A British Chain Home Airborne Radar radar station with

three metal Radar is one of the major AESS disciplines and has been transmitting towers a major focus of papers and articles in this magazine. Our 360 feet high; and Jubilee issue (October 2000) included an excellent

four wooden receiving summary of the progress in ground radars in the US. In towers 240 feet high addition we have had a number of articles about radar

development in other countries. There are a number of excellent full-length books that cover radar history as a whole or in part, (see Bibliography). As this issue focuses on the Wright Brothers and aviation, this section emphasizes the evolution of airborne radar systems through WW 11.

With another World War looming in the 1930s, the British concluded that an early waming system to detect attacking aircraft was absolutely vital. Development of the critical British ground-based chain home (CH) and chain home low (CHL) radars began in 1935 to provide a warning of daytime bombing and allow the scrambling of Royal Air Force (RAF) fighters in time to visually find and intercept the attackers. A switch from daytime to night bombing was also anticipated and flagged the need for aircraft that could see in the dark. By 1937, the British were facing two radar challenges: attacker location by CH or CHL followed by visual acquisition during the day; and night-time interception using airbome radar. However the long CH wavelengths (- 10 m) precluded installing similar equipment in aircraft and dictated the development of high frequency, shorter wavelength, airborne radar.

IEEE AES Systems Magazine. July 2003 43

German WW II Wurzburg at the Duxford (UK) Air Museum

' In 1937, the British had developed a triode-based airborne set that radiated 100 watts at a 1.5-meter wavelength. With that wavelength the antenna design had to use separate dipoles for transmission and reception. Reflection limited acquisition range to distances equal to

, that to the ground. Receiving antennas were placed on wingtips in order to balance return amplitudes and establish the target direction. In 1939 the shortest

: wavelength airborne radars being developed in Britain, Germany, and America were about 50 cm. A promising microwave transmitter at the time appeared to be the klystron, an early cavity resonator. However, it only generated a few microwatts of power. The only wartime

: IO-cm klystron with substantial output was designed at the University of Birmingham (England) and never became

~ practical.

Airborne Intercept (AI) Radar

The first operational British radar, designated Airborne Interception (AI) Mark II, operated at 1.5 meters. Its wide transmitter pulse prevented the operator from seeing targets closer than 1000 feet and made night acquisition difficult. Even with eyes adapted to the darkness a fighter pilot had to be directed to within a couple of hundred meters before the target became visible. In July of 1940, the AI radar made its one and only kill during the Battle of

44 lEEE AES Sysiems Magazine, July 2003

Britain. The second success was during the Blitz the following November. In spring 1941 ground controllers had begun to use ground control intercept (GCI) radar with plan position indicators (PPIs).

As a result they could now see enemy bomber formations more clearly and direct RAF fighters to them. The AI Mark IV was the first successful British night fighter radar. Improvements in the Mark IV brought minimum range down to.100 yards and boosted power to 10 KW to provide detection at 3-4 miles. Using this radar, heavily armed Bristol Beaufighters destroyed 52 enemy attackers in April 1941 and over one hundred in May. In June, the attacks wound down as Hitler turned his attention to the invasion of Russia. A year later, as German night raids over Britain subsided, the RAF had 25 night fighter squadrons. As the air war subsided, radar development became focused on the submarine menace.

Identification Friend or Foe (IFF) Radar Map of the Wash Identification Friend or Foe (IFF) was developed in the

late 1930s and was used in the Battle of Britain to distinguish between RAF fighters and German attackers. It began as a simple dipole to cause radar echoes to bloom and fluctuate. The radar operator would recognize friendly aircraft by the relative size of the blips. However because of difficulties the design evolved into an airborne transponder that responded when triggered by a CH radar signal. For proper operation, adjustment on both the ground and airborne ends of the link was required. To overcome difficulties and accommodate future centimetric radars, the system was changed to a special ground transmitter separate from CH. The Germans also had difficulties getting their military personnel to operate the IFF equipment properly. Pilots on both sides eventually came to appreciate IFF because it could also be used to direct them home. IFF has since evolved into today’s secondary radar systems (ATCBI, Mode-S) that supplement air traffic control primary radar.

Air to Surface Vessel (ASV) Radar - Sub Detection

Submarines had nearly brought Britain to her knees in WW I and a technique for locating them was badly needed. Submarines were vulnerable at night when they had to surface to charge their batteries. By spring 1941, as the air war over Britain wound down, German submarines began

Radar Map using High Resolution 3-cm Equipment

IEEE AES Systems Magazine. July 2003 45

,to sink merchant ships at the alarming rate of nearly 100 ,per month. At the time, sonar, then called ASDIC, was the only system that could detect submarines, but it suffered from short range and could not detect submarines on the surface. In addition ASDIC had been installed on very few ships. There was a distinct need for an air-to-surface vessel (ASV) radar to counter this threat.

The British ASV Mark ll (1940) at 1.5 m was one of the most important sets of the war for radar detection of surface vessels. ASV made the curious old Swordfish biplane possibly the most successful torpedo bomber of the war. (In a flyby of WW II aircraft at the 50th anniversary of the end of the war, the Swordfish had the honor of leading.) These sets used separate transmitting and receiving antennas and employed lobe switching for accurate direction. In April 1942 the high power 10 cm magnetron-based AI MK Vlll airborne radar went into service. The submarine hunter equivalent of this AI radar, the ASV MK 111, went into service in 1943. With its vastly 'greater power aircraft could fly at 5000 feet at night and detect subs on the surfact at 5-6 miles. However at short 'range, as the aircraft drew near, the submarine was lost in the sea clutter. To overcome this aircraft began to switch on a powerful searchlight from a distance of about a mile, greatly increasing kill probability. In 1940 the Tizard Mission provided the magnetron and ASV design to the US. By mid-1942, after the US entered the war, more than one hundred merchant ships had been sunk along the US East Coast. The Navy ordered 7000 of these magnetron-based sets, denoted as ASE, as eyes for their PBY Catalina flying boats.

'Radar as a Navigation Aid - H2S

In conducting early radar tests with ships scientists discovered that land could be readily distinguished from water on a PPI. ASV tests with the higher frequency Randall and Boot centimetric magnetron showed that landscape features such as rivers, rail lines, and roads, were even more clearly identified. At Churchill's direction, a new radar system called H2S (the origin of this name is still being debated) began to be installed on British bombers as a navigation aid to view the ground through clouds. However this turned out to be a good newslbad news scenario. On the plus side, H2S with the magnetron and PPI constituted a much-improved system for submarine

An external view of the actual magnetron

taken to North America by the Tizard Mission

in August 1940. This is probably the only example of the original batch of 12

still in existence; it is now on display in

the Museum of the National Research Council, Ottawa.

(1990)

46 lEEE AES Systems Magazine. July 2003

Lichtenstein radar antennas on a Junkers, JU-88.

The SCR-584

detection (ASV), but it failed to increase bombing accuracy as much as expected. In fact, Lubaffe night fighters soon learned to detect and home on these H2S transmissions and wreak havoc on the bomber formations.

German Airborne Radars

In 1944 Allied bombers were attacking the European mainland almost daily, so that the Germans began to mount a radar defense. Initially networks of FreyaMlurzburg ground radars plus searchlights illuminated the bombers for antiaircraft guns or fighter attack. Equipment improved and tactics changed from month to month. Development and deployment of the German 50-cm Lichtenstein airborne radar provided independent location of bombers, and reduced the need for searchlights. One of the reasons for introducing Window (chaff) was to blind the 50-cm Lichtenstein, however the Luftwaffe was already transitioning to longer wavelength sets that were unaffected by Window. This was because fighter pilots were asking for sets with greater range so that they could find the bombers even without ground radar. As a result a new radar, the Lichtenstein SN-2, was developed and installed in nearly all night fighters. It operated at a wavelength of 3.3 meters with a higher transmitter output power, covered a 120-degree sector, and could detect targets up to 40 miles away. The antenna consisted of four dipoles mounted on the nose fed by a rotating capacitor to provide a conical scan. By then, British bombers were flying above 20K feet to minimize losses from anti-aircraft fire. The combination of more accurate Wiirzburg tracking radars and the longer wavelength Lichtenstein ended up taking a terrible toll on RAF bombers and personnel.

Radar Altimeter

From the beginning of the war, radar was used to determine altitude. Energy reflected off the ground or sea directly below provided an absolute reading of altitude. Altitude was determined by modulating the radar carrier with a triangular wave and measuring the phase delay. This was contrasted with barometric data, where altitude is referenced to sea level. A related radar development detected returns from a multibeam Doppler system as frequency shifts, which are a measure of aircraft velocity.

IEEE AES Systems Magazine, July 2003 47

N” 18,170 A.D. 1904

Date ofAp&Ilicafion, fOth June, WO1-Accepted, 22nd Sept.. fUO4

CO3fPL#TR SPECIPICATION.

“Xertpian-wave Projecting and Bewiving Apparatus Adapted to Indicate or Give Warning of the Premnoa of a MetaUio Body, awh U a Ship or a Train, in the Line of Projeoth of ouoh Waver ’’.-

Figures 2,3, and 4 from the 1904 patent of Christian Hulsmeyer (UK number 13,170). The receiver (top) and transmitter (middle) are mounted to a compass box “c” to allow the action of gravity to maintain a vertical orientation when the ship

rolls and pitches (Figure 2). The Hertzian dipole “h” radiates from a funnel-shaped projector “I” backed by a concave reflector “m” (Figure 3).

The receiving antenna “0” backed by a semitircular reflector “n” (Figure 4). A metal partition “t” isolates the receiving antenna from the transmitter.

The entire apparatus can be rotated in azimuth.

IEEE AES Sysfams Magazine, Juk MO3