When an invading force of night-flying enemy bombing planes is damaged by the defenders to a decisive degree, we have an occasion of great importance. The American press, both newspapers and magazines, has taken care to see to it that the public knew that this is what happened in England and that it was not just a coincidence. Some time ago it was announced by the British government that the long-rumored secret weapon against night bombers was an actuality and that it was a locating device which spotted enemy aircraft and plotted their courses before they reached the coast of England. Naturally, this left the invading force wide open to attack by defending planes long before they approached their target.
Other than to say that the new instrument is a radio device, that it operates in the microwave portion of the frequency spectrum, and that it borrowed from television technique, very little information has been given concerning its construction and operation. Speculation has built very strongly upon this data using a knowledge of the characteristics of extremely high frequency radio waves (of the order of 1000 MHz and higher) which are well known, and of the operation of a television system.
First of all, it is known that microwaves are reflected from objects the same as light waves are. Therefore, a searchlight could be replaced with a microwave transmitter and a radio receiver used to pick up any reflected energy to indicate the presence of an object such as an airplane in the microwave beam. This method has the advantage that the enemy airman is unaware that his presence has been discovered. This, however, would not be the startling secret weapon now in use because it would be only as useful as a single searchlight with the advantages of greater range and being unknown to the enemy.
All further elaboration of the design and operation of the radiolocator must be considered as pure speculation by your editor and is presented only to give a general idea of how such a device might be made to operate. In developing the idea of a radiolocator it would be well to build upon the characteristics of light waves with which we are familiar and later switch over to a system using radio waves. As the first speculation consider the effect of lighting the entire area of the sky. We would have the effect of daylight and all objects would be visible to the eye, possibly with the aid of a telescope. This, however, is a physical impossibility. Next, consider the effect of a narrow beam of light passing through the sky, very rapidly and to such an extent that each portion of the sky is lighted momentarily. If this process is repeated at very short intervals, the effect on the eye will be the same as if the sky is continually illuminated and all objects within the range of the light beam will be visible. This is also physically impossible.
The foregoing paragraph, while describing impossible or at least highly impractical methods, point the way to an electronic system for the location of enemy aircraft. It is known that a microwave radio beam will be reflected by an airplane or anything else in its path. It is also possible to radiate a microwave in a narrow beam. (This would undoubtedly be one of the major problems.) Thirdly, the direction of radiation of such microwave beams can be rapidly changed in any desired manner. Making use of these phenomena together with some television technique, a workable locating system could undoubtedly be devised. This is a good point to note that no new principles are involved and that scattered throughout the literature of radio and physics of the last five or six years there have been a not inconsiderable number of articles devoted to the location of aircraft using radio waves. One such article appeared in the September 1935 issue of Electronics and was entitled “Microwaves Detect Aircraft." This is mentioned only to indicate that the chances are that no radically new fundamental law of nature has been harnessed, and not to belittle the new instrument. Rather, the development of the radiolocator represents the brilliant application of known methods to the solution of a new and intensely practical problem.
Continuing with the development of our speculation, our theoretical radiolocator consists of a microwave transmitter of special design and a microwave receiver to indicate that the microwaves are being reflected by an object in their path. The method of showing that an enemy plane is within range is to feed the reflected energy, amplified in the receiver, to a cathode-ray tube. This will produce a spot on the cathode-ray tube screen and in itself is not very much. The location of the plane can be determined in the following manner. Borrowing from television, the radiated microwave beam is made to scan a large area of the sky in an orderly manner, similar to the scanning of a television screen. This means that a certain portion of the sky is chosen for examination, the size of which at the moment is not important, but may be of large dimensions such as 10 miles square at a distance of 10 miles from the transmitter. The microwave beam is made to sweep across the uppermost limit of this area. As it reaches the end of the sweep it is returned back to the starting edge, but some distance below the first sweep path. It sweeps across the area again, this time slightly below, just enough so that there is no overlapping. The process is repeated until the entire area is scanned when the cycle repeats itself. Scanning of the field under examination is repeated at very short intervals of the order of about 1/30 of a second.
In synchronism with the scanning of the sky, the screen of the cathode ray tube is also scanned. Thus, for anything in the scanned area of the sky which reflects the microwaves, a light spot, or a dark spot depending on the polarization of the circuit, will appear in the corresponding position on the cathode-ray screen. With a system of coordinates the position of the plane in the scanned field can easily be determined. This, however, is not the complete story. To determinate the exact geographical position of the plane and its altitude it is necessary to use two of our hypothetical locator units separated by a considerable distance. The location and altitude are then determined by the triangulation methods of everyday navigation. It is also possible to determine the distance from a single locator to a plane by the method used in the terrain clearance indicator developed several years ago by Bell Laboratories.
This article was originally published in the "Aviation Radio Dialing the Air Waves with Craig Walsh" column in the September, 1941, issue of Aviation magazine, vol 40, no 9, pp 95, 140.
The original article includes a thumbnail portrait of the author, a photo of a British plotting room, and a diagram showing use of two RADAR stations for triangulation.
Photo credited to International