Radar and the Fighter Directors

By David L. Boslaugh, Capt USN, Retired

Table of Contents

  • Introduction
  • Preface
  • Chapter 1. Chain Home
    • From Death Ray to Radio Location
    • Chain Home Low
    • The Plan Position Indicator
    • Identification Friend or Foe
    • The Dowding System of Control
    • References
  • Chapter 5. The Beginnings of Naval Fighter Direction
    • Fighter Direction in the Royal Navy
    • The Tizard Mission
    • Finally a Name
    • A Radar for Small Ships
    • Early Experience With the CXAM
    • Wildcats and the Thatch Weave
    • A Tentative Doctrine for Fighter Direction from Aircraft Carriers
    • Origin of Fighter Direction in the U.S. Navy
      • Getting the School Ready
      • The Curriculum
      • The Graduating Class
  • Chapter 7. Midway
    • A Plan to Annihilate the U.S. Pacific Fleet
    • Many Planes Heading Midway
    • Akagi, Kaga, and Soryu Mortally Wounded
    • Yorktown Under Fire
    • Hiryu Goes Down
    • More Criticism of Fighter Direction
    • Camp Catlin
  • Chapter 8. The Solomons
    • Guadalcanal Invasion
    • The Battle of the Eastern Solomons
    • The Battle of Santa Cruz
    • Agonizing Reappraisal
    • A New Task Force Air Defense Organization
  • Chapter 11. The Night Fighters
    • Development of USN Airborne Radar
    • Night Fighter Direction Training
    • First Night Fighter Squadrons
    • A Shift to Night Torpedo Attacks
    • The Night Operations Carriers
    • Back to Day Carriers


Introduction

They were very unusual young men. They had to be able to carry a huge amount of rapidly changing information in their heads and think fast on their feet. They had to remember myriads of details like the maximum speeds and cruising speeds of the various types of fighters under their control, as well as their present state such as the fuel and ammunition remaining in their fighters. They also had to be able to instantly remember the capabilities of the various types of enemy airplanes that were at present converging on their task force. They had to have an innate sense of the relative motion between the fighters they were directing and their airborne targets, as well as the ability to predict the relative disposition of these aircraft many seconds or minutes in the future. Their rapidly formulated decisions and resultant commands could mean the difference between survival or destruction of ships in their task force. Such were the responsibilities of the World War II naval fighter director officers (FDOs). One would think that only very experienced senior naval officers would be able to handle managing the anti-air defense of a naval task force under the onslaught of hundreds of enemy airplanes intent on destroying their ships. But such was not the case. Most of the naval fighter director officers, even those in charge of the air defense of large task groups and task forces, were relatively young junior officers; for it had been found that older more experienced men just couldn’t handle such a fast moving job. This is the story of those young FDOs, and how they used their two basic tools: voice radio and newly invented radar.

The history of the naval fighter director officers (FDOs) is inextricably woven together with the history of radar. If radar had not have been invented, there would never have been FDOs. Although radar development by the U.S. Navy began at a low level of effort in January 1931, the British shore-based Chain Home radar project was authorized only later in February 1935. But under the threat of German invasion, British radar development was greatly accelerated. The result was the Royal Air Force’s (RAF) chain of coastal radar sites that went into operation in May 1937. Furthermore, Royal Navy ships were operating radar at sea by late 1938, whereas, U.S. Navy ships were not getting their first radar sets until the fall of 1941. This story therefore begins with the RAF’s Chain Home radar development project and then proceeds to chronicle the RN radar project that was begun when naval scientific authorities learned about the top secret RAF project almost by accident. It then proceeds to the British development of airborne radar and their invention of the astounding cavity magnetron that enabled the generation of heretofore unbelievable radio wave power at microwave frequencies. This, in turn, enabled the design of compact airborne radars, as well as precision pointing shipboard radars.

Next we review the invention and development of U.S. Naval radar, done in total secrecy and isolation from the British radar projects until the British Tizard technology exchange mission in September 1940. Thanks to the Tizard mission, by early 1941 U.S. naval aviator observers were flying in RAF and RN fighter squadrons and attending their fighter direction schools, leading to some of the returnees starting USN schools at Norfolk, Virginia, and San Diego, California, in August and September 1941. We will see that the San Diego school was truly a “do it yourself” project. We then follow the adventures of one of the graduates aboard the carrier Lexington when it was lost in the Battle of the Coral Sea. USN high command was not particularly pleased with fighter direction during that battle. Although the Battle of Midway a month after Coral Sea was considered a great American victory, senior commanders attributed the loss of the carrier Yorktown to deficiencies in fighter direction, although they realized the problem was not the FDOs but rather the appalling lack of the right equipment and facilities.

The Solomons Campaign brought on two more Japanese carriers-versus-American carriers sea battles with even more disappointing fighter direction, and a resulting demand for better shipboard facilities and improvements in radar so that the bountiful information that radar produces could be effectively used. This resulted in Admiral Nimitz’s decree that every Pacific Fleet combatant ship would have a new properly equipped radar plotting room to be called the “Combat Operations Center (COC);” later changed to “Combat Information Center (CIC).” The commanding officers of existing ships were left pretty much up to their own crew’s ingenuity in designing and equipping these new centers, whereas new-construction ships were finally getting well thought out and equipped CICs and improved radars. The Guadalcanal Campaign from August 1942 to February 1943 saw fighter direction move ashore, and also move from the carriers to cruisers and then to destroyers. We then review the initiation of the special radar equipped “Argus” units intended to go ashore with invasion forces to provide shore based CIC and fighter direction services.

In early 1943 new fleet carriers of the Essex Class and light carriers of the Independence Class began showing up in the Pacific. They were equipped with new highly capable radars, including height finders, and spacious, well equipped combat information centers. Furthermore, three new amphibious force flagships were commissioned in September and October 1943. They were equipped with the latest in radars and had CIC facilities adequate for providing fighter direction to protect invasion forces. The performance of the fighter director officers progressively got better. The next chapter summarizes the development of airborne radar in the U.S. Navy, spurred on by the wondrous cavity magnetron tube. Airborne radar, in turn, fostered the development of radar equipped night fighters to help counter the highly developed Japanese skill in night aerial attacks.

The period from mid 1943 to October 1944 might be called the “happy times” for the fighter directors. In June 1944 we have the aerial victory of the Battle of the Philippine Sea, also called the Great Marianas Turkey Shoot, where everything seemed to go right for the fighter directors and the fighter pilots. Here, Japanese carrier aviation was reduced to almost impotence by experienced USN fighter pilots flying new American Grumman Hellcat and Chance Vought Corsair fighters teamed with very effective fighter direction. The happy times would culminate with another great aerial victory at the Battle of Leyte Gulf in October 1944 that opened the invasion of the Philippines. But here the Japanese introduced an ominous new form of aerial attack, the suicide pilots, calling themselves Kamikazes. This would be the greatest challenge for the FDOs, even with their new radars and well equipped CICs. The Kamikaze attacks would rise to a crescendo during the Okinawa invasion from early April to mid June 1945. The picket destroyers, stationed around the island with fighter direction teams aboard, would bear the brunt of the attacks. Finally, the Japanese high command realized they were losing airplanes and pilots at an unacceptable rate, and pulled back the remainder of their aerial assets to prepare for the defense of the homeland. Lastly, we look at what the fighter director training experts decided were the right qualifications in a candidate to make a good FDO. Maybe not surprisingly, relative youth with attendant quick thinking ability comes to the fore.

Preface

The Naval Tactical Data System (NTDS), the first digital computer automated weapon system to be installed in U.S. Navy ships, had two principal war fighting functions, namely gun & missile system weapons direction and air intercept control. A secondary function, called threat evaluation and weapons assignment, analyzed attacking air targets as tracked by search radars, ranked them in order of threat, and recommended the most threatening for assignment either to shipboard guns, missile systems, or to airborne fighter interceptors, as appropriate. If a recommendation for assignment to guns or missile systems was accepted by the Ship’s Weapons Coordinator (SWC) officer, NTDS fed the coordinates of the target to an analog fire control computer and coached the fire control radar to target “lock on.” If a recommendation for assignment to an interceptor under control of one of the ship’s Air Intercept Controllers was accepted, NTDS generated interceptor heading, speed, and altitude commands to complete the intercept. During World War II, and for a number of years after, what is now called air intercept control was called fighter direction, and was done by specially trained officers called fighter director officers (FDOs) using radar and voice radio as their principal tools.

When I wrote the history of the NTDS project, titled “When Computers Went to Sea,” I wanted to begin with a few pages about the fighter director officers (FDOs) and their work as a way of introducing the reader to the NTDS air intercept control function. Search as I might, I found precious little in the literature about fighter directors, let alone naval fighter directors. The one exception was Barrett Tillman’s excellent article titled “Coaching the Fighters” in the January 1980 U. S. Naval Institute Proceedings. While writing the book, I interviewed a number of people who had been associated with the NTDS project, including Captain Robert P. Foreman who was in charge of the TERRIER surface missile system project in the early 1960s. I had worked closely with CAPT Foreman and his staff as an assistant NTDS project officer while incorporating the TERRIER missile system’s weapons direction function into NTDS. As the interview drew to a close, I told him of my frustration at finding so little about WW II naval fighter directors in the published literature.

CAPT Foreman responded, “Well maybe I can help you. I was an FDO in the Pacific during the ‘great war.’ I was in a fighter direction team that rode radar picket destroyers on stations around Okinawa when we were battling the Kamikazes.” He then regaled me for a couple of hours with sea stories about his fighter direction adventures. He concluded by saying, “What you need to do is get in touch with Captain Nick Hammond who is writing an article about WW II naval fighter directors.” I did, and CAPT Hammond provided a quantum leap in my search for FDO literature. He not only sent me his half completed article on fighter directors, but also much reference material including a ninety-three page article titled “History of Naval Fighter Direction” published in three parts in the April, May, and June 1946 issues of Combat Information Center (CIC) magazine by the Office of the Chief of Naval Operations. My problem had been that CIC magazine was classified confidential when published, and had only been declassified in 1972. By that time there were only a few incomplete sets left in some obscure archives. Today, thanks to the efforts of the Historic Naval Ships Association most copies of the magazine are available on line, however, they are still searching for a few missing issues to complete their collection.

Thanks to CAPT Nicholas J. Hammond I suddenly had more background on the naval fighter directors than I needed for my NTDS history effort. CAPT Hammond’s thick black notebook sat on my shelf for fifteen years, and I would thumb through it from time-to-time and wonder if this great material would be someday worked into a book so that a wider audience could know the story of those most unusual young men. Surely, someone would eventually write their story, but it never seemed to appear. I thought about sending the notebook off to some appropriate archives, realizing that most likely its contents would never see the light of day. Then it dawned; if their story was going to be told, I had better get busy and write it. This is not intended to be a comprehensive coverage of the Pacific Air War, but rather will focus on the training and work of the fighter director officers in that conflict.


I have already mentioned the trove of fighter direction reference material provided by CAPT Nicholas J. Hammond. Second in importance as references were two books written by John B. Lundstrom: The First Team - From Pearl Harbor to Midway, and The First Team and the Guadalcanal Campaign. These cover the activities of U.S. naval aviation during these two periods in great detail. Fortunately Lundstrom considered the fighter directors just as much on the first team as the fighter pilots, and their names and contributions are well documented. Other books covering the Pacific War offer glimpses into the actions of specific FDOs on specific ships. I am greatly indebted to volunteers supporting such organizations as the Naval Historical Foundation, the Naval History and Heritage Command, the Pacific War Online Encyclopedia, the Navy Department Library, Wikipedia, Historic Naval Ships Association, and others. These volunteers have found and posted historical material on the internet, and have tirelessly scanned historic WW II documentation to make it available on their web sites. Their efforts include scanning every illustration in WW II Navy manuals at high quality so that they can be individually downloaded as illustrations. For example, they have posted every U.S. Navy radar bulletin of the WW II period (including the fighter director’s manual), action reports, most copies of the Navy’s Combat Information Center magazine, and much other very useful reference material that would have been virtually impossible to find before the advent of the internet. My thanks goes to CAPT H. Stanwood Foote who endured my interviews and wrote many letters about his experiences as a fighter director in the Pacific War. Special thanks goes to Mrs. Rachael Casey of the Fleet Air Arm Museum, Yeovilton, England, for her research and finding photos of fighter director training tricycles, and to Mr. Peter Marland, Weapons Historian in the Royal Navy Naval Systems Department, for his research and provision of photos, and his guidance to me in finding photos

I also wish to thank my next door neighbor Art Gomes, my across the street neighbor Captain Jerome J. Fee, USN Ret., and Captain Donald L. Leichtweis, USN Ret. for their thorough proof reading of this manuscript and their many useful suggestions.

Chapter 1. Chain Home

From Death Ray to Radio Location

A death ray? It was January 1935 at the National Physical Laboratory’s Radio Research Department at Slough, England, and Scientific Officer Arnold F. Wilkins had been given a very unusual assignment. He was to determine if enough energy could be concentrated in a radio beam to incapacitate a bomber pilot by making his blood boil. In a few quick calculations he could see that there was no generator of radio waves in existence that could even raise a human’s body temperature to fever level, let alone come to a boil.

Why had he been given this strange assignment? There was concern at the upper levels of British Government that England was highly vulnerable to enemy bomber attack. So vulnerable that the Air Ministry’s director of scientific research had established a committee of eminent scientists to conduct a “scientific survey of air defense” to be led by Professor Henry T. Tizard, Rector of Imperial College, London. The committee’s assignment was to identify and evaluate all scientific breakthroughs and technologies that could possibly be used to bolster the aerial defense of Great Britain. They were not to dismiss any idea, no matter how bizarre, until it had been seriously evaluated. [14, pp.54-55]

Death rays had been written of in novels and the comics for quite some time, and were on the list; therefore Wilkin’s assignment. He reported back to his boss, Dr. Robert A. Watson-Watt, Superintendent of the Radio Research Department that the idea was not viable but, if the Air Ministry wanted to use radio waves in aerial defense, there might be another way. Wilkins recalled stories by British Post Office engineers about their experiments in 1931 and 32 to set up a radio link between the Scottish islands and the mainland. Passing airplanes had caused disruption of their radio link, most probably by their radio waves being reflected from the planes and combining both destructively and constructively with the direct wave. If reflected radio waves had this kind of power, they might be capable of reflecting from a bomber with enough energy to be detected at some distance. With some quick calculations Wilkins showed Watson-Watt that existing radio transmitters had enough power to bounce waves off a bomber’s skin, and that the reflections could be detected many miles away from the bomber. [14, p. 55]

Watson-Watt realized that an electronic device making use of this phenomena could give advance warning of the approach of attacking aircraft, and at ranges greater than existing sound detection systems. Perhaps it could even be refined to give range and bearing of approaching aircraft; he had some ideas along those lines. On 28 January 1935 he sent off a memo, along with Wilkin’s calculations, to Mr. Harry Wimperis the Director of Scientific Research at the Air Ministry, in which he described a radio detecting system that could give advance warning of approaching bombers. Wimperis, in turn, forwarded the memo to his boss, Air Vice-Marshal Hugh Dowding. From 1930 to 1935 Dowding had been the Air Ministry’s Air Member for Supply and Research; then in 1935, when this department was divided in two, he became Air Member for Research and Development.

Hugh Dowding, promoted in 1937 to Air Chief Marshal. Ministry of Information official photograph

Dowding had learned to fly in 1912, and by the end of World War I he had flown most types of aircraft in the RAF and had just about every kind of flying experience imaginable. In 1918 he was promoted to Brigadier General based on his achievements, ability, and especially his comprehension of technology. He was sort of a maverick in the RAF because, unlike most senior officers who placed their faith in bombers because they thought “the bomber will always get through,” he was a strong proponent of fighter defense against bombers. This would cause him the ire of senior RAF officers throughout his career. As officer in charge of research and development he pushed the development of two advanced eight-gun monoplane fighters that would eventually emerge as the Hurricane and Spitfire. He also sensed that fighters used for continuous patrols to detect an oncoming enemy attack would be a waste of resources, and what was needed was a more effective advance warning of attack. He was, therefore, very interested in Wason-Watt’s memo, but he was also a technically knowledgeable skeptic. What he wanted first was a demonstration to prove that radio waves could be reflected from a bomber and detected at a useful distance; then he would provide funding. [9, pp.50-52]

Watson-Watt gave Wilkins the job of setting up the demonstration. It had to be done in a couple of weeks, and it could not cost much. Wilkins had already calculated that the best wavelength to send the strongest echoes from a bomber should be about equal to its wing span, or about twenty-five meters. But he despaired of being able to quickly build a transmitter that could generate a powerful enough signal on that wavelength. He did have a tunable receiver and a cathode ray tube (CRT) that would suffice for the operator display. He recalled that the British Broadcasting Company’s Daventry short wave broadcasting station sent its signal at a forty-nine meter wavelength, which was close enough for a demonstration, and it was of high power. He had his transmitter for free. For a receiving antenna he devised an arrangement that barely showed the direct wave from the Daventry transmitter, but was very sensitive to a reflected wave coming from the other direction.

As arranged with the RAF, a Handley Page Heyford bomber showed up over Daventry on the morning of 26 February 1935 and began flying a prearranged north and south path over Wilkin’s equipment van parked in a field. In the van were Wilkins, Watson-Watt, his young nephew, and an Air Ministry representative, Albert P. Rowe, assistant to the Air Ministry’s Director of Scientific Research. As the bomber approached, the small green spot on the display tube rose to make a vertical line. They were receiving a strong reflected wave. That night Rowe wrote his report to Wimperis, who briefed Dowding the next day. Dowding was convinced he could build the missing piece of the fighter defense system, and released a sum of 10,000 pounds that day to start the project. The Committee for the Scientific Survey of Air Defense had its first real project. [22, pp.61-63]

Number 102 Squadron, Honington, Handley Page Heyford Mark III heavy bomber, 1938. It would later be found that spinning propellers were strong radio energy reflectors. Ministry of Information Second World War Press Agency Print Collection

The radio location project was to be done under the tightest security, and as secretary of the Committee for the Scientific Survey of Air Defense, A. P. Rowe was delegated to come up with an unclassified code name. He decided upon “RDF” which to the insider meant Range and Direction Finding but to the outside world it had the long familiar unclassified meaning of Radio Direction Finding. A site where the engineering work, experimentation, and component design could be done in secrecy was also needed, and the Air Ministry found one. Watson Watt was to take his newly formed team to a coastal spit of land called Orfordness, and owned by the RAF, where secret aviation research had been done during World War I. It still had habitable buildings and basic utilities. [9, pp.53-55]

The RDF project engineers spent a few months experimenting to verify Wilkin’s and Watson Watt’s design ideas and preparing specifications for prototype equipment. Watson-Watt had already used a pulsed radio system to measure the height of the ionosphere and it was not too difficult to adapt it to the RDF project because it transmitted at a wavelength of fifty meters, which was close to what they wanted for aircraft detection. This corresponded to a frequency of about six million cycles per second. Each pulse of emitted radio energy would last for roughly 200 microseconds, and would be transmitted repeatedly at about 100 times per second. Radio waves travel at about 186,000 miles per second, meaning a pulse would have traveled out about 1,860 miles before the next pulse was transmitted. This gave plenty of time for an echo from an aircraft to return to the antenna before the next pulse.

Next was the problem of measuring the distance of the aircraft, which would be the speed of light multiplied by half the time it took for a pulse to travel out to the target and the echo to return. The ever useful cathode ray tube was adapted to show this. The tube’s electron beam would be driven horizontally across the tube’s face at a constant rate until it reached tube’s edge. At this time it would be snapped instantaneously back to starting position to start another trip across the tube. The distance from side-to-side of the beam travel would represent maximum range of the RDF system, and the beam swept back and forth so fast that it left a glowing horizontal line across the tube. This they called a time base. In reality the receiver would be tuned so that natural radio noise in the atmosphere would show up as an irregular vertical widening of the time base. This atmospheric return looked like a sideways view of a field of grass, and “grass” is what they called it. But an echo reflected from an aircraft would appear as a spike rising clearly above the grass, and the spike’s location across the time base would be a measure of target range. This type of display would eventually be called an A-scope. [22, pp.111-115]

They came up with a system having separate transmitting and receiving antennas. The transmitting antennas were held aloft by 350-foot tall steel towers that held a screen of suspended antenna elements between them, and down their outsides. A minimum antenna was defined as two towers, and more towers could be placed in a row, as required by a particular site. The antenna radiated in all directions, so a reflecting array was suspended behind the transmitting array to conserve power and prevent inland transmission. At first the system could only indicate that a target had been detected somewhere in the field of transmitted energy, and what its range was. What was needed next was a way to determine target bearing and elevation, and Wilkins had a possible answer. He had previously built a receiver to measure the direction of lightning strikes using antenna dipoles (metal rods) crossed at ninety degrees. By measuring the relative strength of the lightning pulse in each dipole he could use simple trigonometry to measure bearing. The RDF receiving antennas were mounted on 240-foot tall wooden towers because a steel structure would cause unwanted reflections. The receiving antenna elements were crossed rods set at ninety degrees to each other and the rods were initially oriented north/south and east/west. The direction of a reflected return could be determined the same way the bearing of a lightning strike was calculated. To improve bearing measurement accuracy, the crossed dipoles were eventually mounted on a motor driven rotating base so that the operator could rotate the assembly to find the direction of maximum return. [9, pp.53-55]

The Chain Home RDF station at Poling, Sussex, 1945. The three steel transmitting towers are at left, and the transmitter building is in front of them. The transmitting antenna elements were suspended between the towers. The four receiving towers are at right in a square formation, with receiver building in the center. Imperial War Museum photo CH 15173, Source http://www.iwm.org.uk/collections

Estimating elevation angle of the return was yet another problem. There was a stretch of earth between the ocean side and the antennas, and Wilkins used the return bounced off this stretch to estimate elevation. In addition to the receiving elements at antenna top, he installed a second set about one third of the way up the structure. The total signal strength of direct echo and ground bounced echo varied as a function of elevation angle and by measuring the relative return strength in each set of elements, elevation angle could be estimated. [9, pp.53-55]

Watson-Watt’s team at Orfordness was tracking aircraft out to sixty kilometers by the end of July 1935, and in December of that year the Treasury allotted the Air Ministry 60,000 pounds to build five RDF stations around the Thames Estuary. The Air Ministry gave the series of stations the code name “Chain Home (CH).” The RDF project staff had to be greatly augmented for this new phase of engineering, and the Orfordness quarters were now inadequate. A new suitable location, Bawdsey Manor on the Channel shore, was found about 15 miles southwest of their present location. The Manor was well isolated for secrecy, had ample living and laboratory quarters, and was conveniently located just a few miles from the village of Felixstowe. It also had plenty of real estate to accommodate the large antennas, and in March 1936 the staff moved to the new “Bawdsey Research Station.” In the end, Bawdsey would become one of the Chain Home stations. [9, p.53, pp.56-57]

The RDF system was not simple to operate. First, range had to picked off the A-scope by cranking a cursor out to the target “blip.” Next, target bearing was measured by turning the receiving antenna element with a goniometer knob. Sometimes, the operator had to switch among antenna elements to find the one with the strongest return. Then the goniometer would be switched to elevation angle measurement. These measurements were all relative to the RDF site and a plotting team had to plot range and bearing on a paper sheet to convert relative measurements to actual north and south geographic locations. Next, target altitude had to be determined by trigonometry using elevation angle and target range. Every site had measurement idiosyncrasies that had been determined by laborious flights of calibration aircraft and comparison of their actual geographic positions with the RDF site measurements. These fixed errors were laid down in correction tables which the plotters had to apply to the site measurements. All these measurements and corrections had to be done in less than a minute to keep the measurements from becoming stale.

The manual calculating and correcting system was barely workable, and G. A. Roberts of the Bawdsey engineering staff devised a better way. He designed an analog computer that could mechanically pick off the various operator measurements, compute target bearing and height, and then apply the correction tables. The operator could also enter an estimate of the number of aircraft in a raid by pushing buttons on the computer. This labor saving device had some similarity to a slot machine, which in British slang was called a “fruit machine.” So that is what the operators called it. [9, p.54] [22, p.183]

Chain Home WAAF radar operator Denise Miley plotting aircraft on the cathode ray tube of an RF7 Receiver in the Receiver Room at Bawdsey CH. With her right hand she has just positioned the direction/height finding switch and her left hand is ready to turn the goniometer to measure elevation or direction angle to be sent to the calculator. Imperial War Museum photograph CH 15332, Source http://www.iwm.org.uk/collections

By May 1937 the first Chain Home stations were turned over to RAF personnel, and on 1 January 1938 fighters were being directed out to make RDF guided intercepts of Lufthansa and KLM flights coming into Britain. As of July of that year, five Chain Home sites supported the RAF’s August air defense exercises. By this time the sites were operating at wavelengths from seven and a half to fifteen meters, because the original fifty-meter wavelength had been found to interfere with communication signals. [9, pp.54-55]

Chain Home Low

The relatively low Chain Home operating frequency resulted in pronounced vertical lobes in the strength of the echo return from an air target. These changes in echo strength were caused by target return echoes that bounced off the earth and at some locations constructively reinforced the direct return echo and at other locations destructively dminished the direct return. This caused “fade zones” at some elevation angles where target returns were not received. The lobing characteristic is what gave the system the ability to measure elevation angles, but this also resulted in almost no ability to detect aircraft flying below 2,000 feet until they were dangerously close to the site. It would be a group of Army radio location engineers, established at the Bawdsey station in October 1936, who provided the solution to this problem. By this time the Air Ministry had let the Army and Royal Navy in on the secret of radio location because the technology might be useful to the sister services. The Army’s first application was for coastal defense which required a much shorter wavelength to pick up surface vessels. They chose to build equipment operating at 1.5 meters, and by May 1939 they had a system, with rotating antennas, that could detect a 2,000 ton ship at fifteen kilometers and with a bearing accuracy of fifteen minutes of arc. The equipment could also detect low flying airplanes. [9, p.59] [40, p.411]

Artist’s depiction of fade zones in the radio waves reflected from an air target. From Radar Bulletin No. 3 - Radar Operator’s Manual p 1-52, Figure 1-51.

The Air Ministry decided that an RAF variant of the Army system placed on towers located between the Chain home sites could solve the low flyer problem. The new crash project was designated Chain Home Low (CHL), and the first site was in operation by 1 December 1939. Early in 1940 teams of engineers and RAF personnel were busy erecting Chain Home Low sites at fifty-mile intervals around the British Isles. They extended on both the east and west coasts from South Wales up to Scotland, and by August both CH and CHL were operational in the critical areas. The CHL sites transmitted their range and bearing information to the nearest Chain Home site, where the information was combined with the CH report and sent to Fighter Command Headquarters. [40, p.411] [9, p.59]

A Chain Home Low RDF site at Hopton-on-Sea. Early sites used separate transmitting and receiving antennas rotating in synchronism. The transmitting antenna is at the top of the 185-foot steel tower, and the receiving antenna is on a twenty-foot tall wooden tower next to the operations building. Imperial War Museum photo CH 15183

The Plan Position Indicator

The CHL system would spawn another technical breakthrough in the history of radio location; it would be called the plan position indicator (PPI). Dowding realized that if the Chain Home system were successful in stopping daylight attacks, the enemy would probably resort to night bombing, and this would call for special night interceptors fitted with airborne intercept radio location equipment. It was also realized that the three-minute delay time required from detection at a CH site to processing at Fighter Command HQ, to transmission to a fighter was too long for successful night intercepts. It was one thing to bring fighters within visual range of attacking bombers in broad daylight (about five miles), but it would be quite another thing to coach a fighter close enough on the tail of a night time attacker to get a detection at the short ranges expected of early airborne intercept RDF sets. What was needed were ground controlled intercept (GCI) sites that could communicate directly with night fighters.

The CHL radio location equipment was much more compact than a CH site, in fact small enough that, if fitted with a smaller rotating antenna, it could be mounted on a trailer to become a mobile self-contained GCI site. But, something else was needed. Time was of the essence, and the delay in stopping a rotating antenna upon detecting a target, picking range off an A-scope, and target bearing from a train indicator was too time consuming. It was necessary to develop an indicator that would immediately show target range and bearing on the same display. With the outbreak of war with Germany, there was some concern that the RDF research site at Bawdsey Manor could be vulnerable to a commando raid, and forthwith in September 1939, the research staff was moved to new quarters in Dundee, Scotland and renamed The Air Ministry Research Establishment (AMRE). Once relocated, one of AMRE’s first tasks was to solve the problem of a better RDF display.

Chain Home RDF Coverage September 1939 and September 1940. From British Official Histories (History of the Second World War)

What they came up with was yet another adaptation of the cathode ray tube. This time they took the A-scope’s time base and put its sweep starting location at the center of the CRT with the the sweep extending from the center out to the periphery of the tube. Next, they devised circuits to rotate the time base in synchronism with antenna rotation. The result was a glowing line rotating around the tube face like a spoke. A target indication showed up as a glowing short length of arc that could be made to last for a number of seconds by using a persistent phosphor. Target bearing was at the center of the glowing arc, and target range was measured as the distance of the arc (called a pip) from tube center. The new indicator thus displayed a plan view of target locations with respect to the radio location site, and was thus called the plan position indicator or PPI.

By the end of May 1940 AMRE had a prototype PPI in operation, and in late August a Chain Home Low RDF set was fitted with a PPI indicator for trials at the Worth Matravers CH Station in Dorset County on the south coast of England. Here for the first time, a fighter was under direct control of a ground RDF station. Air Ministry officials who watched the demonstration decided on an immediate program to build a number of mobile versions of the new system. The mobile GCI sites would not have height finding capability, but would receive height information by phone from the nearest CH site. [40, p.412]

Identification Friend or Foe

In 1935, soon after starting the Chain Home project, Watson-Watt penned a memo stating the need for an electronic way to distinguish friendly airplanes from enemy. His concern was, once friendly fighters had pounced on enemy bombers, there would be no way for RDF operators to distinguish friendlies from enemy airplanes in the mixup. It was especially important when the fighters headed back home to know if they were friend or foe. It would be very wasteful to deploy even more fighters to intercept the incoming unknowns. Wilkins first tried mounting a dipole antenna on a test airplane to enhance the returned radio echo. It worked, but not well enough to definitely discriminate between aircraft with and without the dipole. His next idea was a beacon device to be carried in the airplane. When it sensed a Chain Home pulse, it would send out a distinctive pulse at the CH frequency which showed up on the A-scopes a known distance behind the airplane’s echo. The beacon was tried out in RAF air defense exercises of August 1939. It worked and was named Identification Friend or Foe (IFF). The Air Ministry ordered production of IFF beacons to be installed in all British aircraft.

The term IFF was really a bit of a misnomer because in reality it only identified friendlies, but if the locations of all friendly aircraft were known, it could be fairly safely assumed that any plane not showing the IFF return could be considered enemy. To avoid interference with other sites, each CH site operated at a slightly different frequency, thus the initial IFF sets, designated IFF Mark I, automatically scanned the band of CH frequencies until it sensed a CH pulse, then it responded at the same frequency. The Mark I had been rushed into production and it had flaws. For example, it needed constant adjustments in flight. The later Mark II version was an improvement, and it featured a wider band of frequency scanning to accommodate newly emerging army and navy RDF sets.

By 1940 there were so many different RDF installations operating at so many different frequencies that a new approach to IFF was needed. It was no longer feasible for one beacon to search through so many frequency bands. The solution was a new IFF system invented in 1940 by engineer Frederick C. Williams at the British Telecommunications Research Establishment (the new name for Air Ministry Research Establishment, and by then located near Swanage, England). The new IFF would not depend on each specific RDF frequency, but would operate on its own assigned frequency. It would be a separate adjunct to each RDF set wherein the IFF antenna would be mounted on and move with the RDF antenna. It was designated IFF Mark III, and each beacon would operate on the same frequency. The Mark III was given additional features such as the ability, on pilot command, to send back a “Mayday” signal. It would become the standard IFF system for most of World War II. [22, p.126] [14, p.459]

The Dowding System of Control

On 14 July 1936 the Air Council reorganized the RAF into four separate commands: Bomber Command, Fighter Command, Coastal Command, and Training Command; and Air Vice Marshal Dowding was assigned to lead Fighter Command. In 1937 Dowding was promoted to Air Chief Marshal, and he had his work cut out for him. While Watson-Watt was busy developing the Chain Home RDF system, Dowding had to come up with a system that could use and effectively disseminate the CH data. He had to turn massive amounts of data into information useful for guiding the fighter pilots to their quarry. He realized that the radar data from each individual CH site could not be sent directly to the fighters. For example, different adjacent sites might detect a given target at slightly different locations due to operator and system errors. Somebody needed to see the big picture made up of the data coming from all the sites, and resolve such errors and ambiguities. The big picture was also needed to allocate fighter resources. They needed to see the location and size of simultaneous raids to determine from where, and how many fighters should be scrambled to intercept each raid. [9, p.52]

There was yet another complication. The reader will recall that the CH RDF sites could not see back over land. Once raiders were past the coast, Dowding depended on the Ground Observer Corps to track them, and their observations had to be added to, and correlated with, the Chain Home data. He envisioned one central location that would receive and “filter” all the data to build the big picture. This location would be in a bunker located beneath Fighter Command Headquarters, and would be connected to all CH sites and ground observers by a network of phone lines. Fighter Command HQ was located at Bentley Priory, a large estate located at Stanmore in the northwest London suburbs. The key part of the system was the operations room which had a large table with a map showing part of the continental coast, the channel, and England as far in as attacking aircraft could reach. [35, p.24]

The underground Operations Room at Headquarters Fighter Command, Bentley Priory, Middlesex. Imperial War Museum photo C 1870, Source http://www.iwm.org.uk/collections

Women’s Auxiliary Air Force (WAAF) plotters stood around the table armed with long wooden sticks resembling croupier’s rakes. They received location reports of the attackers from CH sites and from ground observers, and then positioned blocks at the prescribed grid reference coordinates with the long sticks. The blocks were a truncated triangle in shape with the apex pointing up. On each side were grooves that could hold small number and letter cards. Colored tags with even more numbers could be fitted into holes in the tops of the blocks. The numbers and letters denoted information such as raid number, raid size, and altitude. The plotters also placed colored direction arrows behind the blocks as they were moved across the table. A large master wall clock had a face with each five-minute sector colored either red yellow or blue, and the color of the direction arrows laid down in any five minute span matched the color of that clock segment. Thus, at a glance, observers could tell how current the information on the block was. The operations controllers in charge of the battle were in balcony above the plotting table where they could see the map plot and make decisions regarding what group should be assigned to each raid. [35, pp.24-27]

Dowding divided Fighter Command into four geographic groups numbered 10 to 13, and the groups were further divided into sectors. Each group HQ had a replica of the Bentley Priory map table located in an underground bunker surrounded by a balcony from which the operations controllers worked. The sector headquarters were equipped with a smaller map table generally showing the area covered by the group. The following table lists the area of coverage of each group, the location of their headquarters, and the resources they controlled.

Group Coverage HQ Location No. of Sectors No. of Squadrons
10 SW England & South Wales RAF Box, Box, Wiltshire 2 12
11 SE England & London RAF Uxbridge, Hilingdon, Middlesex 7 27
12 N of London to mid England RAF Watnall, Nottinghamshire 5 15
13 N England & Scotland RAF Newcastle, Newcastle on Tyne 5 13

[64 - Wikipedia, the free encyclopedia, RAF Fighter Command Order of Battle]

The plots showed not only the positions and information on raiders, but also the positions of friendly defending aircraft. The group operations controller could also see a series of status boards giving the state of each squadron under their control. With a series of colored lights the boards conveyed such squadron information as ‘detailed to raid,’ ‘enemy sighted,’ ‘ordered to land,’ ‘refueling and rearming,’ and ‘at standby,’ The group operations controllers decided what raid should be assigned to what sector, how many aircraft should be used, and when they should be scrambled. The Sector Station commanders then decided what squadrons at what locations should join the battle, and sector fighter directors coached the fighters to their targets. The whole plotting, filtering, and ordering process from target position measurement by a CH operator to heading and altitude orders transmitted to a fighter had to be done in less than three minutes. And it was. [35, pp.24-25] [21, p.39]

The Dowding System of Fighter Control. Diagram by the Author

The operative word in the phrase, “Dowding System of Control” was “control.” The pilots in the air were under strict orders to follow the directions of the fighter directors. It was only when the flight leader could see the enemy and had given his “tallyho” report (telling who was reporting, the enemy’s location with respect to his heading, the type of aircraft, the number in the raid, and their altitude) that he was released from control. Then he was free to give his fighters directions for engaging the enemy. [21, p.39] Click here to go to "Radio Location Goes to Sea - Chapter 2 of Radar and the Fighter Directors."

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