Can Passive RFID Readers Measure Time of Arrival?

Are there any models that can measure TOA from a particular responding tag?

—Name withheld

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Passive ultrahigh-frequency (UHF) transponders measure received signal strength (RSS), and can determine a tag’s relative distance from a reader based on how strong the tag’s signal is. This is how, for example, the “Geiger Counter” feature in Motorola‘s handhelds functions. As RSS grows stronger, the unit beeps more frequently, thereby indicating that the user is getting closer to a particular tag.

To my knowledge, passive UHF readers do not measure TOA, since the tag constantly responds to the reader. This is necessary, in part, in order to singulate tags using anti-collision algorithms. If I’m wrong, or if there are any experts on passive UHF readers that can provide additional information, visitors are invited to post their comments below.

—Mark Roberti, Founder and Editor, RFID Journal