Often when one thinks about determining location using RFID what pops into mind is RTLS, or Real-Time Location Systems. RTLS use active RFID tags and transponders to determine the location of objects, often to very high accuracy levels.
However, the concept of location is intrinsic to asset tracking in general, and there are three major ways to determine the location of an object.
By Association
Objects often have a known association with each other. Take a pallet of cases for example. If we know that certain cases of goods, which are tagged with RFID labels, are contained in a certain pallet, which also is tagged, then whenever we detect the pallet, we know the location of all the cases. We don't need to look for each case -- we can just find the pallet and update the location of all associated objects.
By Proximity
In the field of RFID there is a type of RFID reader known as a fixed reader. It's called fixed because we put it in a known location, like bolted to a wall. (A known location is one we can find on a map.) The RF signals from that reader have a range. If that reader detects transient tags -- tags that move around -- we know those tags are proximate to that reader, so we know their location.
Passive RFID has a relatively short range, so the location accuracy of proximate tags is about fifteen feet, which is fine for many purposes. You might think that it's difficult to locate a tag proximately with active RFID. However, active RFID transponders (and recently, passive transponders as well) provide a measurement called Receive Signal Strength Indication, or RSSI. Since signal strength declines with the square of distance, RSSI is a measure of distance. So RSSI can help us determine how close a transient tag is to a known location, and give us a tie-breaker between two locations when a tag has been detected by both.
By Inference
If we have more than one transponder that can detect a transient tag, we can use the known locations of the transponders to infer the position of the tag. With three or more transponders this is known as trilateralization. We can use RSSI to give us an idea of how far away the transient tag is from each transponder, but RSSI becomes less accurate as distance increases. A better measurement is Time Difference of Arrival, or TDOA (also known as Time of Flight). TDOA can allow for very accurate distance calculations, but it requires very accurate equipment, because we are working with time intervals of nanoseconds, or billionths of a second!
