

The principal research establishment of the OSRD activity was 
the Radio Research Laboratory at Harvard University. 

In addition there were numerous essential contributions made by 
the industrial laboratories and contractors; and the producers of 


equipment in the electronics industry of the country have a unique ~ 


story of their own to tell of the problem and accomplishments of 
manufacture. 

This release is meant to be a basic report on electronics warfare 
from which writers and editors may derive a variety of individual 
stories. 


- 


THE Jornt Boarp ON ScIENTIFIC INFORMATION PoLIcy. 
WasHinetTon, D. C., 29 November 1946. 


1. Introduction 


THE PERFECTION of new weapons is one of the most important—and 
spectacular—of wartime tasks. We have seen in this war how greatly 
new scientific developments have increased the effectiveness of the 
fighting forces; in some cases new devices may be said to have decided 
the outcome of an entire campaign. Moreover, we are aware of the 
enormous scientific effort which must be expended in the perfection 
of these new devices. 

A task of equal importance is the preparation of counter measures 
to new methods of warfare introduced by the enemy. For every 
weapon there is a counterweapon. ‘Thus, the chemical mask is per- 
fected as an answer to poison gas, and the antiaircraft gun is arrayed 
against the buzz-bomb. It can be truly said, that whatever scientists 
devise, other scientists can to some extent undo. 

For this reason, it has been found necessary in World War II not 
only to assemble large groups of scientists whose sole task is the devel- 
opment of new weapons, but also to assemble similar groups whose 
assignment is to devise means of preventing the enemy from making 
full use of new weapons he may bring into action. 

Here is a further refinement of the scientific war. On the one hand 
we have scientists racing to perfect new weapons. On the other, 
scientists racing to destroy the effectiveness of these devices. 

This is the story of one phase of that second race—a story of our 
successful campaign against the German and Japanese radars. It is 
a story that is unique in many respects. Of all the many wartime 
scientific developments, radar countermeasures were perhaps the most 


closely tied in with actual military operations. Their successful ap- 


plication depended not only on the enemy’s tactics, but also on our 
own. Because of the close connection’ with both operations and 
intelligence, radar countermeasures have been shrouded in the utmost 
secrecy from the very start. Now, for the first time much of the story 
can be told. 


2. The Nature of the Problem 


Rapar and related electronic devices have completely changed the 
tactics of modern warfare by making it possible to ‘‘see” and attack 
an enemy hidden by smoke, fog or night. 


= 


When the United States entered the war, thousands of German 
radar sites defended the continental empire of the Third Reich—sites 
armed with radars developed before the invasion of Poland. There 
were chains of long-range stations to give the enemy early warning 
of the approach of Allied bombers and to enable him to plot the posi- , 
tion of our formations en route to their target. There were air-borne 
radars which enabled the German fighters to find our aircraft at night. 
Still other kinds of radar were used to direct searchlights and anti- 
aircraft fire, all as part of a plan to make the war in the air prohibi- 
tively expensive to the Allies. The German Navy was similarly 
equipped. As time went on, chains of sea-watching radars covering 
the entire Atlantic and Mediterranean coasts of the Continent were 
erected to discover our shipping or signs of invasion. 

German radar research had an early start, and the results presented 
a serious threat to the Allies. Fortunately, because the Germans 
were confident of a short war, they standardized early on a few types, 
virtually ending all development and research for an all-important 
2-year period after the fall of France. This fatal error in strategy 
gave the Allies time to develop and apply effective countermeasures 
against a relatively static system of radar defense. 

The Italians never got very far with radar. The best sets in their 
possessions were some rejected models sold them by the Germans. 
Upon Germany fell the burden of the radar defense of Italy. 

Japanese radar at the start of the war was far behind that of the 
Allies or of Japan’s Axis partner, Germany. Nippon had sets for 
early warning, searchlight control, antiaircraft direction, ana some 
for surface watching, but the majority were poor copies of early 
Allied equipments, and most of these were in short supply. Later the 
Japanese developed more modern equipment, but little of it was 
produced in time for use in combat. 

Both the Japs and Germans, suffering under the impact of our 
countermeasures, belatedly tried the same thing against us. They 
were too late. It can be said that we had radar, and got the most 
out of it; the Axis also had radar, but because of our countermeasures 
got very little out of it. 

The entire countermeasures program has been a race against time, 
its success has been dependent upon the closest haison between the 
laboratories, the Services, the manufacturers, and the fighting fronts, 
for only in this way could early intelligence of enemy plans result in 
the production of equipment with the speed necessary to get it into 
operational use in time. 

To insure a maximum of speed in dealing with problems whose 
details changed from day to day, secret missions were sent to the 
major fighting fronts; transoceanic teletypewriter conferences were 

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held, and equipments designed to do special. jobs were ‘crash- 
produced” in model shops and flown directly to the scene of operations, 
often accompanied by the inventor to insure their effective use. Then 
there was the all-important planning of the method of use of the new 
devices; military personnel and civilians, working shoulder-to-shoulder, 
cooperated more closely than ever before in solving the operational 
problems as they arose. 


~3. The Achilles’ Heel of Radar 


Dr. VANNEVAR Busou has said that ‘‘the new eyes which radar has 
supplied can sometimes be blinded: by new scientific developments.” 
Let us examine some of the weaknesses of radar which lend themselves 
to exploitation. 


It will be remembered that radar works on the principle of echoes. 
_ Just as a man, by shouting loudly, can hear the echo of his voice 
returning from a cliff, so a radar, figuratively speaking, first sends out 
a loud electrical ‘‘sound”’ and then listens for the faint echo to return. 
Instead of a sound, of course, a radar transmits a radio signal. In 
both cases, the original disturbance must be loud if the weak returning 
echo is to be heard at all. Radar stations therefore send out radio 
impulses of tremendous strength; all that is needed to detect or hear 
these signals is a special radio receiver which will tune to the extremely 
short wavelengths used by the radar. 


This represents the first weakness of radio location; because it is 
constantly sending out strong radio signals, a radar set can be heard 
at a great distance—in fact, at a much greater distance than the 
furthest range at which it can detect an object. Thus the radar 
itself can be easily detected. A radar with a 70-mile range could easily 
be “heard” well over 100 miles away. An operating radar, in effect, 
continuously advertises its presence. It is about as quiet, electrically 
speaking, as an artillery barrage is acoustically. 


Second, a radar set betrays not only its existence, but also its 
exact location by the signal it sends out. It is always possible, by 
means of a radio direction finder, to determine the direction from 
which radio signals are coming, just as it is possible to tell the direction 
from which sound waves are coming by pointing an ear trumpet in 
- different directions until the received sound is loudest. If the direc- 
tion to a radio station can be measured at widely separated points, 
and the lines of bearing drawn in on a map, the position of the radio 
station will be at the intersection of these lines. Once a radar signal 
has been tuned in.on a radio receiver, it is possible by means of an 
attachment to the receiver to measure the bearing of the radar and 
thus to determine its location. 


A third weakness of radar sets is the fact that the echo returned 
from most targets is so weak in strength. The sound returned from 
the cliff is many times weaker than the man’s original shout. A 
fairly weak noise, therefore, would suffice to cover up the echo. A 
second man, standing on the cliff and shouting continuously, would 
prevent the first man from hearing the echo of his own voice. 

Radars can be blinded in the same way. It is only necessary to 
provide the target with a device which sends out a radio signal capable 
of covering up the signals reflected back to the radar by the target. 
This process is known as electronic jamming. Because each radar 
set operates on a particular frequency channel, it is necessary that the 
jammer—which is fundamentally a small radio transmitter—be 
tuned to that same channel. 

A practical radar jammer consists of a tunable radio transmitter 
provided with a type of modulation which is especially suited to 
drowning out radar echoes. Any home radio owner who uses an 
electric razor has a good idea of what such interference means. It 
has been found that the most effective radar jamming signal is simply 
a hissing noise similar to the background noise heard in sensitive 
radio receivers when no program is being received. Such a signal is 
said to have a ‘“‘random noise modulation.” 

When picked up on a radio receiver equipped with a loudspeaker, 
a noise jamming signal sounds like a hiss. A radar, however, pre- 
sents the information it received not aurally but visually. Signals 
appear as patterns on the face of a cathode-ray tube called a “scope.” 
As seen on this tube the ‘‘noise’’ looks like many fine blades of grass 


moving about in a random fashion. Echoes from airplanes, which ~ 


are usually displayed as vertical spikes on the radar scope, simply 
disappear and become lost in the “grass.” 

A further weakness of radar sets is the circumstance that they can- 
not distinguish the nature of small targets. One small object, capa- 
ble of returning an echo, looks to a radar just about the sameas another. 
To a radar, an airplane or a ship is a small object. It has been tound 
that a number of thin metallic strips, cut to a length proportional to 
the wave length used by a radar, can return a remarkably strong echo 
to that equipment. In fact, several thousands of these thin metallic 
strips, packaged in a small bundle weighing less than 2 ounces, will 
give a radar echo signal equivalent to one bomber, when the strips 
are ejected from a plane and allowed to fall freely through the air. 


oe Pe =. 


ECHO FROM 
AIRPLANE 
OR SHIP 





BEFORE AFTER 


ANTENNA 





TRANSMITTER 


The sketch shows the pattern which might be seen on the face of the radar scope 
before and after jamming by means of random noises. Below it is a line 
drawing of a typical radar jamming transmitter widely used both aboard ships 


and in aircraft. 


985462 O- 52 - 2 





SINGLE UNIT A SINGLE STRIP TRIPLE UNIT 
(ONE BOMBER) (THREE BOMBERS) 


WINDOW 
(DEHYDRATED BOMBERS) 


ECHO FROM 
AIRPLANE 
CR SHIP 





BEFORE AFTER 


The drawings at the top of this page show typical bundles of ‘‘Window”’ while the 
“before and after’ illustrations below indicate the way that the image on the 
enemy radar scope is confused by the strips of ‘‘Window” as they drop freely 
through the air. 


The phenomena is one of resonance. The metallic strips, designated 
by the code name ‘‘Window,” are resonant at the frequency of the 
radio waves sent out by the radar; in this way a relatively small 
number of strips can return an echo equal to that from a large metal 
object such as an airplane. 7 

If a number of Window packets are dropped out of a plane in succes- 
sion, a trail is produced in which a radar can no longer distinguish a 
real target. The echo from an aircraft is simply lost among the 
echoes from the Window. It is much as if the planes were being 
concealed by a smoke screen of metallic foil. 

In brief review, the weaknesses of radar which can be exploited 
are these: First, a radar is really a powerful radio transmitting station 
which can be heard at a considerable distance. Second, like any 
radio station, its direction and location in relation to the listener can 
be determined by means of radio receivers equipped with direction 
finders. Third, since the radio echo that they receive from most 
targets is very weak, relatively little power is required to cover up 
that echo by sending out a jamming signal from the target. Fourth, 
radars have difficulty in distinguishing between actual targets and 
free falling strips of foil cut to the proper length. 

To knock out the enemy’s radar, all four of these weaknesses can 
be exploited. It is also necessary, insofar as is possible, to prevent 
him from doing the same to you. One reason the Allies achieved 
their victory as quickly as they did was that they were always one 
_-step ahead of the enemy in the jamming war. 


4. December 1941 


AMERICA’S ENTRY into World War II brought about immediate 
changes in our military policy in relation to radar. Prior to Pearl 
. Harbor, our task had been to develop radar, and to evaluate its possi- 
bilities. After Pearl Harbor, with definite enemies and a definite 
order of battle in mind, the emphasis shifted to the quantity produc- 
tion of our radar developments as required. 

Radar countermeasures followed much the same course. Prior to 
the war, research work of a long-range character had been carried on 
in our Service laboratories. After Pearl Harbor, it became clear that 
if the enemy’s radar was as useful to him as we expected our own would 
be useful to us, it would be necessary to put a major effort into the 
developing of countermeasures against specific German and Jap 
equipments. This feeling was reinforced by the experience of the 
British, who had already used countermeasures successfully in the 
defense of their homeland, and attached a high and increasing impor- 
— tance to this activity. 

In the realization that an expanded research and development pro- 


: 


eram would be required, the Services formally requested, late in 
December 1941, that the National Defense Research Committee 
undertake a project in the field of radar countermeasures. * 

The research and development work carried out represent a very 
considerable contribution to the radio art. ‘Tunable receivers were 
designed, capable of continuous coverage up to frequencies 100 times 
as high as those attained by any prewar commercial equipment. 
Transmitters operable over unprecedently wide frequency ranges had 
to be developed, and made to give power outputs far larger than those 
previously deemed feasible at frequencies higher than ever before 
reached by such equipment. Tubes of special characteristics, 
embodying entirely new principles, had to be designed and produced 
in order to make many of these transmitters possible. 

Unusual antennas, capable of operating over wide frequency 
ranges without any adjustment, presented another difficult problem 
which was successfully overcome. 

In addition to this strictly radio development work, new fields 
had to be explored in connection with the metal foil radar reflectors; 
not only was it necessary to carry out research on the principle of 
operation of these devices; it was also necessary to create the new 
machines and new manufacturing processes needed to produce and 
to dispense from airplanes the many billions of foil strips required by 
our forces in their successful prosecution of the war. 

In all, Service orders for countermeasures equipment totalled 
upwards of $300,000,000, representing more than 500,000 individual 
items exclusive of ‘‘Window,”’ of which some 20,000 tons were pro- 
duced. Approximately two-thirds of the dollar value of the Service 
orders represented equipment developed with the help of the NDRC. 

The Service and NDRC research and development program in the 
field of radar countermeasures cost roughly $35,000,000. Of this 
total, the NDRC spent approximately two-thirds, or $23,000,000. 
The NDRC funds were allocated as follows: $15,000,000 to a central 
University-sponsored laboratory ; $7,500,000 to industrial laboratories, 
and $1,000,000 to a field laboratory located in England. 

The various organizations engaged in this program under Service 
and NDRC sponsorship worked in the closest collaboration. Certain 
groups specialized in the development of new sources of jamming 
energy—others specialized in the development of equipments and 
techniques utilizing these sources of power for countering radar or 
other electronic weapons devised by the enemy. 


5 (a). Alaska and the Jap Radar That Wasn't There 


In June of 1942 the Japanese made their first and only landings 
on North American soil, on the islands of Kiska and Attu in the 
Aleutians, 


8 


From the standpoint of the Japanese defenders, these Aleutian 
islands were provided with excellent natural fortifications; from the 
standpoint of the United States invaders they presented an impos- 
ing military problem. The steep shores of islands such as Kiska 
meant that landings could only take place at a few beaches where 
attackers would be exposed to direct fire from the hills above. It was 
realized from the start that special efforts would have to be made to 
neutralize the enemy’s defensive power and to increase the element of 
surprise with which our attack would be carried out. 


In view of its isolated location and high terrain, Kiska represented 
an almost ideal location for a radar station. This meant that the 
Japs, by using radar, could be forewarned of the approach of an 
invasion force when such a force was still many miles out at sea. 


During the invasion of Guadalcanal in August 1942, United States 
Marines had captured some Jap radar equipment. One of the fea- 
tures of this gear was a billboard-sized antenna whose construction 
resembled that of a bedspring. Later on in 1942, service intelligence 
officers were puzzled by two suspicious looking structures which 
showed up in photographs of Kiska taken by photo-reconnaissance 
planes. An alert countermeasures officer remembered the Guadal- 
canal radar—the hunt was on. 


It is very desirable to confirm the suspected nature of these in- 
stallations, and it was particularly desirable, from the standpoint 
of future countermeasures, to determine the operating frequency if 
they were radars. The quickest way to get this information would 
be to equip an airplane with the necessary radar intercept equipment 
and speed it to the theater. 

In December 1942 the Army Air Forces ordered a B-—24 fitted 
out with the latest equipment available for this purpose. 

In February 1943 this plane was already based at Adak in the 
Aleutian Islands and was ready to fly radar investigation missions 
westward over Jap-held territory. Operated by two AAF counter- 
measures operators, the receiving equipment did its job: two un- 
familiar radar signals were heard, and their origin traced to a point 
which coincided with the suspected site. It was the first time, in 
fact, that enemy radar signals had been heard on American-built 
countermeasures equipment. A new method of securing military 
intelligence had been born: radar reconnaissance had taken its place 
beside photo-reconnaissance, 

From that time on, the Kiska radars were kept under constant 
observation. One quadrant of their beam was found to be shadowed 
_ by the island’s volcano: knowing this blind direction, our bombers 
were able to fly an approach course which brought them in undetected. 


9 


5 (b). We Prepare to Invade Sicily 


In THE MEDITERRANEAN theater, the business of systematically 
spotting enemy radars by listening for their electronic ‘‘shouts” really - 
began shortly after the North African landings. Reports of highly 
accurate gunfire from German coastal batteries became a matter of 
concern not only to the Theater Commander, but also to Headquarters, 
Army Air Forces, at Washington. It also became apparent that 
enemy early warning radars and radar-controlled gun batteries, par- 
ticularly those along the coast of Sicily and Sardinia, could be instru- 
mental in removing the element of surprise from the contemplated 
Sicilian invasion, and could greatly increase our ship and landing 
craft losses during the actual landing operations. 

Cables were exchanged on this subject in late 1942 and early 1943. 
By March 1943, the need for a United States program to investigate 
enemy radar defenses in the North African Theater became urgent, 
and high priority was placed on the equipping of a second search plane, 
which by this time could be equipped with improved gear easily 
capable of spotting the German radars. This plane, known popularly 
as a ‘‘Ferret,’’ was fitted out in slightly over a month’s time. 

In addition to prototype radar intercept equipment built by the 
NDRC, this plane carried with it to the theater a civilian engineer of 
high professional standing who had been requested by the Army Air 
Forces to accompany the expedition as its technical advisor. Within 
a month after the North African Theater’s request, the Ferret was 
flying on regular operations off Sicily. 

These radar investigational flights, carried out at night in blacked- 
out, black-painted planes, had a considerable air of mystery about 
them. Flames from exhaust pipes were carefully shielded to prevent 
visual observation by enemy night fighters, which constituted the 
chief danger, since the Ferrets tried to avoid enemy flak as much as 
possible. An added precaution against enemy interception was a 
specially installed radio device which would cause warning lights to 
flash in the pilot’s compartment when the Ferret plane was being 
tracked by German night-fighter’s radar. 

The night investigational flights usually followed a course along a 
line generally parallel to that section of the enemy coastline under 
observation. Then, if a German radar signal was heard, its general 
direction was determined by means of the radar direction finding 
equipment aboard the plane. Several such readings, or “fixes,” 
on the same radar station usually provided data of sufficient accuracy 
so that photo-reconnaissance planes could later be directed to the 
area to take pictures of the actual radar site. 

Before the arrival of the Ferret mission in North Africa, observa- 
tions of German radar signals were being ¢arried out in the Mediter- 


10 


ranean area by an RAF squadron of Wellington bombers which had 
been fitted out with British intercept equipment. This equipment 
was not designed to determine the direction from which intercepted 
enemy radar signals were coming; the fact that the RAF planes 
nevertheless did locate a number of radar stations represented a real 
tribute to the gallantry of their crews. British ‘“‘Wimpeys” were 
accustomed to cruising around at low altitude in the vicinity of a 
suspected radar site and locating the radar hy ascertaining the area 
in which the signals were loudest. In order to locate the enemy sets 
with any sort of accuracy, it was necessary to fly entirely too close to 
the well-defended radar sites for comfort. Needless to say, the experi- 
ence of the British investigational operators proved a great help to 
the newly arrived American group. 

Since the wave lengths on which most radars operated were far 
shorter than those for which commercial radio direction finders had 
been designed before the war, entirely new equipment was developed 
for the job. The first American attempts to locate the source of 
intercepted radar signals were crude enough; they took the form of a 
system of homing. Fixed receiving antennas were used, and the 
direction to the unknown transmitter was determined by altering the 
airplane’s course until two audible tones were equal in strength. 
However, when this occurred the plane was headed directly toward 
the radar under observation, and unless the procedure was carried 
out at some distance from the site, it was likely to become dangerous 
since flying too close to an enemy radar is not exactly healthy. 

There was another serious disadvantage to the homing method. 
Since more than one bearing had to be taken in order to establish 
the location of a given radar station, the search planes had to zigzag 
a good deal, which made navigation on their night flights extremely 
difficult. 

Besides locating the German radars, the original Ferret and its 
many successors unearthed a good deal of information about German 
radar strategy in the Mediterranean. The following is a good 
example. At one point in planning the invasion of Sicily, it became 
necessary to know whether the German coast-watching radars, which 
were primarily used to plot Allied shipping, could also be used to plot 
aircraft. One evening, the Ferret airplane, equipped this time with 
jammers in addition to receivers, flew out across the Mediterranean 
to a point near the coast of Italy, where it was brought under observa- 
tion by several German radars. By tuning in the different signals 
in his receiver, the countermeasures officer on the plane knew that 
his plane was being looked at by at least three early warning radars. 
He also heard one or two coast-watching radars about their usual 
business of sweeping back and forth on the lookout for ships. The 


11 


latter sets had up until that time displayed no particular interest in 
passing aircraft, which were investigated exgluatvaly by the sets used 
for aircraft Sheen tint 

On this occasion, the countermeasures officer turned on his jam- 
ming transmitters and put the three early warning radars out of 
action one by one until the German ‘“‘eyes’”’ were thoroughly blinded. 
Then, after a slight delay, the expected happened. The German 
ship-watching sets ceased their normal business of scanning the sur- 
face of the Mediterranean; they eagerly pointed in the direction 
from which the jamming came in order to see what was going on. 
Wherever the Ferret flew, the coast-watchers followed it. The evi- 
dence was conclusive that both early warning and coast-watching 
radars would have to be jammed in order to prevent the Germans 
from getting an idea of the extent of our aerial attack during an inva- 
sion. The officer had matched wits with the German radar operators 
on the ground many miles away, and had won. | | 3 

The enemy radars thus located by the well-equipped Mediter- 
ranean Ferrets were marked for destruction before the landings in 
Sicily and Italy itself. Some were destroyed by bombing; others 
were pounded with Naval gunfire. The enemy had to be denied 
advance notice of the strength and disposition of our forces. 

For the landings at Salerno, 50 newly developed radar jammers 
were flown direct from the factory to the Mediterranean theater, 
there to be installed on landing craft by a United States Navy counter- 
measures team. These jammers made it doubly sure that the Ger- 
mans got no help from their electronic ‘‘eyes.”’ 


5 (c). Flak Evasion—How To Ruin a Billion-Dollar Investment 


THE OVER-ALL Allied plan for the air phase of the European war 
had two basic parts: the British would area-bomb Germany by night, 
while the Eighth Air Force in England, and the Fifteenth Air Force 
in Italy, would pinpoint-bomb by day. This plan, it is now clear, 
was soundly conceived. But it is easy to forget that the success of 
the American attack once hung in the balance. 

Committed to daylight operations, the U. S. B-17’s and B—24’s 
went into battle in tight defensive formations which allowed their 
heavy armament to provide mutual protection against fighter attacks. 
Moreover, with large, tight formations a very desirable concentration 
of bombing could be achieved. These advantages, however, were 
not obtained without cost, for our compact air squadrons provided 
the German antiaircraft guns with especially juicy targets. 

From early in the war on, the Germans had been preparing to meet 
the Allied air attack, and their antiaircraft defenses enjoyed the very 
highest priority. Starting in 1940 with 88-mm. antiaircraft guns, 


12 


their artillery was soon improved by the addition of an improved 88; 
later on, 105- and even 128-mm. cannon appeared. By the end of 
the war, it was estimated that the Germans had 16,000 heavy guns 
in action, and their important targets were defended by the largest 
concentrations of antiaircraft weapons in the history of warfare. 

As important as the guns was the equipment for fire-control. The 
Germans had standardized early in the game on a satisfactory pre- 
dictor and optical fire controller; by April 1940, the first of what was 
to become a standard line of fire-control radars appeared. Radar, 
optical controller, and predictor were used as integrated units. At 
first four guns were assigned to each of these units; later, as gun 
production began to outstrip radar production, this number rose in 
some cases to 8 and even to 16. 

The radars, known by the code name ‘‘Wurzburg”’ were used not 
only at night, but also in the daytime when visibility was poor. They 
were capable of tracking aircraft with a precision equal to that obtain- 
able with optical control. In 1945, the Germans had about 4,000 
Wurzburgs or Wurzburg-type radars in service. Taking into account 
the 10 trained operators required to run each set, the Wurzburgs 
represented an investment of roughly one billion dollars. 

All in all, the flak defenses of the greater Reich were extremely 
formidable. They were particularly menacing to the special type of 
bombing formation used by the USAAF. Any antiaircraft gun, when 
steadily aimed and fired at a fixed point in space, will land the majority 
of its shells within a certain space enclosing the point of aim. (In- 
herent inaccuracies in fuse setting, etc., cause a certain proportion of 
the shots to miss.) Now it happens that the German antiaircraft 
guns, in firing at a fixed point at the same altitude as our bombers, 
could land over 80 percent of their shells inside a space which just 
enclosed the standard United States bombing formation. The 
German fire-controller had only to aim at the center of our formations 
in order to assure 80 percent of.his shells an excellent chance of hitting 
one of our planes. As a result of these and other characteristics of 
the German guns, an individual United States plane in a tight forma- 
tion ran a several times greater risk of being hit by flak than did a 
single plane in one of the RAF’s loosely knit ‘‘bomber streams.” 

Although radar control of antiaircraft was used by the Germans 
during many daylight engagements (in view of haze, local smoke 
screens, etc.), the United States Air Forces’ need for radar counter- 
measures became really acute when bombing with the aid of radar was 
begun. As soon as our heavies received fighter protection, the 
Germans’ only defense against bombs through the overcast was radar 
controlled AA fire—and radar is susceptible to jamming. 

The British had been faced with the same problem in the course of 


985462 0 - 52-3 13 


their night attacks, and their planes had first begun to drop the 
metallic foil strips, known as Window, during the saturation raids 
on Hamburg in July 1943. Results were spectacular; the RAF’s 
losses were cut to a small fraction of those sustained in previous 
attacks. German radar operators were heard to exclaim: ‘The 
planes are doubling themselves!” 

By May 1943, it had been demonstrated by United States engineers 
working with the United States Air Force that in England the German 
gun-laying radar was indeed used against daylight raids, and by 
October of that year the AAF was ready for an operational test of an 
electronic jammer known as ‘“‘Carpet.”’ 

This equipment has been designed by an NDRC laboratory early 
in 1942, with the support of the Signal Corps well before the Army 
Air Forces had even begun their operations in Europe. Based on 
intelligence received from the British and originally thought of as a 
solution to the British countermeasures problem, the Carpets turned 
out to be a “natural” for the United States heavy bombers. Theo- 
retical considerations showed that when carried one-to-a-plane—at 
least in the early days—the Carpets ought to be able to send out an 

“electronic raspberry” quite capable of knocking out the Jerry radar 
fire control. 

On the basis of these calculations only, and in advance of an actual 
Theater requirement, Headquarters Army Air Forces had authorized 
the procurement of a number of prototype Carpets. 

Procured in record time by the Signal Corps, these sets were flown 
to England in the summer of 1943 and were installed in planes of two 
United States heavy bombardment groups, with the aid of United 
States civilian scientists who were at that time attached to a British 
countermeasures laboratory. 

In October 1943, the Germans got their first taste of United States 
jamming. Dahings the first raid to Bremen, Germany, the Carpet- 
equipped planes suffered losses less than one-half those of the non- 
equipped planes. The Carpets had clearly done their job well. 
Radar countermeasures in the Strategic Air Forcés had come to stay. 

Meanwhile, the Mediterranean theater had not been idle. Air 
Forces countermeasures officers, who had come over with the first 
Ferret planes, had been spark-plugging requests to Washington for the 
new jamming equipment. The Germans were to find their southern 
antiaircraft radar defenses also jammed. 

The American Air Forces, who has been the first to exploit electronic 
jamming of these radars, added the second countermeasure Window 
to their bag of tricks in December 1943. Since the growing size of our 
forces required the combat formations, or ‘‘boxes’’ to fly in line, 
Window tossed out by the first box laid down a trail in the sky in which 


14 


the German radars were powerless to track the following planes. 
Carpets were eventually in every plane, and the combination of Carpet 
and Window jamming proved to be far more effective than either type 
alone. 

The need for flak radar countermeasures grew steadily after their 
introduction. The average number of operational days per month 
increased, with the advent of radar bombing, from roughly 9 in 1943 
to around 22 in 1944. During the winter months of November and 
December of 1944, and January 1945, the percentage of missions 
carried out from England with the aid of radar bombing ran 94, 67, 
and 73 percent, respectively. 

Blind bombing greatly increased the weight of the air attack on 
Germany, not only because of the increased number of days on which 
operations were possible, but also because the flak losses sustained on 
blind missions were lower. This fact permitted more attacks to be 
made; for example, on D-day, the strategic air forces in England 
(which was usually able to put up about 1,500 planes on maximum 
effort missions) flew 2,490 sorties. 

It will be seen that radar countermeasures played a major role in 
keeping these losses down. ‘The decline of the German fighters left 
radar-controlled antiaircraft fire virtually the sole air defense of 
Germany on overcast days. Although the losses to fighters had far 
exceeded the losses to flak in the first part of the air war, by the late 
summer of 1944 this situation had reversed itself; in August and 
September 1944, 445 planes were lost to flak, whereas only 198 fell 
victim to fighters. 

Although the effectiveness of our countermeasures was clearly 
demonstrated by the reduction in losses during the first raids, it _ 
became harder and harder to draw valid conclusions from loss data, 
since no two attacks are run off exactly alike under exactly the same 
conditions of weather over the target, etc. The first real evidence of 
the success our Carpets and Window ‘‘Flak Pills” were having, came 
-after the fall of Rumania, when Italy-based AAF countermeasures 
officers seized the opportunity to interrogate German gun crews 
' captured near the heavily defended Ploesti oil fields. Later, when it 
became possible after VE-day for Army officers and civilian scientists 
to cross-question members of the Luftwaffe, from the generals right 
on down to the Wurzburg operators, the answer became abundantly 
clear: the advent of countermeasures on a large scale reduced the 
effectiveness of Wurzburg-controlled antiaircraft fire to 25 percent of 
normal. 

How much grief this caused the enemy can now be pictured. Com- 
mitted as they were to an enormous investment in their Wurzburg 
radars, the Germans had tried feverishly to keep their sets in operation. 


15 


Antijamming attachments were hastily devised and rushed out into 
the field in the hope that they would cut down the effectiveness of our 
jamming. In all, some 13 of these devices saw service; 19 additional 
schemes were under development at the end of the war. 

There were two basic ideas behind these attachments: to avoid 
our electronic jamming, the Germans tried to shift the operating 
frequency of their Wurzburgs. ,To avoid Window, they built gadgets 
which would enable the radars to distinguish between moving and 
stationary targets. 

However, the anti-Carpet devices made their radars vulnerable to 
Window and the anti-Window devices made them vulnerable to 
Carpet, because of difficulties in using both devices at once. For this 
reason, our decision to combine the two countermeasures was a partic- 
ularly happy one. The answer to all the German attachments was 
simply more jamming. As it turned out, the constantly growing 
scale of the Allied countermeasures made the German antijamming 
devices more or less obsolete before they could be placed in service. 

More and more of the German scientists with radar and electronics 
experience were pressed into the search for workable antidotes to our 
jamming. At one time, toward the end of 1944, the frantic search 
had reached a peak. According to a reliable German estimate, 90 
percent of their ultra-high frequency engineers were working on anti- 
jamming attachments. An average figure was certainly 50 percent, 
or roughly 4,000 people, whereas only about one-tenth that many 
trained United States engineers were employed in devising radar 
countermeasures. The Luftwaffe, in desperation, even announced a 
public competition, with prizes totalling 700,000 Reichsmarks (free of 
all taxes) for the best solution to the problem of Window. 

In their rush to save the Wurzburgs, the Germans were distracted 
from the development of microwave radar, which had so brilliantly 
been exploited by the Allies. Although a working sample of this 
Allied equipment had been recovered from a crashed bomber as 
early as January 1943, the Germans were unable to take advantage 
of a golden opportunity to copy the equipment which had fallen 
into their hands and modify it for antiaircraft use. Although this — 
possibility was understood, the urgent need to save some small part 
of the Wurzburg investment diverted too many men. To this extent 
it may be said that Allied countermeasures not only jammed the 
Wurzburg radars, but also the German scientists. 

Since at least 10 men are required to keep each Wurzburg in 
operation, our countermeasures tied up roughly 40,000 trained enemy 
troops who could have been better employed elsewhere. What had 
been a billion-dollar antiaircraft defense, turned out to be a liability. 

By VE-day, at least two radar jamming transmitters were carried 


16 


in every heavy bomber of the Strategic Air Forces. The scope of 
this undertaking becomes clear, when it is remembered that these 
equipments were invented, fabricated, and shipped to an operating 
theater, where they were installed, and the air crews trained in their 
operation—all in the period after America’s entry into the world 
conflict. 

In all, some 10,000,000 pounds of aluminum foil in the form of 
Window strips were dropped over Europe by our British-based bombers 
alone. As a result of this demand, United States aluminum foil 
production was tripled during the war. Seventy-five percent of this 
total capacity was devoted to the manufacture of Window. The 
aluminum foil, which our cigarette packages and candy bars did 
without, now lies cluttering up the fields of Germany. 

During the period from September 1944, until May 7, 1945, heavy 
bombers based in England flew 30,000 sorties against heavily defended 
strategic targets under blind conditions. According to official figures, 
the average flak loss rate for this type of mission was one-half of 1 
percent, thus accounting for a total of 150 planes in the period under 
consideration. 

Since all evidence has shown that the effectiveness of German 
radar-controlled antiaircraft fire against blind strategic missions during 
those months was approximately 25 percent of normal, it can be 
said that radar countermeasures undoubtedly saved the United 
States forces in England roughly 450 planes and 4,500 casualties. 
The cash value of 450 four-engined aircraft alone is approximately 
-$150,000,000, or about twice the cost of the entire flak countermeasures 
program. Roughly, the same considerations apply to our strategic 
Air Force in Italy whose size was fully half that of its British-based 
counterpart. 


5 (d). Breaching the Radar Ramparts of Fortress Europe 

Tuer INVASION of Normandy has taken its place as one of the 
greatest military operations in the history of the world. The success 
of this frontal assault on the best defenses which could be erected by 
an ingenious and resourceful enemy can largely be credited to superb 
military planning. 

It was impossible to conceal the intentions of an invasion from the 
Germans. However, the exact time and place could pe concealed, 
and it was of paramount importance to the success of the operation 
that this information be withheld from the enemy until the last pos- 
sible moment. 

As an integral part of their N Anite France defense system, the 
‘Germans had erected hundreds of radar stations, which, if unmolested, 
would not only have given warning of the apivennl of Allied phates 


17 


and ships, but would also have made possible the direction of gunfire 
against the attacking force under “blind’”’ conditions. 

To give some idea of the density of these radar installations, along 
the 200 miles of coast line between Dieppe and the tip of the Cher- 
bourg peninsula, no less than 50 air-warning and coast-watching 
sites were found, each with an average of 2 radars per site. In one 
section, there was an average of 1 radar every 1% miles. 

Moreover, the Germans took unusual pains to diversify their 
electronic defenses: no fewer than 12 separate and distinct types of 
radars were to be found along the so-called invasion belt! Often these 
sets would be arranged for double duty: early warning and anti- 
aircraft fire control sets were adapted for the control of coastal defense 
guns. 

It is no wonder that the impressive German radar defense was one 
of the major topics for discussion at the invasion planning conferences. 
It was given added importance by the fact that operation was sheduled 
to begin under cover of darkness. By jamming the radar, or otherwise 
putting them off the air, it would be possible to prevent the enemy 
from finding out where our forces were concentrated. There were 
two main countermeasure problems: diversions, and the jamming of 
coastal gun-laying radar. 

In the discussions on how best to put the enemy radar out of action, 
two possibilities were considered: bombing or strafing plus jamming. 
It was agreed that both of these propositions would be exploited: 
as many radars as possible would be shot up, and the rest would be 
jammed. Complete reliance on bombing would be unwise, since it 
was perfectly possible for the Germans to hold carefully camouflaged 
radars in reserve—radars that would not be turned on until the need 
arose. And even a few radars, left unjummed, could be dangerous. 
Accordingly, the Air Forces were assigned the task of attacking the 
radars which had so carefully been spotted by low-flying British and 
American photo-reconnaissance planes and by radar search receivers. 
In addition, both Navies undertook the tremendous task of fitting 
their many hundreds of ships with jamming equipment. 

Diversions require careful planning. If all the radars in one par- 
ticular area are completely jammed, the enemy will suspect that some- 
thing is happening in that vicinity, and will usually send forces out to 
investigate. However, in order to be successful, the jamming must 
be 100 percent complete—something hard to achieve in practice. 
Each jammer can take care of one radar channel. If many radars 
are present, each on a different channel, a proportionate number of 
jammers is required. One or two radars, not jammed, can give the 
show away. The problem presented by the many types of German 
radar was not easy. 


18 


A more successful sleight-of-hand is one in which small targets are 
made to look like large ones. In this way a few planes will resemble 
a formation, or a few ships a fleet. We have seen how 2 ounces of 
aluminum strips can give a radar echo similar to that from a plane; 
10 Window bundles thrown out all at once will give a fair imitation of 
10 planes. Moreover 10 bundles released close to the water cannot 
be distinguished from a ship. However, once the strips are falling 
freely, their forward velocity stops, and an alert operator notes that 
his “‘targets’”’ are standing still. If, however, a complicated situation 
is developing rapidly and the radar screen is partially cluttered with 
jamming, even the best operators will be hard put to it to say exactly 
what is happening. If an enemy operator reports large forces ap- 
proaching when only small ones are actually there, our purpose has 
been accomplished. Most diversions are planned on such a basis. 

In the case of ships, confusion and screening can be produced by 
firing ‘‘Window” rockets or shells from the guns of the vessels them- 
selves. ‘‘Window” can also be dropped from airplanes overhead. 
Still another possibility is the use of large metallic screens so arranged 
that a flat surface is always presented to the enemy radar; devices 
such as this are known as corner reflectors and under some conditions 
can make a small ship look like a large one. 

Once the over-all plan has been adopted, preparations for the actual 
event were pushed full speed. The Air Forces and the two Navies 
had an installation problem of the first magnitude. 

In order to determine the requirements for ship protection, a 
section of coast along the Firth of the Forth in Scotland was picked 
out for its resemblance to the Normandy littoral. Captured German 
radars of the three main types were set up on this practice shore. At 
sea, countermeasures-equipped Navy ships, landing craft and aircraft 
played a game of hide-and-seek with the radar operators on land 
while British and American civilian engineers stood by as umpires. 

The United States Navy’s own installation program was well 
handled by teams of officers and enlisted men specially trained for the 
job and operating as independent units for reasons of secrecy. 

The British, who were depending heavily on American equipment 
for their part of the radar jamming operation, found themselves short 
of men familiar enough with the United States gear to do a good job of 
installing and testing it. Therefore, NDRC and United States 
Service civilian engineers assisted the British in the installation of the 
American countermeasures equipment. These engineers were divided 
up among half a dozen British Naval bases, and by D-day, 90 percent 
of the proposed installations had been completed satistactorily. 

The invasion really began with the Air Force attack on the German 
radar sites. This took place shortly before D-day, and in order to 


19 


avoid betraying the location of the coming attack, radars had to be 
put out of action along virtually the entire channel coast. Rockets 
punched out nice round holes in parabolic antennas and in transmitter 
huts, and 50-caliber machine gun bullets played havoe with the 
delicate equipment. 

Yet in spite of this tremendous dressing down, when the invasion 
actually started, a certain portion of the radars were observed to be 
still on the air. The night before D-day, airplanes carrying jammers 
cruised up and down England’s south coast and jammed the German 
long-range early warning radars so that the Germans were prevented 
from seeing our air squadrons forming over England, and approaching 
the continent. 

Early in the morning of D-day, a small force of bombers flew a 
jamming and Window diversion inland from the Dover-Calais area in 
an effort to simulate a large bombing raid headed for Germany; 
many enemy fighters are known to have taken this bait, and to have 
spent much of the night circling fruitlessly inland near Calais to the 
east of the main invasion forces. With the aid of air diversions 
intended to simulate additional air-borne landings, our American 
air-borne troops were able to complete their landings on the Cher- 
bourg peninsula with very little opposition, and with phenomenally 
low losses—one-half of 1 percent. The diversions provided a good 
part of the answer to a question almost universally raised at the time 
of the invasion as to the whereabouts of the Luftwaffe. 

Naval diversionary forces from Folkstone approached the French 
coast just south of Calais; other forces from Newhaven crossed the 
channel south of Dieppe. Aircraft flying low over these ships dropped 
enormous quantities of Window in order to supplement the increase 
in ship “‘size’’ provided by special reflectors. The additional Window 
made the separate ships look like one huge convoy. 

The Royal Navy sent a group of plucky HDML’s—Harbor Defense 
Motor Launches—out on a screening mission, during which they 
spent some hours off the French coast north of Normandy. These 
craft, plying their countermeasures vigorously in an effort to seem 
like a major fleet, had the satisfaction of drawing a considerable 
amount of inaccurate German gunfire. 

A United States cruiser—one of the ships which had not received 
its quota of jamming equipment—found itself the target of accurate 
gunfire while standing far offshore. In view of the distance involved, 
the fire was obviously radar-controlled. The Naval Task Force 
Commander then ordered that this ship change positions with one 
fitted with the latest jamming transmitters. As soon as the swap had 
taken place, the shore fire became inaccurate, and soon stopped 
altogether. 


20 


The Germans later attributed much of the success of the invasion 
to their inability to meet the landing forces with all the reserves at 
their disposal. Certainly the confusion was considerable. The 
German radio announced on several occasions during the operation 
that landings had been repulsed at points where they had not, in fact, 
been attempted. One enemy radar operator, whose set commanded 
Omaha beach, told interrogators that he had known something was 
coming, but had had no idea what it was. 

The invasion of southern France followed much the same pattern 
as the Normandy operation, only this time it was primarily an Ameti- 
can show. The good work done by the radar-hunting Ferrets in the 
Mediterranean theater paid off during ‘Operation Anvil,’ even 
though the harrassed German operators had been forced into foxy 
tactics, such as not tracking single planes at night, and pulling in 
special retractable radar antennas when photo-reconnaissance planes 
were about by day. The Germans had reason to fear our radar 
reconnaissance, for prior to the invasion itself, some 500 air attacks 
were made on 22 known enemy radar sites. 

The Navy part of the invasion included a main force, a support 
force, and two diversions. Radar countermeasures received consid- 
erable attention in the diversionary forces, for the attack was pur- 
posely planned to begin under cover of darkness, in order that the 
enemy might not discover the nature and scope of the operation until 
it was well under way. Countermeasures equipment were installed 
in about half that number of British and American ships ranging 
from landing craft to cruisers. 

The air-borne portion of this show was also protected by diversions 
using Window and by planes fitted with electronic jammers. To add 
further to the confusion, a ground-based Signal Corps radar jamming 
and intercept installation located in Corsica also joined in the chorus. 
This outfit, located on a mountain top at the northern end of the island, 
had for months been carrying out a round-the-clock study of the 
intimate habits of the German radars dotting the southern French 
coast. From this study, much valuable information had been ob- 
tained on the German tactics. 

As was the case in Normandy, interrogations afterward showed that 
our countermeasures had been successful. The vaunted radar ram- 
parts of Fortress Europe had not only been rendered useless to the 
Germans, they had been rendered worse than useless, for when 
needed most, they had become a source of completely misleading 
information. 


5 (e). Electronic Ears To Go With Radar Eyes 


AS WAS POINTED out earlier in this account, radars send out very 
strong radio signals which can be picked up at considerable distances 


21 


by means of a suitable radio receiver. Just to be able to hear a sig- 
nal, however, is seldom enough, since the question, ‘‘Is there a radar 
there?’’, is usually followed immediately by ‘‘Where is that radar?’’. 

As we have seen, this problem first became serious in connection 
with the Sicilian campaign and the business of plotting German coastal 
radars that might be dangerous to our invasion forces. The experi- 
ence of those early days was well applied at home in speeding research 
on aircraft direction finding systems which did not depend on a change 
of aircraft heading for their successful operation. 

In the end, a very excellent device of this sort was developed. Not 
only did it tell you direction, but it also told you this direction instan- 
taneously and automatically. 

This was done by taking advantage of the fact that for radar fre- 
quencies, a directional antenna—i. e., one which can only pick up 
sionals coming from one particular direction—can be built in very 
compact form. If one of these antennas is made to rotate so that it 
scans the horizon, signals from a distant radar will be heard only when 
the antenna is pointing directly toward it. Although the antenna 
could be rotated by hand, and its heading noted when a signal was 
received, this is awkward, and it is much better to drive the antenna 
continuously with a motor and display the received signals (if any) 
on something very similar to a radar scope. ‘The signals show up 
on this scope as radial deflections outward from the center of the 
tube. A signal, looking something like a spoke in a wheel, points 
toward the target’s bearing. The exact direction away from center 
is controlled by the instantaneous position of the pickup antenna. 
When the deflection of the scope trace points vertically from the 
center to the top of the tube, the received signal is coming from dead 
ahead; if the trace is horizontal and to the right, the radar is on the 
starboard side, and so forth. 

In the fall of 1943, intelligence reports contained disturbing news. 
The indication was that German submarines were being fitted with a 
new and considerably improved radar. A high-performance set of 
this tvpe could constitute a very serious threat to our own radar- 
equipped search planes which had driven the U-boats down and which 
had helped to break the back of Doenitz’s undersea campaign. If the 
submarine could see an approaching plane, it would then have ample 
time in which to submerge to a safe depth. Our planes would still be 
able to force the subs down, but the enemy radar could remove the 
element of surprise and thus prevent our planes from making kills. 

The Navy was very much interested in air-borne radar search 
equipment capable of operating in the frequency range understood to — 
be used by the German sub radar. The new direction finder under 
development at an NDRC laboratory seemed to provide an ideal 


22 


answer. It was felt that submarine radar transmissions might be 
intermittent; hence, any device which displayed the direction of a signal 
instantaneously on a cathode ray tube in much the same manner as 
a radar displays its echoes seemed to fit the requirements perfectly. 
This the new direction finder could easily do. 

In September 1943, a laboratory prototype direction finder was 
installed aboard a Navy sea-search PB4Y-1 patrol bomber on the 
very highest priority. 

One month later, the PB4Y1 left for a base in North Africa, from 
which place it flew over 500 hours in submarine searches over the 
U-boat hunting grounds in the Bay of Biscay and in.the Mediter- 
ranean. 

Throughout many long and tedious flights the equipment func- 
tioned perfectly—but no enemy signals were heard. Back home, 
anxiety over the supposed submarine radar menace subsided. It 
appeared that the new German radar was either no good or was not 
being used. 

At the end of the war, the correctness of the latter conclusion was 
verified. Interrogations of captured submarine crews revealed that 
the U-boat skippers were downright afraid to turn on their equip- 
ment. By preference, the German radar was used only in regions 
close to the homeland, and then only for navigation in narrow pas- 
sages. Out in the open ocean, it was almost never used. 

This German fear of being overheard could be interpreted as a 
tribute to our radar countermeasures organization—a tribute which 
was perhaps not quite justified at the time. However, the fact re- 
mains that countermeasures—or the threat of countermeasures—pre- 
vented the German U-boat commanders from making use of their 
radar—a weapon which has made our own submarines many times 
more effective. 

A number of other prototype direction finders were built on a rush 
basis and turned over to the Navy for air-borne operational use in the 
Pacific. 

On one occasion, one of the Navy’s radar-direction-finder-equipped 
bombers was on patrol in the neighborhood of a crippled United States 
submarine being shepherded back to its base by two others. The 
weather was overcast and the visibility poor. Suddenly the counter- 
measures officer called his skipper on the interphone and reported 
that he had picked up what was unmistakeably the signal of a Jap 
radar-equipped plane. He reported the bearing of the enemy aircraft 
to the skipper. 

The skipper immediately headed the patrol bomber in that direc- 
tion. As they flew along, the signal picked up by the countermeasures 
operator grew louder and louder, and presently he advised the skipper 


23 


that they must be closing in on the Jap plane. At that very instant, 
the radar operator reported contact with an unidentified aircraft, and 
the patrol bomber was soon maneuvered into attack position. 

Nosing down through the overcast, the patrol bomber broke out 
of the clouds and found a Jap plane directly beneath it. The Betty 
jettisoned his depth charges and tried to escape, but was soon 
splashed—only 8 miles away from the United States submarines. 

The need for long range patrol aircraft capable of carrying out both 
radar and radar intercept searches was recognized early in the war by 
the Navy. The first plane expressly designed for this purpose was 
the PB4Y2. Known as the Consolidated “Privateer,” this aircraft 
consisted of a single-tailed B—24 whose fuselage had been extended in 
the nose section by some 7 feet. The extra space thus made available 
was shared equally by the latest in radar, long-range radio communi- 
cations, and radar countermeasures equipment, including direction 
finders. Enough antennas were installed in each PB4Y2 to permit 
radar intercept work at all frequencies which might possibly be en- 
countered. So many of these plastic-dome-covered antennas were 
provided, that the prototype ‘ Privateer’”’ was nicknamed the “‘ Wart- 
hog.” 

Space, power, and antenna facilities were provided in the electronics 
compartment for a flexible installation of search receivers as well as 
jamming transmitters. For the first time, a coordinated counter- 
measures installation had been designed as an integral part of an air- 
plane. 

NDRC engineers, working in cooperation with the Navy, took an 
active part in the planning of the PB4Y2 installation from the very 
earliest days. Newly developed laboratory equipment was installed 
in a prototype plane and its performance studied under actual flight 
conditions. 

PB4Y2 planes, of which some 600 were ordered at one time, were 
playing an active part in Pacific operations at the conclusion of the 
war. ‘The first to arrive in that theater operated out of the Naval 
base at Manus in the Admiralty Islands in February 1945. A sub- 
stantial number were on duty in the Philippine area by March. 

A ship-borne version of the radar direction finder was developed 
shortly after the air-borne model. During the course of its first trial 
run aboard a destroyer escort, the remarkable capabilities of this 
equipment were clearly demonstrated. During maneuvers off shore, 
the radar operator of the DE made contact with a target at a certain 
range and bearing. The countermeasures operator, meanwhile, 
picked up a number of different signals all coming from the same 
bearing. From the characteristics of these signals, as well as from 
the frequencies at which they were tuned in, he deduced that the 


24 


vessel on the bearing in question must have been a United States 
cruiser. When this information was reported by him to the bridge, 
the captain’s amazement was profound. That gentleman must have 
decided the advantages of radar were now complete: not only could 
these electronic devices tell the range and bearing of a target, but 
now they could identify the nature of the target as well. 


5 (f). Fleet Radar Countermeasures 


In Evropg, special teams of countermeasures experts who were 
used for the execution of the antiradar phases of amphibious opera- 
tions, since these were not numerous or separated by vast distances. 
The Pacific theater, however, presented Navy planners with an en- 
tirely different problem in logistics. 


Not just a few but many landing operations were in the cards. 
Moreover, the timing of these events was such that they had to be 
planned and rehearsed many months in advance, thus permitting 
prompt execution at locations widely separated from each other. 
Teams of experts could not be moved from place to place fast enough 
to keep up with the parade of D-days. For these reasons, a bold 
decision was made: radar countermeasures would be made a Fleet- 
wide activity rather than an electronic specialty. Radar officers 
became electronics officers; in addition to their other accomplishments, 
they became experts in radar countermeasures. 


This decision resulted in an extensive installation program, desizned 
to make every combat ship in the Fleet self-protecting from an 
countermeasures point of view. Ships from destroyer-size on up, 
carried both intercept and jamming equipment; now countermeasures 
antennas sprouted on masts and yardarms like mushrooms. 


Even landing craft had their complement of the new equipment. 
Every effort was made to protect these vulnerable ships from radar- 
aided aerial attack or radar-controlled shore fire. Pretuned, unat- 
tended jammers installed aboard the landing ships were arranged to 
play havoc with any Jap radar which might be encountered. 


Careful plans were made to coordinate the operational use of the 
Fleet-wide installations. Special communications channels were set 
aside for the use of operators reporting enemy radar intercepts. This 
information, like the data supplied by our own radars, was fed into 
the CIC, or Combat Information Center, of each ship. There it was 
found tbat radar and countermeasures data supplemented each other 
perfectly. If on the same bearing an enemy radar signal was heard, 
and an unidentified radar contact reported, there was little doubt as to 
the nature of the “bogey” or unknown target. On the other hand, if a 
fricadly radar signal was heard coming from the direction of an un- 


25 


known target, it was a good idea to hold back fire and investigate a 
little further. 

The logging and checking of Allied radar signals represented an 
important function of the countermeasures intercept equipment. 
Oftentimes intercept operators could detect troubles in radars of their 
own task force even before those troubles became apparent to the 
radar operators themselves. 

Radar intercept work in our surface ships was begun in the fall of 
1943 during attacks on the Marshall Islands. The Jap radars, kept 
under surveillance at that time, were actively taken into account 
during the next operation at Paiau. Plans were made, and equip- 
ments were ready, in case the Jap radar presented a real threat. 

Considerable radar reconnaissance preceded the Marianas oper- 
ations. An approach lane was plotted, through which our forces 
could slip without fear of detection by enemy radar. 

The preinvasion bombardments of Iwo Jima were carried out by 
cruiser task forces which relied heavily on radar intercept gear during 
‘their approach to the target. -In order to prevent being detected 
themselves, the cruisers ‘‘secured’’—that is, turned off—much of 
their low frequency radar which might conceivably be intercepted by 
the Japs. By listening to Jap shore radars, the cruisers were able to 
tell if they had been detected by the enemy. As long as the Jap 
radars swept aimlessly back and forth, our forces knew no warning 
had been given. 

During the actual landings, countermeasures intercept and jamming 
activities were coordinated by a control officer stationed in the am- 
phibious force commander’s flagship. As soon as an enemy threat 
appeared, this officer was empowered to rearrange the available 
countermeasures in order to meet it. 

The Japs were much behind the Germans in electronic develop- 
ments. However, as the Pacific campaign developed and our surface 
forces approached closer and closer to the mainland of. Japan, a new 
form of Japanese radar put in its appearance—a radar potentially 
very dangerous to our Naval task forces. This equipment consisted 
of a crude but effective air-borne search set patterned after the original 
British equipment which did yoeman service in1941. This gear was 
mounted in a number of different kinds of Jap aircraft, ranging from 
their long-range patrol ships to their torpedo bombers. 

Aided by their new radar, the Japs did, in fact, succeed in locating 
our forces with greater ease. The most common procedure consisted 
of sending out a “snooper” plane which would intermittently shadow 
our ships, taking care—out of a healthy respect for our task force air 
patrols—to come no closer than the greatest distance at which it 
could establish radar contact. If the Japanese then elected to launch 


26 


a full-scale air attack, the snooper would radio instructions to the 
attacking aircraft, thus enabling them to find the position of the 
American task force without difficulty. Few of the planes actually 
pressing home the attacks were equipped with radar, although often 
a single radar-equipped plane would lead several others in. 

The Japs never perfected the radar-aided torpedo attack as did the 
American Naval forces with the aid of their superior equipment. 
However, they were able by means of radar to bring their planes into 
fairly close range from which point the attacks could be carried on 
visually. Flares were often used at night. 
~ By November 1944, when the Jap air attacks began to assume some 
importance after the introduction of suicide tactics, the United States 
Navy was well equipped with radar search and intercept equipment, 
much of which had been developed jointly by the NDRC and by 
Service laboratories earlier in the war. 

In general, radar intercept equipment was manned on a 24-hour-a- 
day basis when there was fear of Jap air attack. The particular fre- 
quency ranges in which the Jap sets were known to operate were 
closely scrutinized. 

The radar search equipment would usually intercept the signals 
of an approaching Jap torpedo plane as much as half an hour before 
that plane itself was able to detect the presence of the American forces. 
Moreover, the signals would also be heard many minutes before the 
powerful United States Fleet radars could pick up the approaching 
enemy. Often the first warning our Naval forces had of the approach 
of hostile aircraft was the tell-tale indication picked up by the counter- 
measures operator. 

By listening to these signals, a skilled operator could almost read 
the enemy pilot’s mind. Wearing a pair of earphones, or watching a 
scope much like a radar’s, this man would spend his time tuning his 
receiver back and forth on the lookout for new transmissions amid the 
welter or familiar radio signals from ships and planes of his own task 
force. There was no mistaking the sound of an enemy radar signal: 
that high-pointed whine meant only one thing: Jap air attack. At 
first, these tones would be weak and wavery: that meant the Jap 
was still searching from side to side looking for our task force; if 
the signal grew strong and steady, that meant look out for trouble— 
he’s found his target. 

Often it was possible, on the basis of countermeasures information 
alone, to send out a night fighter in the proper direction in advance 
so that the final interception by means of radar could be speeded up. 
The best trick of all was to shoot the enemy down before he had 
located the task force. This could be done because our ship radars 
outranged the snoopers. It was not easy to do, but the effect on 


27 


Jap morale was very satisfying. The extra warning provided by the 
countermeasures gear was invaluable. 

Countermeasures operators not only could tell that enemy planes 
were approaching, but also could give the direction from which to 
expect the attack. This information enabled interceptors to find 
their targets sooner, and also helped the ships’ radars to pick up the 
enemy more quickly by telling them the direction in which to look. 

As the Pacific war approached a climax, the tactics of the United 
States Naval task forces grew more and more daring. On many occa- 
sions our carriers cruised for several days in the vicinity of Japanese- 
held territory, and no longer depended on their speed to keep them 
out of danger. During that period a serious threat to our operations 
was the Jap night torpedo bomber, especially the kamikaze version. 
It was important to deny them the benefit of radar. 

When these radar-aided attacks first became serious, it was found 
that the frequency used by the Japs could not be reached by the 
tubes used in existing shipboard jammers. Representatives of the 
Navy turned to an NDRC industrial laboratory in October 1944 
with an urgent request for a tube that would do the job. Fifty were 
needed, and 1 week was the time limit. 4 

Experiments performed in the laboratories that very day showed 
that the design of the tubes previously used could be modified to. 
accomplish the purpose. Instructions were given to the manufactur- 
ing department, and the tubes were delivered in time. Not long 
afterward, as a result of remarkable teamwork all down the line, the 
new tubes were in successful operation. 

When jammed, the Jap radar-equipped planes reacted very defi- 
nitely. An approaching enemy bomber, guided by radar, could be 
tracked as it came in by our ships’ own radars. When the Jap came 
close enough to show clearly which ship he had chosen for a victim, 
the countermeasures operators on that ship would turn on their 
jammer. Robbed of his radar, the Jap pilot usually gave up and 
went home. Operators on our ships, watching the enemy bomber on 
their radar scopes, had the satisfaction of seeing the enemy waver and 
finally turn back. 

Many grateful skippers authorized their countermeasures operators 
to paint a Jap flag on their jamming transmitters after each successful 
action. 


5 (g). Superforts and ‘Porcupines” 

By THE TIME our Pacific Air Forces had begun to mount relatively 
heavy attacks against the Japanese home islands, the Jap air defense 
system included reasonable effective searchlight control and antiair- 
craft fire control radars. One of these radars was based on an early 


28 


United States set captured in the Philippines; another on British 
equipment captured at Singapore. Both sets had already been en- 
countered in Formosa and in the Manila area. In both these places 
they had, in fact, already been jammed. Sample fire control radars 
had even becn captured and shipped to the United States for analysis. 

However, before the B-29’s began their raids on heavily defended 
targets in Japan itself, few opportunities had presented themselves 
for investigating the antiaircraft defense systems of these objectives. 
It was felt that the best Jap equipment would probably be hoarded for 
a last-ditch fight. The number of these sets, their deployment, and 
their method of use against day and night attacks by America’s newest 
and most effective bombardment aircraft, were unknown quantities. 

One of the better ways to get this type of information is to carry 
radar intercept equipment in the aircraft participating in the strikes. 
The Jap radars had always displayed a considerable degree of nervous- 
ness when plotting single planes; they showed an understandable 
but irritating tendency not to turn on all their available equipment 
until a full scale attack was in progress. 

In anticipation of this strategy, provision had been made for the 
optional installation of intercept and jamming equipment in all 
production Superforts. Space was reserved and power facilities 
provided in each plane. In actual practice, two or three planes 
of a group scheduled to take part in a mission would be selected to 
carry radar intercept receivers operated by AAF countermeasures 
officers. . 

During the course of their strikes over Japan, the B-—29’s soon 
began to encounter uncomfortably accurate flak through dense clouds, 
and effective enemy searchlight control at night. The counter- 
measures search planes, by correlating signals heard to accurate Jap 
fire, showed plainly that radar was responsible for the Jap blind fire 
control. Moreover antiaircraft sets in considerable numbers were 
discovered. As a clearer picture was gained of their employment by 
the enemy, plans for appropriate counteraction were made. 

Early in April 1945 when the Mariannas-based B-29’s were flying 
500-plane raids, a full-scale jamming program was undertaken. By 
the time the war ended, every Superfort carried at least 1 jammer, 
some carried 2, and operational plans called for the installation of 2 
jammers in every B—29 delivered to the Pacific theater. Simul- 
taneously, the B-—29’s started the large scale use of a new form of 
Window, called ‘‘Rope.”’ Instead of many short strips of foil, Rope 
consists of 400-foot aluminum foil ribbons suspended from parachutes. 
This countermeasure proved to be especially effective against low 
frequency radar like that employed by the Japs. The protection 
provided was considered so important that. ultimately nearly 600 


29 


pounds of aluminum foil in the form of rope were to be carried in every 
B-29 on every mission. 

On daylight raids, the B—29’s flew in close formation, for mutual 
protection and better bombing accuracy. Under these conditions, a 
single jammer in one plane can protect all the other planes in the group 
against an enemy radar. If each plane carried a jammer, there would 
always be a safe margin of jammers over enemy radars within range of 
the formation. 

At night, the problem was entirely different. The B—29’s flew in 
very long, loose formations, with approximately a mile between 
aircraft. Under those conditions, each plane could only hope to pro- 
tect its immediate neighbors. While it was theoretically possible for 
every bomber to carry enough jammers to screen itself from all the 
radars which might be encountered, the weight of equipment required 
made this arrangement uneconomical. 

It was decided instead to fit out a few special jamming aircraft 
whose sole job would be to cruise around in the target area and jam 
any radars which might come on the air during an attack. This was 
technically possible because the Jap low frequency radars had broad 
beams which could be jammed from any point in a wide area. 

These B-—29’s, which carried in place of a bomb load as many as 18 
jammers, as well as the necessary receivers and operators, were nick- 
named ‘‘Porcupines”’ because of the many spinelike antennas pro- 
jecting from their fuselages. Their task, during attacks, was to fly 
over the target area along a course parallel to the bomber stream and 
at an altitude somewhat above it. In this manner the Porcupines 
stayed out of harm’s way while protecting the main bomber force. 
Again, it was a battle of wits between the men in the jamming 
planes and the radar operators on the ground below. The Japs lost. 


5 (h). How To Heckle the Japs 


ALTHOUGH ouvR high-altitude strategic bombers in the Pacific played 
a very important part in the campaign against the Japanese, much of 
the pin-point bombing was carried out at medium altitude by carrier 
planes of the United States Navy. During invasions, particularly 
those at Iwo Jima and Okinawa, it was necessary for our carriers to 
come in close to shore for extended periods in order to give our beach- 
heads continuous support. These valuable ships presented all too 
vulnerable targets to Jap land-based planes. To protect them, as 
well as the invasion forces, all Jap air fields within range had to be kept 
under continuous attack. This was done both day and night by 
planes based on our carriers. 

The procedure usually consisted of sending out two or three air- 
craft—normally two fighters and a torpedo plane—to each airfield. 


30 


These planes would then maintain a continuous patrol, and if any 
signs of activity developed on the ground below, one or more of them 
would go down and shoot things up a bit. However, the Japs, aware 
of the importance of their airfields, had not failed to concentrate anti- 
aircraft defenses in their vicinity. At night, our hecklers encountered 
accurate searchlight control and antiaircraft fire. 

To cut down this particular hazard, encountered in the course of an 
all too hazardous job, our carrier hecklers were given radar counter- 
measures protection. Jamming transmitters were installed in each 
torpedo plane and its radio operator doubled in countermeasures. 
The Pacific variety of Window known as ‘‘Rope”’ was also provided in 
considerable quantity. 

This improved the night hecklers’ sport. Not only could they shoot 
up all sign of life below, but they could now jam the Jap radars off 
the air as well. On many occasions when the jamniing came on, the 
Japs actually shut down their equipment, presumably to find out 
what was the matter with it. They thought it curious that so many 
of their honorable radars should give trouble at once. 

The Rope was equally successful. It is an unpleasant feeling— 
and unhealthy—when flying along at night, to be caught without 
warning in the glare of a number of searchlights. When a plane is 
properly ‘“‘coned”’ it is virtually impossible to shake off the lights 
unless there is a cloud nearby to hide in. Our Navy airmen on these 
occasions, put their planes into a dive, tossed out some of the magic 
bundles of Rope, and had the satisfaction of seeing the Jap radar- 
direct-searchlights follow the Rope, which was soon left far behind. 

The night ‘‘hecklers’’ were not the only carrier aircraft to receive 
radar countermeasures protection. During raids over heavily defended 
targets—such as those in Hongkong, Formosa, and Japan itself— 
Naval aircraft were almost universally supplied with Window or 
Rope, and the larger planes also carried jammers. In some instances, 
automatic Window dispensing machines were installed in fighters in 
order to relieve the already overburdened pilot from the chore of 
throwing out packets by hand. 


5. (i). Ferrets and Black Cats 


By REASON of the nature and geography of i war in the Pacific, 
radar might have been an extremely effective defensive weapon fan 
the Japanese. That it was not, is due to two reasons—first the slow 
development of Jap radar, and second the timely application of 
radar countermeasures by the Allies. 

Because, in the island warfare of the Pacific, our planes had to 
fly long missions—many of them as long as 18 hours—the extra gas 
required meant fewer bombs. It also meant fewer evasive detours 


31 


from the straight line that is the shortest distance between home 
base and the target. A few radar sets, strategically located on small 
Jap-controlled islands, could serve to alert the enemy and assure our 
bombers a hot reception. 

Moreover, our bombers in this theater flew until very late in the 
war, without benefit of fighter protection. There was no attitude of 
“let ’em come up so we can shoot ’em down”’ as in Europe. 

Much of the South Pacific war was an antishipping war—a struggle 
to cut the Jap supply lines. An important part was the mining of 
harbors to rob the Japs of safe anchorage—a hazardous task which 
was largely done by planes at night. 

Because of the long missions over water, air-sea rescue work had 
to be assigned a high priority. Many daring rescues were carried 
out by patrol planes, also operating at night. 

All these activites could be seriously hampered if the Japs were 
allowed to have full use of their radar. The importance of counter- 
measures was emphasized. 

Army B--24 ‘Ferret’? planes, Navy day and night patrol aircraft, 
and Fleet submarines were given the assignment of locating enemy 
radars. (Navy Catalina Flying boats were nicknamed ‘Black Cats’’ 
because of their black paint and nocturnal habits.) These mobile 
listening posts spotted and pin-pointed Nipponese air warning sets 
scattered all the way from the Solomons to the China coast and sited 
at virtually every strong point. 

Maps showing the probable coverage of these enemy equipments 
were drawn up and supplied to the operating forces. This meant 
that aircraft missions could be planned to avoid detection by radar 
(when a choice of courses was available). For example, the night 
mining of the heavily-defended Balikpapan harbor was carried out 
with the aid of these maps by aircraft based in Australia. In the 
event that the course could not be changed, and only one or two radars 
presented a threat, they would be put out of action by bombing and 
strafing planes known as ‘radar busters.’”’ Some of these planes 
themselves were fitted with countermeasures equipment which enabled 
them to “home’’ on radar signals, thus using the enemy radars as 
navigational beacons. 

In some cases more drastic action was necessary. <A few days be- 
fore the Leyte landing in October 1944, one of the Ferret planes dis- 
covered a new Jap radar which proved to be located on Suluan 
Island, at the mouth of the Leyte Gulf.. This set commanded the 
approaches to the very strip of Leyte coastline on which we were to 
land, and it was absolutely essential that the Japs be deprived of,its 
use. This unfortunate radar and its crew were eliminated by United 
States Rangers in a neatly executed Commando raid. As an inci- 


32 


dental, several other radars on Mindanao were also demolished in 
order to prevent the Japs from inferring that we had our eye on Leyte 
Gulf. 

When enemy radars were too numerous to be ‘“‘busted,”’ the Ferrets 
and Cats were called upon to do some jamming in addition to their 
normal duties. In one case a Cat accompanied a group of low-flying 
mine-laying planes into Manila Bay on an operation considered certain 
suicide for all. The judicious use of jamming transmitters, coupled 
with the dropping of ‘‘Rope,”’ so confused the Jap radar defenses that 
many of the stations went off the air, guns were fired wildly, and the 
attacking planes were able to complete their mission without a scratch. 

Radar search receivers were installed on our submarines as soon as 
satisfactory sets became available.. Seldom has the introduction of a 
new equipment been greeted with as much enthusiasm by the operating 
crews. Use of the new receiving gear was immediately made a part 
of each submarine’s operating plan. The skippers swore by radar 
countermeasures, and it was not long before every sub that could 
carry the new gear had received its quota, for it played an important 
part in every phase of submarine operations. 

Our subs were given the job of scouting the enemy in addition to 
their offensive mission against Jap shipping. Surfacing at night deep 
in enemy waters, our undersea craft took time to study Jap radar 
defenses, learning not only what sets the enemy had, but where he 
placed them. The information brought back was distributed to all 
fleet units, thus forewarning countermeasures operators of new enemy 
signals and frequencies. 

During the first part of the sub attack on Jap shipping, enemy 
vessels were plentiful and easily found. As operations successfully 
continued, an understandable scarcity of targets resulted. However, 
the Japs, fearful of unpleasant surprise attacks, began to equip their 
vessels with radar to give warning of United States air or surface 
attack. These radar signals, of course, were easily detected; our subs, 
attracted by the Jap transmissions, could turn off their own radars to 
silently move in for the kill. In many instances, the Japs were re- 
turned to their ancestors without knowing what had given them away. 

Jap submarines were given radar—a crude air search set not very 
good at detecting surface targets. During one classic patrol, a 
United States submarine commander located three Jap subs in suc- 
cession by means of their radar transmissions: in each case, he fol- 
lowed the signals in until they abruptly ended—silenced by United 
States torpedoes. For this remarkable performance, the United States 
skipper and his crew were given a citation in which radar counter- 
- measures were credited with a direct ‘‘assist.”’ 

When Jap aircraft were fitted with radar “eyes” with which to see 


33 


at night, our submarines were prepared. A strict watch was kept on 
the enemy wave lengths, and all signals noted. Should a wavering 
tone steady down and begin to increase in strength—a sure sign the 
enemy had detected you—our subs would leisurely switch on their 
own, radar just to check the range, and then prepare to dive. They 
had ample warning, for the enemy could be heard even before he 
could be detected by the sub’s own radar. Moreover, our subs were 
not obliged to advertise their presence by continuously operating 
their radars—they could depend on their electronic ears to avoid 
detection at night. 


5 (j). Tuba and the German Night Fighters 


ONE OF THE most ingenious countermeasures developments of the 
war was a device known as ‘Tuba’’—a tremendously powerful 
jamming transmitter developed for use against German night fighters. 
In addition to its countermeasures application, ‘‘Tuba”’ has interesting 
peacetime applications. It was certainly one of the outstanding 
individual scientific achievements of the war. 

The problem was to create a jamming device to blind the German 
night fighters, which in 1942 took a heavy toll of British night bomb- 
ers. The German fighters used an air-borne interception radar 
known as ‘‘Lichtenstein” for close-range location of their targets. 
Agéinst them the British found it impractical to use jammers carried 
in their bombers, because the jammer itself provided a signal which 
the German fighters could use to locate the bomber. A radio signal, 
including a jamming one, betrays the direction from which it comes, 
and even though a jammer might blot out a German scope, making it 
impossible to find the range, the German could find the bomber 
simply by following the signal in. 

But the German night fighters usually did not reach their prey 
until after the British bombers were flying home from their mission. — 
Someone suggested, why not set up a very high-powered jammer in 
England to blind the German fighters’ radar as they flew toward it 
in pursuit of the homeward bound bombers? It would be a blinding 
beam—‘shining”’ in the German “‘eyes’”’ (i. e., their radar antennas)— 
through which the bombers could fly to their bases in safety. 

A jammer of this sort obviously would require enormous power. It 
was calculated that it would need power a thousandfold more than 
any previously attained in the frequency range of operation involved, 
which in itself was 10 times higher than that used for frequency 
modulation and television. 

It appeared possible to solve the power problem by means of a very 
remarkable vacuum tube, developed in the United States, known as 
the “resnatron.”’ This NDRC development, sponsored by the Signal 


34 


Corps, was so promising that the British forthwith placed a lend-lease 
order in the United States for a complete jamming system based on 
its use. 

Since the experimental model used a huge parabolic antenna, the 
project was promptly nick-named ‘‘Tuba,” to distinguish it from 
smaller projects already known as ‘‘Piccolo,”’ ‘‘Flute,” and the like. 

All sorts of difficult technical problems had to be solved. It was 
necessary to build:a resnatron that would be tunable over a wide fre- 
quency range (because the Germans could change the frequency of 
their radars by slight modifications). This had been thought incom- 
patible with high power, but a tunable resnatron was produced in one 
of the industrial laboratories. Also it was necessary to find a pay to 
modulate the resnatron’s output with the random ‘“‘noise’’ necessary 
for jamming, but this too was accomplished and ie January 1944 a 
workable instruinent had passed its test. 

Its power was comparable with that of the most powerful United 
States broadcasting station (50,000 watts), yet the frequency of oper- 
ation was 500 times as high. The whole instrument, together with all 
its associated equipment and its primary power generator, was loaded 
in seven Army trucks. 

By June 1944, the complete jamming system had been shipped 
to England and was in operation against the enemy—a remarkable 
achievement when one considers that the equipment was ‘still in the 
blueprint stage when work began early in 1943, NORC scientists were 
flown to Great Britain to help set up the equipment and train the RAF 
in its use. 

It is now known that in June 1944, the Germans changed over to 
an entirely different type of night-fighter radar. If there was any 
doubt in their minds as to the desirability of making the shift, the 
first blast from Tuba must have convinced them. The British were 
profoundly satisfied with the project, and ordered two more equip- 
ments in addition to the first. 

The power output developed by Tuba is of such unforeseen magni- 
tude that our planning for frequency channel allocations in the ultra- 
high-frequency range will be directly affected. Tuba, by raising the 
sights on what is possible, has made existing thinking obsolete, and 
will tremendously advance the development of ultra-high-frequency 
broadcasting. 


6. Epilogue 
Special Problems Encountered 
To GIvE a better understanding of what is involved in a counter- 


measures development, let us trace a particular device from its incep- 
tion to its final operational use. 


35 


First, it is necessary to find out not only that the enemy has a new 
electronic equipment, but also its exact nature. Based on this, engi- 
neers propose a possible countermeasure. This device must then be 
built in the laboratory and its feasibility tested against some equip- 
ment which is a reasonable facsimile of the enemy gear. If the equip- 
ment passes these tests, it is then necessary to consider its use in relation 
not only to operations currently being carried out in the theaters of 
war, but to those operations which will be in progress by the time the 
new countermeasure can be manufactured and introduced on a wide 
scale. , 

Once the decision is made to manufacture the device, a manufac- 
turer 18 brought in, an order placed, a model built. This model is 
tested by the Army or Navy to determine its ruggedness and suita- 
bility; regular production is then begun. The equipments, together 
with all necessary accessories, must beshipped to an operating theater; 
the forces must be trained in its use, and operational plans or tactics 
devised or modified accordingly. Finally, the results obtained by 
the use of the countermeasure must be assessed by operational research 
of various kinds. 

Countermeasures research and development was carried out in 
Navy, Army, and NDRC laboratories. These laboratories were sup- 
plied with information on the latest enemy electronic developments 
through Service intelligence channels. In many cases, entire enemy 
equipments were picked up by field intelligence teams and shipped to 
United States Service laboratories where Ahoy were pieced together — 
and placed in operating condition. 

The information thus gained on the performance and vulnerability 
of these equipments was often invaluable. 

As an example, a Jap air-borne radar captured at Hollandia, New 
Guinea, was shipped to a Navy laboratory in the States, reconditioned, 
mounted in a United States airplane, and returned to the Fleet at 
Pearl Harbor, arriving there in time to participate in a full-scale 
rehearsal of the Iwo Jima landing operation. Shipboard counter- 
measures operators thus got the best possible training—they learned 
in advance not only the characteristics of the enemy radar signal, but 
also the details of the performance of the radar itself. 

The Army Air Forces maintained a field proving ground in the 
United States at which captured enemy radars were set up and their 
weak points determined. One of the German antiaircraft fire-control 
radars, reconditioned by Signal Corps experts, was set up at this radio 
research proving ground. ‘The latest in countermeasures was tried out 
against the latest in enemy equipment. 

In the field of radar countermeasures, the Services have been aided 
in various ways by NDRC assistance. This has not been confined to 


36 


laboratory research and development along; special arrangements have 
been made to assure manufacturers a maximum of service from the 
developing agency in order to keep the time required for bringing a new 
device into production at a minimum. After the production equip- 
ment arrives in the field, the problem of fitting it in planes and ships in 
the available time between operations and other duties is not easily 
solved. Training of operating and maintenance personnel must also 
be carried out with dispatch. To help in these phases of countermeas- 
ures work, civilian engineers have been sent to almost everyone of the 
fighting fronts. 


As countermeasures equipment was perfected it was necessary also 
to develop a full line of new electron tubes, of which the resnatron, 
mentioned in the previous section, was only one. Although very short 
radio waves, of high frequency, had been produced in the early stages 
of the radio art, most of the work with high power had been done on 
much longer waves. Because of the fact that the short ones can more 
easily be focused into a beam they had been applied more and more in 
radar and consequently, countermeasures equipment had to operate 
with waves of the same size. Many new tubes had been devised for 
radar, but these worked by pulsing—that is, in a short burst of about 
a millionth of a second, followed by a much longer rest period. Tubes 
for jamming had to produce their power continuously, so the require- 
ments were much more severe. This work, the designing of such tubes, 
was performed mainly in industrial laboratories. Much of it was 
carried out in advance of actual Service requirements. 


One of the most important tubes in this work was the magnetron. 
Before the NDRC research program was started, however, there were 
no magnetrons in existence which could supply the necessary power at 
the required frequencies. At the end of the war, as a result of indus- 
trial research for the NDRC, there was a complete line of magnetrons 
of high power covering a wide range of frequencies. 

Although radar was one of the most important electronic weapons 
of the war, there were others that made use of radio and electronic 
principles, and which were also susceptible to countermeasures. 


Postwar Possibilities 

THERE HAS been a steady trend, in past radio development work, 
in the direction of higher and higher frequencies. First came broad- 
cast radio, then short-wave radio and finally FM. This trend has 
been enormously accelerated during the war by the need for radar 
and allied devices. However, much of the radar technique—such 
as the pulse-echo method itself—is highly specialized. Counter- 
measures development work, on the other hand, has been concerned 


37 


with continuous-wave techniques similar to those used in ordinary 
radio communication. 

The new developments embodied in the design of countermeasures 
equipment represent the very developments which will be needed to 
solve many postwar problems and to make possible many postwar 
developments. 

Basically, much countermeasures research was directed toward 
the improving and developing of radio transmitters and receivers of a 
type very similar to those which will be used in postwar FM, tele- 
vision, and radio relay transmission. 


U. S. GOVERNMENT PRINTING OFFICE : O—1951 


For sale by the Superintendent of Documents, U. S. Government Printing Office 
Washington 25, D.C. - Price 20 cents 


38 





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