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It would depend on how precisely you needed to measure the distance from an object. Passive UHF RFID has a read range of 30 feet or so with standard tags and readers, and there are larger tags and phased-array antennas that can extend this read range to 100 feet or more. Radio waves can bounce off floors, ceilings and walls, however, as well as objects within a room, so it would be difficult to obtain precise distance readings using passive UHF RFID. If it was acceptable to know that a tagged object entered a read zone that was, say, 20 feet square, then passive UHF RFID might work for your application.
If you wanted to know that a person was precisely 100 feet from an object, then tagging both the individual and the item with an active ultra-wideband system would work. UWB overcomes the issue of radio waves bouncing off things and can locate an object to within a few inches or centimeters. You would need to install readers around the perimeter, tag the people and objects you wanted to track, and set up software rules to alert you if a person came within a set distance of an object.
If you have any further questions, feel free to email me at firstname.lastname@example.org.
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