The next early use of RFID was the Identification-Friend-or-Foe (IFF) system. In 1934, H. E. Wimperis from the British Air Ministry approached Dr. Watson-Watt, the head researcher for the Radio Research Station at Ditton Park, about creating a death ray. At the time, Wimperis informed Watson-Watt that the Germans had such capability, and he felt the British Air Ministry was falling behind in the weapons technology battle.
Dr. Watson-Watt actually tested such a device in a laboratory and knew the enormous amount of energy it would take to create a device that would satisfy Wimperis's request. Watson-Watt felt that, instead of a death ray, research should focus on the ability to identify aircraft through the use of radar and IFF. Because of this, Dr. Watson-Watt promptly responded to the query from Wimperis with the following:
Meanwhile attention is being turned to the still difficult, but less unpromising, problem of radio detection, and numerical considerations on the method of detection by reflected radio waves will be submitted when required. (The Detection of Aircraft by Radio Methods, by Watson-Watt, February 12, 1935)
The British invented the IFF transponder around 1939 and used it during the Battle of Britain in World War II to distinguish between friendly and enemy warplanes. In 1940, the British put an active system (designated the Mk I) into service. The Mk I used a receiver aboard each aircraft that broke into oscillation and acted as a transmitter when it received a radar signal. Because of the variety of radar frequencies used, it had to be mechanically tuned across the radar bands in order to be triggered by any radar that was illuminating it. This mechanical tuning requirement and other factors limited its performance.
However, before this technology, ground personnel had to identify aircraft only by silhouettes displayed in individual aircraft-recognition handbooks. In low-light or night conditions, identification was difficult, if even possible at all.
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