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[6.0] Microwave Radar At War (3)

v2.0.5 / chapter 6 of 12 / 01 feb 13 / greg goebel / public domain

* The momentum of discoveries in the field of radar technology did not slow down in the last years of the war. Further improved systems were introduced, many of which would see little or no combat, but which would prove their value in the postwar period.

TBM-3W Avenger AEW


[6.1] SCR-584 REVISITED
[6.2] AN/APQ-7 EAGLE / AN/CPS-1 MEW
[6.3] PROJECT CADILLAC: AN/APS-20
[6.4] THE END OF THE BEGINNING: CHAIN HOME & BIG BEN

[6.1] SCR-584 REVISITED

* The SCR-584 was an outstanding piece of equipment, and it not only led to other radars, but opened the door to other applications for radar. Even before the SCR-584 was fielded, the US Navy was interested in obtaining a version of the set. The Navy's CXAM was a fine early-warning set, but it couldn't track an aircraft accurately in three dimensions. The SCR-584 could, so the Navy decided to mount it on their warships and send it to sea.

The prototype of the ocean-going SCR-584, the "CXBL", was mounted on the new carrier USS LEXINGTON on March 1943, while the production version, the "SM" -- built by General Electric and mentioned earlier -- was operational on the carriers USS BUNKER HILL and USS ENTERPRISE by October 1943. The fast schedule was possible because the M-9 director wasn't required for the application, though the shipboard installation was complicated by the need to gyrostabilize the antenna.

This complication could be dealt with by sheer brute force, and the SM ended up weighing 8.2 tonnes (9 tons), the weight also being increased by the need to use a heavy-duty dish that could stand up to sea storms. Mounting such a mass high up on a ship tended to make the vessel top-heavy, and so SM was only used on the big carriers. The Rad Lab put their minds to it and managed to develop the "SP", which only weighed half as much and so was installed on smaller vessels, beginning in late 1944.

The Navy also introduced a number of small, short-range gun-laying radars for lighter anti-aircraft gun installations, particularly the quadruple 40 millimeter Bofors gun mount, that used a dish antenna with conical scanning. The first was the "FJ" or "Mark 9", and it was followed by the "FL" or "Mark 10".

* The SCR-584 was compact and capable, and followed the troops as they moved forward. One of the problems with front-line combat was directing strike aircraft to accurately hit the enemy along the battle lines, which was particularly troublesome because there was a good chance that they might hit friendly forces instead. SCR-584s were used to locate Allied strike aircraft and give them position corrections so they would know where to pounce. The troops also used them to observe movements of enemy ground vehicles at night or in foul weather, with artillery called in on suspicious targets. One SCR-584 managed to pick up German vehicles at a distance of 26 kilometers (16 miles).

Yet another application of the SCR-584 was to track the trajectories of artillery shells. This had been used early on to adjust the ballistic tables for the US 90 millimeter gun, but it was also used in Italy to follow the trajectory of German mortar rounds for counter-battery fire.

The SCR-584 was too heavy to be used at the very front lines, which tended to shift rapidly, so lightweight air-warning sets like the AN/TPS-1 were used for the task. They couldn't track a mortar round's trajectory as well as the SCR-584, but they could obtain enough data to allow calculation of the trajectory using a custom slide rule-type calculator. The scheme was only used experimentally during the war, but led the way in the postwar period to the development of light radars specifically designed for the counter-battery task.

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[6.2] AN/APQ-7 EAGLE / AN/CPS-1 MEW

* Although work on the Rad Lab's Eagle radar had preceded work on H2X, Eagle did not reach operational status until long after H2X was in production. Eagle was a remarkable invention, but it was a new approach and took major effort to get to work.

Luis Alvarez came up the idea for Eagle after a chat with Taffy Bowen in November 1941 on the need for accurate targeting radars. Alvarez wanted to build a 6 meter (20 foot) long linear array of 250 transmitting dipole antennas into the leading edge of the wing of an aircraft. If the antennas were properly arranged and controlled, their emissions would interfere with each other and focus the radar beam into a tall and narrow swath, ideal for targeting. Interference effects could also be used to electronically steer the beam over an angle of 60 degrees in front of the aircraft.

The Rad Lab environment was open to new ideas, and the energetic Alvarez quickly got the project moving. At first, the design team called it "EHIB (Every House In Berlin)", but at Lee DuBridge's insistence in early 1942 it was renamed "Eagle". Eagle was eventually redefined as a winglike antenna that was carried below the bomber, operating in the X-band at 3 cm (10 GHz). The antenna was to be linked up to the Norden bombsight, though it was designed to be easily connected to other bombsights as well.

Alvarez had originally envisioned the array as a waveguide, a hollow pipe with a square cross section through which microwave energy could flow, with slots cut into it at intervals to act as transmitting antennas. In practice, an array of dipole antennas was used instead, but initial tests showed that such an array didn't produce a neat forward beam. It also generated "side lobes", beams that went off at unwanted angles, wasting transmitter energy. Alvarez thought the matter over for a while, and came up with a new array design that featured twice the number of dipoles with half the spacing, with the dipoles alternating in polarity. This "reversed dipole" scheme canceled out the side lobes. Although there was considerable skepticism at the Rad Lab about Eagle and the project was given low priority, a prototype was successfully flight-tested on 16 June 1943.

* While the Rad Lab worked on Eagle, they also used the ideas embodied in it to develop an improved early-warning radar. Longwave early-warning radars had poor resolution and poor coverage of low altitudes, allowing aerial intruders to slip in "under the radar". They were also very poor at tracking large numbers of targets.

The Rad Lab's Morton Kammer went to Britain in January 1942, only weeks after Pearl Harbor, to study the British early-warning system. Kammer hoped to get clues on how an American coastal defense system might be implemented. When he returned to the US, he spoke with Luis Alvarez, who had very helpful suggestions on how a microwave early warning radar system might be implemented, based on Eagle technology.

Kammer was assigned to the project to develop a "microwave early warning (MEW)" radar in June 1942, and the first operational MEW or "AN/CPS-1" was in operation in Britain by January 1944. Six preproduction MEWs were put together by hand at the Rad Lab to get the device out in the field.

Although Eagle and MEW were derived from similar concepts, they had little resemblance. A complete MEW system weighed about 60 tonnes (66 tons), required eight trucks for transport, and drew 23 kilowatts of power from a portable generator. It took 150 troops three days to pick up and move a MEW.

The MEW control electronics included five 30 centimeter (1 foot) scope displays, allowing operators to track large numbers of targets. While Eagle used electronic steering, MEW required 360-degree coverage, and so it used a rotating antenna. The MEW antenna was actually two antennas joined back-to-back, with one antenna covering low altitudes and the other covering high. Each of the two antenna consisted of a linear array with 106 dipoles in front of a solid reflector, in the form of a section of cylinder with parabolic curvature laid horizontally. Each reflector was 7.6 meters (25 feet) wide. The low-coverage reflector was 2.4 meters (7 feet 10 inches) tall, while the high-coverage reflector was 1.5 meters (4 feet 11 inches) tall. They could form a beam only 0.8 degrees wide that could provide extremely precise location of intruders, at least in the horizontal plane. The beam was very tall and so MEW didn't do well at height-finding.

A second MEW was operational in Britain by the summer of 1944. The two radars were very useful in helping to deal with the V-1 Blitz, since MEW's longer range gave greater advance warning of flying bombs, allowing more effective fighter interceptions. Since the V-1s flew at preset low altitudes, MEW's inability to compute heights was not a problem.

The Rad Lab's office at the TRE modified a MEW system to be transportable during April 1944, and this MEW was set up on the Normandy beachhead, arriving in pieces on 12 June 1944. It included a complete fighter-control center, organized around a vertical transparent plotting panel on which plotters marked positions and wrote notes in mirror writing. The MEW helped Allied fighters protect the invasion forces from German intruders, and in particular keep track of the massive flow of air traffic over the area.

The height-finding problems was addressed by adding a British AMES Type 13 CMH radar. The same idea was used with some other MEW installations with the US counterpart to the AMES Type 13 CMH, the AN/APS-10 Little Abner.

Another MEW arrived in France in late summer, but the MEW systems were large and complicated and so were of limited use during the rapid Allied advance west. SCR-584s were more portable and accompanied the armies as they advanced.

A MEW was sent to Saipan, where the USAAF was ramping up Boeing B-29 Superfortress bomber raids against Japan. The system arrived on 21 September 1944, and was not greeted with much enthusiasm. It was big, it was clumsy, and since air traffic in the Pacific was much less dense than in Europe, longwave radars like the SCR-270s seemed able to do the job just fine. Destructive Japanese low-level intruder air raids on Saipan suggested that the longwave radars didn't do the job as well as might be desired, and the MEW was operating on top of Mount Tapochau on Saipan by New Year's Eve. Japanese raiders suddenly lost the element of surprise, and the MEW also served well to locate aircraft that had ditched in the sea around the island.

* An operational prototype of the Eagle was first flight-tested on 16 May 1944. It had a beam width of 0.4 degrees, giving it much improved accuracy relative to H2X. The radar was almost everything hoped for, its narrow beam allowing it to pick out a single large building in a city. It had a range of 260 kilometers (160 miles). Eagle never did see action in Europe, but the "AN/APQ-7", as it was formally designated, was in operation against Japan with B-29 Superfortresses by June 1945.

The major drawback of Eagle was that it was hard to use, and postwar analysis showed that even Eagle didn't have accuracy comparable to that of optical bombsights. However, it allowed Allied bombers to attack in darkness in cloudy weather, and after the war radar bombing would become much more accurate.

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[6.3] PROJECT CADILLAC: AN/APS-20

* The Rad Lab's biggest project never saw combat in World War II. In June 1942, an interservice committee recommended the development of an early-warning radar carried by an aircraft. A flying early-warning radar would be in effect mounted on an extremely tall "tower", extending the radar's view to a comparably wider horizon and improving the ability to detect low-flying intruders. The US Navy really wanted this capability. Jerome B. Weisner was put in charge of the effort, which was named "Project Cadillac", after Cadillac Mountain in Maine, where the sun first rises on the United States every morning. The project grew massively, ultimately soaking up 20% of the Rad Lab's staff, as well as 12% of the lab's outside purchases for its entire existence. The Navy also contributed 160 officers and men.

Putting a radar on an aircraft was almost the least of the problem; that part of the job wasn't all that different from flying an ASV radar on an aircraft. There was the issue of ensuring proper IFF for the large numbers of air and sea targets on the radarscope, and the extremely difficult issue of considering what to do with the information obtained by the radar. The idea of having a operator on board the aircraft radio the floods of information picked up on the PPI to users on the ground was obviously impractical.

RCA had been ready to introduce television just before the war broke out, and provided a TV system that was used to relay the PPI display to a surface station. The normal amplitude-modulated TV signal transmission scheme didn't prove adequate, and so Zenith Radio designed a frequency-modulated TV transmitter system. By May 1943, Rad Lab engineers were able to receive useful PPI images sent by an aircraft to a ground station.

Since the PPI display was centered around the aircraft, not the ship that received the image, the relative locations of the aircraft and ship had to be precisely determined using radar beacons. The high-flying aircraft would also serve as a radio relay for command communications; as long as the Navy had a tall tower, full use might as well be made of it.

* The Project Cadillac team was organized into five sections: shipboard system; airborne system; airborne radar; radar transmitter; and beacons and IFF. System elements were in place by early 1945, and the system began flight evaluations in April. By this time, the project's priority had been raised even further. Japanese "kamikaze" suicide aircraft attacks had inflicted major damage on US Navy ships, and Project Cadillac promised to deliver a bigger and tighter screen against the attackers.

The radar element of the system was the "AN/APS-20" S-band radar, which was fitted to Grumman TBM Avenger torpedo-bombers. As mentioned earlier, the TBM had carried the first operational American airborne radar, the ASB. The converted TBM, designated "TBM-3W", was stripped of armament, armor, and bombing gear, then fitted with a big radome between the main landing gear for the AN/APS-20's rotating antenna. The radome gave the aircraft a "pregnant" appearance, and additional "finlets" had to be added to the outboard section of each horizontal tailplane to keep the aircraft flying straight. The aircraft was littered with various small antennas for other elements of the system, including two VHF radios, IFF interrogator, and the television relay. Although the PPI imagery was relayed to the CIC on board an aircraft carrier or other vessel, it was also monitored by two operators on board the Avenger.

About 40 TBM-3W conversions were performed, but the system was still in evaluation when the war in the Pacific ended, and did not enter service until 1946. It was still the very first "airborne early warning (AEW)" aircraft. The Navy was impressed enough by the TBM-3W that they also had the AN/APS-20 fitted to the land-based Boeing B-17 Fortress bomber, which was given the designation "PB-1W". The PB-1W provided greater range and endurance than the TBM-3W and could operate far more autonomously.

* The AN/APS-20 radar would have a peculiarly long life. It was fitted to a number of other aircraft in the 1950s, most significantly a variant of the Douglas Skyraider carrier-based attack aircraft, the "AD-3W". The AN/APS-20, with some enhancements, remained in first line service until the early 1960s.

It appears that radar technology reached a sort of evolutionary "plateau" in the postwar period. It was still being improved and used in new applications, but the rate of progress was nothing like the incredible explosion of technology seen during the war. Radar didn't really take another big step forward until the 1960s, when digital circuitry and then computing power was integrated with radar sets, giving them intelligence that allowed them to be much more capable and less of a magical art to operate.

However, the AN/APS-20 remained in service with the British for most of the rest of the century. The British Royal Navy obtained the AD-3W Skyraider for operation off their own carriers. In the late 1950s, when the Skyraiders were being retired from British service, the Royal Navy decided to adapt their Fairey "Gannet" carrier-based antisubmarine aircraft to the AEW role, modifying the design and fitting them with AN/APS-20 sets scavenged from the British Skyraiders to create the "Gannet AEW.3".

The Gannet AEW.3 served well into the 1960s. When it was retired, the British were in desperate need of an AEW capability, and in 1971 they scavenged the AN/APS-20 sets again, fitting them to existing Avro Shackleton ocean patrol aircraft. The Shackleton was every bit as much an antique, a four-piston engine aircraft that was a derivative of the World War II Avro Lancaster bomber.

The result was the "Shackleton AEW.2". Twelve were converted and served with the RAF in support of the Royal Navy. The Shackleton AEW.2 was an embarrassment, a flying museum piece, and the only good thing that could be said about it was that it was better than nothing. It was supposed to be a temporary fix while the British developed the advanced "Nimrod AEW" aircraft, but that program proved overambitious and terminally "snakebitten", leading to seemingly endless delays until it was finally canceled. The British would have been better off to use available US AEW radar technology and install it on their own platform, but because of the delays they were forced in the end to buy the complete solution from the US, in the form of the Boeing "Sentry E-3D Airborne Warning & Control System (AWACS)", based on the 707 airliner.

The British got their E-3Ds in the early 1990s and were finally able to retire their exhausted Shackletons. The E-3D was a state-of-the-art machine that was the descendant through several generations of the Project Cadillac Avenger TBM-3W. The RAF jumped through decades of improvement in a single breathtaking step.

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[6.4] THE END OF THE BEGINNING: CHAIN HOME & BIG BEN

* While improved radar systems were introduced during the war, refinements were added to old ones. Chain Home remained in service throughout the conflict, with many refinements added to keep it useful.

For example, a special CRT display was developed to allow operators to cut through jamming. This display was coated with a "short persistence" (quick fading) blue phosphor and a "long persistence" (slow fading) yellow phosphor. The display could be covered with either blue or yellow optical filters to allow viewing of one phosphor or another, but not both at the same time.

The blue filter was used normally. If operators were confronted by jamming, they could switch to the yellow filter and view the long-persistence display. The idea was that jamming was generally and by design simple electronic noise, while radar returns from an actual target were persistent. The track for the actual target would tend to build up on the long-persistence display, while the noisy jamming wouldn't. The scheme worked very well in practice.

However, by late 1943, the Chain Home network was being scaled back. The British were no longer fighting for their lives, and everywhere the Allies were on the offensive. There was not as much need for vigilance at home, and a strong need for radar expertise elsewhere, so Chain Home stations judged redundant were shut down and others were put on reduced operational schedules.

Chain Home still had one major role to play in the war. In the fall of 1944, in response to the German V-2 missile blitz, a radar network was organized to provide early warning. Five CH stations, along with some modified GL Mark II sets, tracked the rockets, which were about the size of a quarter-wave dipole relative to the CH transmission frequencies and so were good radar targets. Procedures were developed to determine the impact area and send out an alert. The warning gave enough time to close floodgates into the Charing Cross tunnel of the London Underground system under the Thames. A hit on the tunnel would have been a major disaster, flooding much of the subway system.

The tracking system, which was known as "Big Ben" after the codename for the V-2, also attempted to pinpoint the launch site so that Mosquito bombers standing by could try to bomb the launchers. Unfortunately, the missiles were fired from mobile launchers, and bombing them proved almost impossible.

There were also an attempt early on to use jamming countermeasures against the rockets, on an assumption that they were radio-guided. That assumption was based on the discreet examination by British intelligence of a V-2 that had fallen on Swedish territory. The V-2 was actually a special prototype, carrying an active radio guidance system for an experimental anti-aircraft missile known as "Wasserfall". However, a normal V-2 had a gyroscopic inertial guidance system and the jamming proved ineffectual; it was quickly abandoned.

As Germany crumbled in 1945, the V-2 launches faded out. The V-2, developed at great expense, did little to help the German war effort, but it gave the Americans and the Soviets the basis for a missile arms race in the postwar period. Chain Home continued its fade into the early 1950s, when it was finally completely phased out in favor of much improved radar systems. It had served its purpose and served it well in the meantime, and in retrospect amounts to a memorial for Robert Watson-Watt, Stuffy Dowding, and all the other British pioneers of modern radar technology.

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