Wilkins and Watson Watt made some calculations. They believed that the best radio wave for detecting a metal bomber of about 75 feet wingspan was one of about the same length, a wavelength of 75 feet. Moreover, in terms of power and reception, such radio waves could be relatively easily transmitted and their echoes, i.e., re-radiated waves, detected on specially built equipment. They reported so at the committee’s first meeting in January 1935. Rowe checked the math; Wimperis checked the math. Indeed, it seemed to be so.
Wimperis asked Watson Watt for more information and within weeks the Scottish scientist presented the committee with a paper called, “Detection and Location of Aircraft by Radio Means.” Now the committee had something to work on and a reasonable amount of money was given to Watson Watt for continued research.
He began work at once. The idea was to create a short pulse, about 0.0002 second, later trimmed to even shorter duration, of radio wave and then to determine if there was an “echo,” that is, a re-radiated wave of the same kind. The time between the pulse and any echo would determine the distance of the object that created the echo. Watson Watt called this system radio direction finding (RDF). It was not until later in the war that the British term was changed to conform to the American term, radio detection and ranging (radar). (Is it more than a coincidence that the term reads the same left to right as right to left, mimicking a wave and its echo?) At this point, there was not so much an attempt to determine height as location and bearing.
Perfecting the RADAR
Watson Watt established an experimental station on an isolated spit of land near the town of Orford on the North Sea, 65 miles from London. He and his crew began setting up transmitters and receivers. The receivers were to capture both the original pulse and the echoes and display them on a cathode ray tube. By summer, Watson Watt’s team of radio engineers was reading echoes from aircraft, but there was much work yet to be done. They had to figure ways of not being fooled by commercial radio waves, or to counter efforts should an enemy detect the radar pulses and send out their own “jamming” pulses on this wavelength. By fall, the committee had decided it would build a line of transmitters and receivers from Southampton on the Channel all the way up the east coast to Scotland. These would be called “Chain Home” (CH) stations (to be constructed in locations that would not “gravely interfere with grouse shooting”). Thus, a curtain of radio waves would pulse outward so that no aircraft could approach Britain without detection. (RDF at the time did not work inland. Once intruder bombers crossed onto British soil, detection was shifted to ground observers who would track their progress.)
To help solve the problem of determining the altitude of an intruding aircraft, receivers were placed at different heights on their towers. The difference in strength of a signal received from an upper and lower receiver was an indicator of the height of the intruder. Altitude determination was crude, and it remained a problem for years, despite the fact that it was crucial for intervening fighters to intercept a group of intruders at higher rather than lower altitudes. During the Battle of Britain, fighter pilots began to make their own adjustments to the radio instruction altitudes they were receiving from Fighter Command.
Direction was another problem, but Watson Watt developed an elegant solution. He had receivers placed at right angles to each other. A plane flying directly at one would produce a strong signal in it, but a very low signal in the other. If flying at an angle midway within the right angle formed by the two receivers, the plane would produce a signal of equal strength in both. Analyzing the signal strength between the two echo receptions thus gave a rough idea of direction. The strength of the echo wave also was an indicator of the size of the plane formation, should there be more than one plane. Nevertheless, accuracy was elusive. Seventy-five miles out, the RDF could only judge that a plane or planes would be in an area of 15 square miles and at an altitude range spanning 2,000 feet. During practice drills, fighters sent out to intercept often could not find the “intruders.” Still, Dowding, now in charge of Fighter Command, championed the work and kept it going.
It was a close thing. The radar effort might have been squashed, and it might have been Winston Churchill’s fault. Churchill’s good friend and science advisor was F.A. Lindemann, a fearless flyer and a professor of physics. Lindemann had his own ideas about dealing with bombers, and he didn’t believe RDF was the way. Detecting them with heat sensors would be better and the way to counter them was either developing aerial mines that floated down on them or steel cables dropped from above that would foul their propellers. Had Churchill been appointed prime minister in the late 1930s rather than 1940, Lindemann may have had more power and thereby scuttled radar in favor of his other schemes.
As it was, by the time of the Munich crisis in September 1938, many towers had been built and five were operating 24 hours a day. Accuracy was still a problem, but operators could detect airplanes taking off in Belgium and France 150 miles away. New stations were added to help detect airplanes approaching Britain at levels below 3,000 feet.
Identification Friend or Foe
Engineers also had to work on communication and coordination, which was just as vital to the defensive effort as detecting the intruder planes themselves. There were problems of determining whether two RDF stations were detecting the same or different groups of aircraft, how to resolve any differences in their estimates of location, bearing, and altitude. Other problems were how to assemble the information, deal with it effectively, route it to the fighter groups, then to the squadrons, and then to the pilots. All this had to be done with enough speed to allow the pilots to arrive over the intruders with their backs to the sun.
This was accomplished in part by laying telephone lines from each of the radar stations to the command center of Fighter Command at Bentley Priory outside London. Here the radar results were sent to an underground “Filter Room,” which would organize and synthesize the information. At the center of the room was a large map of Britain and the nearby coast of the Continent. When groups of aircraft were detected, their number and location were indicated on the map by a red marker. Black markers noted positions of RAF squadrons. Women of the Women’s Auxiliary Air Force (WAAF) used casino croupiers to slide the markers along the maps as the plane groups moved. Watson Watt had suggested that the radar scopes at CH stations, the telephone lines, and the filter rooms (there were others at Group and Sector locations around the country) be operated mainly by women. This was done, and not a few died at their posts when the Luftwaffe bombed Chain Home stations during the Battle of Britain.
Tizard and Dowding championed the new technology with the passion of men who sensed that a conflict for survival was drawing nigh. The RAF continued to test the system with mock air raids. Group commanders learned how to immediately obey any telephoned command for whatsoever number of squadrons to be scrambled, and fighter pilots learned how to accept radioed directions.
There was yet another problem. The RAF planes gave the same echo as enemy planes. Engineers solved this problem by installing in each RAF plane a dipole that altered the echo or re-radiated wave from a transmitter’s pulse. The echo as it appeared on the cathode ray tubes would look different from those emanated by aircraft not containing the dipole, which the engineers called Identification Friend or Foe (IFF). Planes began to be fitted with this device in 1939.
RAF Losses in France Were Staggering
In the same year, Germany nearly gained an understanding of Britain’s advancement and reliance on radar. The Germans, by means of commercial airline overflights, had, of course, noticed the 350-foot towers and deduced their link to radio transmissions. In order to learn more, they equipped a Zeppelin with radio gear and sent it up and down over international waters off Suffolk’s Bawdsey Manor, then the center for radio research after the Orford site had been outgrown, to listen in on the Bawdsey Manor towers’ transmissions. The Zeppelin crew heard nothing, their radio receivers being faulty. The Germans tried again just one month before the war began, but again the Zeppelin crew heard nothing—this time the British radar was under repair. At the time, German radar could detect planes at 50 miles, but the British could detect them at 150 miles and had a superb system for assimilating the information and getting the fighter aircraft to act effectively on it. German air strategists and tacticians never fully appreciated this difference through the whole of the Battle of Britain.