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

By RFID Journal

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Ask The ExpertsCan an RFID Reader Interrogate Two Passive Tags and Accurately Calculate the Distance Between Them?
RFID Journal Staff asked 10 years ago
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

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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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