Radio Tags and Transponders 16.1. Principle of Operation

Transponders were originally electronic circuits that were attached to some item whose position or presence was to be determined. The transponder operated by responding to an interrogation request received from an interrogator, either by returning some data from the transponder such as an identity code, or returning the original properties of the signal received from the interrogator with a minimum time delay. Because the interrogator signal was generally much stronger than the returned signal, the former would swamp the latter unless some characteristic of the response was changed. This was generally the frequency, but it could be polarisation. 16.2. History and Applications Transponders were developed for aircraft in WWII for identification friend or foe (IFF) application. Both commercial and military aircraft still use this technology. Most air traffic control centres rely on the range and altitude information returned by commercial aircraft rather than raw radar information. Since the Gulf War during which unacceptably large numbers of personnel were killed by their own side, transponder technology called Battlefield Identification System (BIS) has been developed for ground vehicles. It relies on interrogator – transponder technology operating at 38GHz. Another important application of the transponder has been in the measurement of distance. Here, the interrogator sends a signal to the transponder which immediately responds on another frequency. By using the time of flight, the range can be determined with great accuracy. This technique is more accurate than using the skin echo as the ultimate signal to noise ratio remains very high. A system called the Tellurometer (see Chapter 5) used this technique with improvements to measure ranges of hundreds of kilometres to an accuracy of a few cm