Can RFID Pinpoint a Moving Object’s Location?

I was wondering how accurate the technology is when tracking moving objects within a well-defined space. Say I have a rectangle measuring 15 feet by 150 feet, and there are 16 objects within that rectangle. How reliably would RFID be able to tell me which of those objects was the closest to a certain point? I was hoping maybe you could direct me to some information that would clarify this. Thanks.

—Matt (Denver, Colo.)

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Matt,

This is not a simple question to answer without knowing more about what you are specifically trying to achieve, as well as the nature of the environment involved. Many considerations must be made when choosing the proper radio frequency identification system—and, as in most situations, there are often tradeoffs. One solution might offer greater location accuracy than another, but at a greater cost. That said, I'll try to answer your question as best I can.

Generally speaking, an active ultra-wideband (UWB) RFID system will be able to determine where a tagged object is located to within a few inches. UWB tags can be fairly expensive, however, so if you are tagging a large number of objects, the system could become costly.

A passive ultrahigh-frequency (UHF) RFID solution will provide about 15 to 30 feet of read range, depending on which tags and readers you deploy, as well as other factors. UHF systems can typically tell you that a tag is within a defined read field, but not specifically where within that field. Given that many applications require you to know where a tag is located in the read field, some companies have developed solutions that deliver greater location accuracy.

Mojix, for example, uses a phased-array antenna system to locate passive tags in three-dimensional space, usually to within about 3 square feet (see Mojix Takes Passive UHF RFID to a New Level). And Impinj recently released its Speedway xArray antenna system, which is mounted overhead and provides some location information, though not to within a few square inches (see Impinj Unveils New UHF Readers for RTLS Applications, Embedding in Other Devices).

Passive systems utilize something called a Received Signal Strength Indicator (RSSI) to determine how close an object is to an RFID interrogator. RSSI can't tell you that a tag is, say, 8 feet 11 inches from the reader, because signal strength can be affected by waves bouncing off the floor or ceiling, but it can tell you that a tagged object is getting closer or further away. So the RSSI for an object closer to the reader within the same environment should be stronger than for one located further away. However, it would be difficult to distinguish the difference in location of tags spaced an inch apart.

I know that's a rather long-winded answer to your question, but I wanted to provide some clarity regarding what can be achieved from implementing a radio frequency identification solution, in terms of determining the locations of tagged objects. If you would like to speak to me confidentially via telephone so that I can understand more about what you are attempting to do, I would be happy to have a brief conversation and point you in the right direction. You can reach me via our contact page.

—Mark Roberti, Founder and Editor, RFID Journal