Most RFID systems can not be utilized for distance detection, because signals from a tag to a reader often take multiple paths to the reader antenna. This makes calculating a tag’s precise distance from a reader challenging. The one exception involves ultra-wideband (UWB) systems, which are able to overcome multipath issues since UWB interrogators communicate with tags over a wider spectrum (exactly why this is so is beyond my limited knowledge of RF engineering, but I would welcome a knowledgeable reader to provide a more in-depth explanation below).
I do know that some researchers are working on passive ultrahigh-frequency (UHF) systems that can determine a tag’s distance from a reader at a fairly close range, for which the multipath problem is not as big of an obstacle. So, for instance, a passive tag on a container could be read by an interrogator on a crane picking up that container, and determine when the reader antenna is four feet from the container tag, then three feet, then two feet and so on.
To my knowledge, however, no precise system is currently available on the market.
—Mark Roberti, Founder and Editor, RFID Journal
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