Would it be possible to use RFID chips to track the positions of a large number of items in relation to each other as they moved in a tight group?
Probably not. Passive RFID transponders can tell you that a tag (or many tags) is (or are) within a read field. Some systems can give you information about how far away a tag is from an antenna, but passive systems are not very precise in pinpointing an object's location, due to something called multipath. The signal from the tag can bounce off a lot of different things, making it difficult to precisely determine location. When you pack a lot of tags into a small area, the problem is compounded because tags can block a reader's energy from reaching other tags.
Ultra-wideband (UWB) active RFID systems are the most precise in determining location accuracy. These battery-powered tags broadcast a signal at regular intervals and use a variety of frequencies to overcome the multipath problem. The tags are about the size of a matchbox (or larger). These systems are accurate to within 10 centimeters. So if you had a lot of tags in a small space, it would be hard to determine how they were moving in relation to one another.
—Mark Roberti, Founder and Editor, RFID Journal
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