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Can an RFID Reader Interrogate Two Passive Tags and Accurately Calculate the Distance Between Them?

Posted By RFID Journal, 01.02.2013
If so, can this be accomplished to within less than 1 centimeter (0.4 inch), and can the system consequently forward the collected information to a controller?

—Name withheld

The short answer is no. Generally speaking, passive radio frequency identification systems can tell you only that an RFID transponder is located within a read field. Some ultrahigh-frequency (UHF) passive RFID readers do employ a received signal strength indicator (RSSI) to determine how far a tag is from a reader antenna. This is mainly used by handhelds to indicate that someone is approaching a tag being sought. A stronger signal from a tag indicates that a user is closer to that tag.

However, the RSSI value is very imprecise. There is no way to determine with certainty that a specific RSSI value indicates a distance of, say, 3 feet 2 inches. The signal strength can be influenced by numerous factors, particularly something known as multi-path. The signal might bounce off a metal floor and increase the RSSI value, or the signal from the floor might cancel the signal from the tag, giving little to no RSSI value.

Active ultra-wideband (UWB) systems utilize different frequencies to offset the effects of multi-path signals. These are active RFID systems designed to operate over longer distances than passive systems, and even UWB systems are accurate only to within a distance of 15 to 20 centimeters (5.9 to 7.9 inches).

—Mark Roberti, Founder and Editor,
RFID Journal

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