Distance can be determined by employing something known as received signal strength indication (RSSI), but it is not extremely accurate. Essentially, the stronger the signal received from a transponder, the shorter the distance between that transponder and a reader antenna.
There is no precise correlation between RSSI and distance, however, because many factors can influence signal strength. For instance, if you are reading a passive ultrahigh-frequency (UHF) tag in a warehouse, and there are steel reinforced bars in the concrete floor, the signal might reflect off the floor and return to the reader. This would create more signal strength and suggest a closer distance than reading the same tag from the same distance in an area in which the signal does not reflect off the floor.
That said, RFID reader manufacturers are getting more sophisticated regarding how to use RSSI to provide better information about tag location in relation to a reader antenna.
—Mark Roberti, Editor, RFID Journal
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