RTLS Tag Types
Real Time Location Systems (RTLS) use a variety of tags defined by the technology that reads them. Active or passive RFID tags can also be used in an RTLS.
Ultra-wide-band (UWB) readers transmit a very wide pulse over a GHz of spectrum. The readers then listen for chirps from the ultra-wide-band tags. These tags have a spark-gap-style exciter that generates a small pulse within them, which creates a short, coded, very wide, nearly instantaneous burst. The readers then report very accurate time measurements from the tags back to a central server. UWB systems are most commonly used inside of barcode scanners for use in warehouses and other sophisticated supply chain applications. When someone goes to scan a box during inventory, pulling the scanner's trigger both reads the barcode and sends out a UWB pulse. The system then localizes precisely where the item is —down to one centimeter —based on that pulse. If you are looking at an ultra-wide-band system, there are a few things to consider. First, all the frequency transmitted gives you tremendous accuracy, but has a weak signal so the frequency doesn't interfere with electronics in the area (i.e., cell phones, Wi-Fi, etc.). This means the range of ultra-wide-band is pretty low. Second, it requires careful calibration, which makes installation complex. And finally, if you use a UWB system, you need a good data connection, because the system is passing a lot of real-time information around. Thus, you need to synchronize all the readers together with very stable and low-jitter IP networks.
A highly energy-efficient protocol first introduced in 2010. It allows devices to run for several years on tiny coin-cell operated batteries. Due largely to the simplicity of the technology, Bluetooth-based beacons can be created quickly and for a lower cost than competitors.
Infrared tags are relatively simple technologically, if somewhat complex in infrastructure. They use light to transmit codes, which are then picked up by ceiling-mounted readers. This makes them more foolproof when compared to radio-based systems. Systems that use radio waves will occasionally pick up a signal from another room, leading to a false positive. Light waves, however, can’t pass through walls, so if the system says an item is in a certain room, it is undoubtedly in that room. As an aside, it’s not common to see infrared RTLS by itself; instead, they’re often paired with either an active RFID or a Wi-Fi system to minimize the inaccuracy issues of those symptoms.
Passive RFID tags are very short-ranged and require many high-power readers in order to function as an RTLS. They’re common in airports (for luggage tracking), shipping facilities (for box and pallet tracking) or in stores and libraries (for anti-theft and point-of-sale metrics). They are less common in the medical field, due to concerns about having too much concentrated RF energy, but there are some exceptions. For example, KitCheck creates boxes that serve as high-power readers for pharmaceutical trays. The items on the tray are marked with passive tags, so the box can read them and tell you which items are missing. The biggest benefit to passive RFID tags is that they are, by far, the cheapest RTLS option.
Active RFID tags transmit a short-range radio signal. Readers pick up this signal and measure the received signal strength (through either a one-to-one match or a trilateration between multiple readers) to get a location for the asset. Some active RFID systems like AirFinder use iBeacon tags for a few reasons. First, they're ubiquitous and can be found anywhere. Second, they're inexpensive and only cost between $2 and $10 apiece. Third, iBeacon tags aren't proprietary. But with all that said, there's nothing special about using a Bluetooth Low Energy (BLE) tag of any sort, as any low-power radio tag will do. One of the considerations for active RFID is that readers may pick up signals through walls or floors, making it more prone to false positives than, say, an infrared location system may be. A great deal of the hard work on our AirFinder system has gone into creating sophisticated algorithms to calm down the floor and room hopping.