radar

radar is an acronym for Radio Detection And Ranging, now embracing a diverse group of systems which employ radio waves to detect the presence or measure the location of distant objects. There exist two essentially different types of radar system, monostatic and bistatic: the former and more common has its transmitting and receiving antennas at the same location, whereas the latter may work with the entire transmitting and receiving systems widely separated from each other. The frequencies utilized by radar sets during the Second World War ranged from 20 MHz to 10 GHz (see Chart). Pulse radars predominated. In a pulse radar energy is transmitted for brief periods of time (of the order of a millionth of a second) separated by relatively long quiescent intervals (generally greater than one-thousandth of a second), during which the echoes from targets can be received and processed.

Between the years 1934 and 1936, radar emerged independently and for military purposes in eight countries: France, Germany, the Netherlands, Italy, Japan, the UK, the USA and the USSR. In most it was first seen as a means of early warning against air attack and as an anti-aircraft gun-laying device (see Table for some of the more notable radar sets).

Three related events, which were very significant for the Allies, were the British discovery of the cavity magnetron in July 1940, which made centimetric radar possible (see electronic navigation systems and H2S); the Tizard mission to the USA two months later, when the cavity magnetron was disclosed to the Americans; and the setting up of the Radiation Laboratory at the Massachusetts Institute of Technology in November 1940. From an objective global viewpoint, the impact of the work of the Radiation Laboratory, both on the operational uses of radar during the latter half of the war and thereafter on its immediate post-war usage, can hardly be over-emphasized.

The radar programmes in the countries listed above will receive brief mention. These programmes were extensive in the case of the UK (and its Dominions), the United States, and Germany and, to a lesser extent, Japan.

France

A bistatic early-warning system using continuous waves and beat-frequency technique was proposed by Pierre David in 1934 and operationally tested in 1937. In 1939 fixed David radar chains protected naval bases along the English Channel, the Atlantic Ocean, and the Mediterranean, and also the north-eastern approach to Paris. A limited number of early-warning pulse radar equipments were manufactured in 1939 by commercial companies. Mobile early-warning and gun-laying radar sets were employed in France by the British Expeditionary Force from October 1939 to May 1940.

Germany

Radar in Germany had its origins in the foresight and determination of Dr Rudolf Kuhnold of the German Navy's Signals Research Establishment. In 1934 he set up the company GEMA, which in 1936 produced prototypes of both the Seetakt series of ship-based search and gun-ranging radars operating at 370 MHz and the Freya early-warning sets operating at 125 MHz. Radar development in Germany, while technically successful, did not follow a clear co-ordinated programme. There is no doubt that much work, particularly in microwaves, was curtailed or delayed because of Hitler's edict prohibiting research which could not guarantee early success.

Radar was used for aircraft reporting purposes as early as December 1939 with radar plots being passed directly to fighter and Flak units. The first night-bombing of Germany by the RAF took place on 15 May 1940. It was a prelude to a sustained air defence campaign of almost five years by the Germans in which searchlights, night fighters, radar—and, for a while, sound detectors—and the Luftwaffe's flak arm played their various roles. General Joseph Kammhuber and the Kammhuber Line figured prominently in this. The area bombing raids by the RAF which began in the spring of 1943 radically affected the efficiency of the Kammhuber Line and this ushered in new methods of fighter control and a re-allocation of the areas of control. This period also coincided with the widespread use of radar counter-measures as part of the electronic warfare being waged by both sides.

Italy

Following observations made in 1933, Guglielmo Marconi built a radar system which he successfully demonstrated to Mussolini and members of the general staff on 14 May 1935. The military authorities enlisted the services of Professor Ugo Tiberio who produced, practically single-handed, a series of experimental sets; the last, EC-3 ter, was completed in 1941 and went into production with the SAFAR company. There appear to have been more than 100 Italian naval radar sets available at the time of the armistice in September 1943: their potential had not, however, been realized.

Japan

Microwave and magnetron research was carried out from 1933 onwards at the Japan Radio Company and the Naval Technical Research Institute, and experiments were also conducted by Professor Okabe Kinjiro of Osaka University in 1936. The first sets, which went into operation in 1941, were continuous wave, as opposed to pulse, bistatic systems operating in the 40–80 MHz band. Serious work on pulse radars began in 1941. A complete lack of liaison between the army and navy led to great inefficiency and duplication of effort (see rivalries). The navy, from 1941 until the end of hostilities, carried out an extensive programme of design and manufacture of successful land-based, shipboard, and airborne metric and centimetric wave sets. One cannot say that the Japanese radar programme was not good: it just could not equal that of the USA and UK.

Netherlands

In 1936, work on the design of a radar began independently in the Philips Physics Laboratory at Eindhoven and in the physics laboratory of the Dutch armed forces. A few prototypes were built. One of these was operating in The Hague, when the Germans invaded in May 1940.

UK

The unease felt by A. P. Rowe in 1934, when, as assistant to the director of scientific research at the air ministry, he examined the state of Britain's preparedness against air attack, set a chain of events in motion. By 31 May 1935 a basic radar transmitter and receiver, at that time called Radio Direction Finding (RDF), a name the British retained until the middle of the war, were operating at Orford Ness on the east coast on a frequency of 6 MHz. In August and September 1935 the early-warning chain of radar stations which became known as the Chain Home (CH) was planned. In the organization of all this, Robert Watson-Watt played a leading part. At the outbreak of war, on 3 September 1939, eighteen CH stations were operational and connected to the Stanmore filter room in north London; two more in Scotland were operating locally. This result was achieved after much effort and the benefit of air exercises. The effectiveness of the CH system throughout the war, but especially during the battle of Britain, lay not only in the performance and reliability of the radars but in the efficiency of the personnel in the radar stations, the filter rooms (where information on a raid was evaluated before being passed to the operations room), and the operations rooms.

From 1940 to 1943, the Chain Home coverage was extended to the south-west and west coasts and Northern Ireland. Its capability was added to by the employment of 200 MHz CHL (Chain Home Low) and 3 GHz CHEL (Chain Home Extra Low) stations which were effective against low-flying aircraft: the latter also reported the movements of shipping.

The setting up of the CH network was purposely given priority of resources. Nevertheless, even as early as 1935, most of the later radar developments were given consideration. In 1937, Dr E. G. Bowen carried out successful AI (aircraft interception) tests using a radar fitted in a Heyford bomber and a co-operating transmitter on the ground. This work eventually led to the development of a number of successful ASV (aircraft to surface vessel) and AI radars.

Naval set development began in 1935 and by 1939 an air-warning set, Type 79, operating at 43MHz, was fitted to some of the larger vessels. The fitting of microwave radar to escort vessels in 1942 was an important factor in the war against the U-boats during the battle of the Atlantic. Serious work on army radars began at the end of 1936. Before the end of the war some 80 types of sets and their variants had been designed.

Radar: Some notable radars

Function	Type/Name	Frequency	Tx Peak Power	Pulse Width	PRF (Hz)	Range km	Range miles
a Same as H2S Mk1 (see electronic navigation systems).
b Although only about 50 aircraft were fitted with this set, it is of great historical interest as it was the first ever centimetre-wave airborne radar tobecome operational: it first flew in a Blenheim on 10 March 1941.
Source: Contributor.
1. Germany
Early warning	Freya	125 MHz	20 kW	2-3 µs	500	200 km	125 mi
Gun Laying;	Würzburg	560 MHz	8 kW	2 µs	3,750	30 km	18.5 mi
Ground Controlof Interception
Sea Search	Seetakt	368 MHz	8 kW	5 µs	500	25 km	15.5 mi
	FUMO22
Aircraft	Lichtenstein SN2	91 MHz	2.5 kW	1 µs	937	4 km	2.5 mi
Interception	FuG 220
2. Italy
Naval:	EC-3 ter	500 MHz	10 kW	4 µs	500	25 km (on	15.5 mi
Sea Search	(‘Gufo’)					battleship)
3. Japan
Early warning (land and naval)	Type 13	150 MHz	10 kW	10 µs	500	100 km	62 mi
Surface Search	Type 22	3 GHz	2 kW	10 µs	2,500	25 km	15.5 mi
4. UK
Early warning	Chain Home	22.7–29.7 MHz	200–800 kW	6 µs–25 µs	25;	300 km	185 mi
	A.M.E.S. Type 1	42.5–50.5 MHz			12 1/2
GCI (Ground Control	A.M.E.S. Type 15	209 MHz	100 kW	3 µs; 5 µs;	300–540	180 km	112 mi
of Interception)				8 µs
Gun Laying	G.L.11	54–85 MHz	80 kW	2 µs	1,200	45 km	28 mi
Naval:	Type 281	90 MHz	350 kW	15 µs	50	220 km	136 mi
Air Search			1 MW	2 µs
Naval:	Type 271	3 GHz	70 kW	1.5 µs;	500	25 km (on	15.5 mi
Sea Search				0.7 µs		battleship)
Air to Surface	ASV Mk11	176 MHz	7 kW	2.5 µs	400	Coastlines	62 mi
Vessel						100 km
	Destroyer	22 mi
	35 km
Air to Surface Vessel	ASV Mk111a	3 GHz	50 kW	1 µs	750	160 km	99 mi
Air Interception	AI MkVIIb	3.3 GHz	5 kW	1 µs	2,500	5 km	3 mi
5. USA
Naval: Air Search	CXAM	195 MHz	15 kW	3 µs	1,640	130 km	80 mi
Precise tracking of aircraft	SCR–268	205 MHz	75 kW	3 µs–9 µs	4,098	36 km	22 mi
Early warning	SCR–270	106 MHz	100 kW	10 µs–25 µs	621	230 km	142 mi
Early warning	EW (Microwave	3 GHz	1 MW	1 µs	320	380 km	236 mi
and fighter direction	Early Warning)AN/CPS–1
Gun Laying (Automatic Tracking)	SCR–584	3 GHz	300 kW	0.8 µs	1,700	Tracking from 29 km	18 mi
6. USSR
Early warning	RUS–2	75 MHz	70–120 kW			150 km	93 mi

Before the outbreak of war, a radar programme for the air defence of British overseas ports existed. Part of this programme was implemented in the early days of the war. Radar was employed effectively in early-warning and in fighter interception in the various defensive and offensive campaigns in the Middle East, in the Far East, in the North African and Italian campaigns, and in north-west Europe. The period June 1944 to March 1945 saw the bombardment of London and the south-east coast by V-weapons and radar played a very considerable part in their destruction. See also scientists at war.

USA

The development of radar in the USA from its origins to the end of the war can be viewed in two stages. It was born in the USA in the Naval Research Laboratory from observations made in June 1930 by Leo Young and Laurence Pat Hyland which eventually led in 1934 to Robert Page's building of a 60 MHz pulse radar set. A development of this, the CXAM, became available in November 1939. Twenty sets were installed on battleships, aircraft carriers, and cruisers in 1940.

Major William Blair, the director of the Signal Corps Laboratories at Fort Monmouth, New Jersey, promoted radar experiments from 1933 onwards. A simple pulse radar was demonstrated in December 1936. By May 1937, a prototype of the first US Army radar, the SCR-268, was built. A long-range radar, operating at 106 MHz, the mobile SCR 270 and its fixed counterpart the SCR-271, went into service in 1940. About 800 were produced between 1939 and 1944. By early 1942 the Aircraft Warning Service had a chain of SCR-270 and SCR-271 radars protecting the east coast from Maine to Key West and the west coast from Washington to San Diego.

At the time of the Tizard mission and the exchange of information with the UK in 1940, the USA possessed a very solid and developing radar programme though it lacked, perhaps, the urgency engendered by a country threatened by war. The second stage of development was initiated by the setting up of the Radiation Laboratory in November 1940, a direct result of the mission.

USSR

The air defence command, dissatisfied with sound-location, contracted with the Leningrad Electro-Physics Institute to undertake research into the radio detection of objects. The result was the experimental set ‘RAPID’, a bistatic continuous-wave system, which in August 1934 detected the presence of aircraft up to a distance of 75 km. (53 mi.). Acute political changes occurred during 1937–8 which undoubtedly affected the radar programme, but from February 1942 onwards the state defence committee actively promoted radar. By the end of the war successful naval surface-search and airborne AI sets were being produced.

Radar as a weapon

Radar systems enhanced the capability of air, land, and sea forces in both defensive and offensive roles. The element of surprise in attack became more difficult to achieve. In some operations or battles the use of radar or a particular type of radar was of decisive importance.

The use of the Chain Home in the battle of Britain was an essential factor in the RAF's victory. It was also the first time in history that early warning and control were used in an air battle. In the assault phase of the Normandy landings (see OVERLORD), the chain again played an essential part: its radar coverage was extended by three specially equipped fighter director tenders, which finally hove to off the coast, and by the GCI (ground control of interception) and other sets which were landed on the bridgehead. The landings, involving accurate blind-bombing and naval bombardment of shore targets, and the expeditious movement of a huge air and sea armada, would have been impossible without a heavy involvement of radar.

The Allied strategic air offensive against Germany went through many phases. Overall, it was by no means a one-sided victory for RAF Bomber Command and the Eighth US Army Air Force. The German radar early-warning system was very effective so that the defences were hardly ever surprised. In the biggest night air battle of the war, on 30 March, 1944, a force of 782 Halifaxes and Lancasters, carrying out a raid on Nuremberg, suffered 13.6% losses due in large part to the effectiveness of the German night fighters' Lichtenstein SN2 AI sets.

The Pacific war, which lasted almost four years, was essentially a naval war. American submarines and radar-equipped aircraft inflicted heavy losses on Japanese merchant shipping and tankers. The balance of naval battles, in both defensive and offensive phases of the campaign, was determined largely by the effectiveness of the American Task Forces' radar directed fighters. The ‘Great Marianas Turkey Shoot’ of 19 June 1944, during the battle of the Philippine Sea, when nearly 300 Japanese aircraft were destroyed for a loss of 30 American aircraft, was an example of the potency of well-organized radar fighter-control.

One instance where the usage of radar was critical for the Allies and in which the outcome of battle was vital to the whole conduct of the war was the battle of the Atlantic. The submarine war on Allied merchant shipping lasted from the outbreak of hostilities until the defeat of Germany. Losses such as those of June 1942 when 141 ships were sunk, could not have been sustained for long. Many factors apart from radar, including code-breaking (see ULTRA, 1), ship-building potential, anti-submarine weapon development, and convoy procedures, denied ultimate victory to the U-boats. The use of radar, particularly the British naval Type 271 microwave set used in escort vessels, proved very successful. At the end of 1940, the ASV MkII was fitted to a variety of aircraft including Wellingtons, Whitleys, Sunderlands, and Catalinas. Later, American long-range aircraft, such as the Liberator, fitted with radar were used in the western Atlantic. At night, the combination of ASV radar which could pick up a surfaced submarine and the Leigh Light (see searchlights) which could illuminate it as the aircraft made its final run in before dropping depth charges, proved a deadly weapon. The 200 MHz ASV MkII lost much of its potency due to the Germans' introduction of listening receivers on submarines. However, the advent in March 1943 of the 3GHz ASV MkIII set, which could not be detected, heralded the defeat of the submarines.

The graph starkly and simply illustrates the flow of the U-boat war, focusing on the ratio of Allied vessels sunk to U-boats lost. When the USA entered the war the U-boats moved into American waters and for many months convoy losses rose dangerously high: the peak in the curve for February 1942 illustrates this.

Sean Swords

Bibliography

Guerlac, H. E. , Radar in World War II (New York, 1987).
Burns, R. W. (ed.), Radar Development to 1945 (London, 1988).