When American troops were occupying Germany after the surrender they were somewhat amazed at their discovery almost everywhere they looked of small glittering objects -- narrow strips of bright metal foil which lays on the fields, in the roads, on the rooftops, and hung from the trees like the familiar tinsel on Christmas trees.
They didn't then that this seemingly innocuous "tinsel" was a product of one of the great secret projects of the war, the development of countermeasures against enemy radar. The full story of this program, which cost more than 24 millions dollars and which is estimated to have prevented the loss of more than 450 bombers and 4,500 lives in the Eighth Air Force alone, was released yesterday by the War and Navy Departments.
Equipment Designed Here
Design of the equipment to carry out these activities was the task given to the Harvard Radio Research Laboratory when it was secretly set up in March, 1942, as the chief research center on radar countermeasures. Operating in a converted wing of the huge Biological Laboratory, the R.R.L., which employed some 800 technicians and spent more than 15 million dollars on research, developed more than 150 anti-radar devices for the Allied armed forces.
By the end of the war, virtually every American heavy bomber and naval vessel carried anti-radar equipment designed by the R.R.L; the United States and Great Britain spent more than 175 million dollars for R.R.L. designed equipment.
Two General Purposes
Radar countermeasures have two general purposes: one is to blind the enemy's radar, or trick it into reporting misleading information; the other is to get information from his radar and use it, as radar itself is used, to locate the enemy, with the result that the enemy's radar boomerangs by revealing his position.
The first step in dealing with enemy radar is to locate it and determine its wavelength or frequency. This reconnaissance is done by "ferret" planes, specially equipped with electronic "search receivers," which pick up and locate operating enemy radar like stations on a home radio set.
These detectors have a great advantage in range over Radar itself, which is limited to the distance at which it can get back a detectable echo of its initial signal. Thus American use of search receivers often deprived the enemy of the use of their own radar, particularly in submarines, because they shut it off to avoid detection.
Once the enemy radar installation has been located and identified, it must be jammed to make it unusable. Two primary methods were developed in action and perfected at the R.R.L.: electronic jamming, and aluminum foil, called "Window."
Electronic jammers are high-frequency radio transmitters which bombard a radar installation with radio waves of the radar's own frequency, thereby obscuring the detectable echo on the radar viewing scope. Allied bombers and ships were armed with these electronic jammers, labeled as "Carpet", for use in foiling detection. For defensive purposes, particularly against enemy air-borne radar, very powerful long-distance, ground-based jammers, called "Tuba", were used.
The other major jamming method, "Window", involves the seemingly simple procedure of dropping huge quantities of light aluminum foil from planes on bombing missions. It was discovered the small strips of aluminum, which is an excellent radio reflector, would, if cut to one half a radar's wavelength, send back a disproportionately strong echo.
A six-ounce bundle of six thousand such trips, called "Chaff" by the American, dropped from a plane and scattered in the air, gives an echo resembling that of three bombers on a radar scope. Large numbers of these small bundles scattered through the sky can effectively screen whole formations of bombers from enemy radar; similarly, the "Chaff" dropped by one plane can present the illusion of a mass raid where there is no raid at all.
The Germans managed to devise some counter-jamming measures, but a combination of "Carpet" and "Window" jamming was never able to answer. So effective was the Allies' jamming system--it reduced German anti-aircraft efficiency by 75 percent--that by the end of the war almost 90 percent of Germany's high-frequency radio experts, some 7,000 men, were diverted from other urgent work to the single job of finding a way to prevent jamming of German radar.
The Germans were not without their own radar countermeasures. They jammed American radar badly at the Anzio beach-head, and it was German jamming that enabled the Seharnhorst and Gneisnau to escape through the English Channel, by blocking out British radar along the whole Channel coast. After these early successes, however, the Germans and Japanese never again succeeded in seriously jamming Allied radar, because our radar was adapted to much higher frequencies -- microwaves -- which are much harder to jam.
All through the war, the Harvard and M.I.T. laboratories, just a mile apart, carried on a friendly but deadly serious rivalry: the Tech Radiation lab developed better and better radars, and the Harvard R.R.L. produced better and better jammers. From the roof of the Bio Lab, the Harvard technicians took delight in jamming Tech radars across the roof tops of Cambridge.
"Window" experiments were conducted secretly out at sea, but trials of electronic jamming were harder to conceal. Once the researchers tied up the whole Boston police communication system for half an hour by accidentally jamming the patrol cars' short-wave radios. On another occasion, the New Haven Railroad electrified lines were stopped in their tracks by a form of Window called "Rope"--rolls of aluminium tape, which fell from planes and draped themselves across high-tension wires, short-circuiting them