Do any companies make tags that can report their positions?
Yes. In 2009, RFID systems supplier Savi Technology unveiled an intelligent hybrid tag combining active RFID, satellite communications and Global Positioning System (GPS) technologies. The tag is designed to track goods anywhere within a global supply chain, whether they are waiting within a warehouse, being loaded onto a ship or sitting in a desert at a bare-bones military outpost (see Hybrid Tag Includes Active RFID, GPS, Satellite and Sensors). This, however, would be true of a normal GPS device.
RFID tags do not know where they are, so they cannot report their location. They simply send their unique ID and other data stored on a chip to a reader. With real-time location systems that employ active RFID tags, readers receive signals from tags and pass that information on to a back-end software application that calculates the tag's location based on information provided by readers, such as time of arrival, angle of arrival, triangulation or other methods.
Passive ultrahigh-frequency (UHF) systems generally provide a figure known as the received signal strength indicator (RSSI), which tells you how strong a signal from a tag is. A stronger signal means the tag is closer. This is an imprecise method of locating tags, however, because the signal could, for example, bounce off metal objects and reach the reader, thereby increasing the RSSI. This makes it difficult to get a precise calculation of a tag's distance from a reader.
With passive systems, you generally create a zone or a portal. Then, as you read each tag, you can know it either is within the zone or has passed through the portal. There are a few passive UHF fixed reader systems, such as Mojix's Star system (see The Brightest Star) and Impinj's Speedway xArray reader (see Best in Show: RFID Gets xArray Vision), that use antenna arrays to determine location. But again, the location is determined by the reader, not reported by the tag.
—Mark Roberti, Founder and Editor, RFID Journal
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