Real-time locating systems (RTLS), also known as real-time tracking systems, are used to automatically identify and track the location of objects or people in real time, usually within a building or other contained area. Wireless RTLS tags are attached to objects or worn by people, and in most RTLS, fixed reference points receive wireless signals from tags to determine their location. Examples of real-time locating systems include tracking automobiles through an assembly line, locating pallets of merchandise in a warehouse, or finding medical equipment in a hospital.
The physical layer of RTLS technology is often radio frequency (RF) communication. Some systems use optical (usually infrared) or acoustic (usually ultrasound) technology with, or in place of RF, RTLS tags. And fixed reference points can be transmitters, receivers, or both resulting in numerous possible technology combinations.
RTLS are a form of local positioning system and do not usually refer to GPS or to mobile phone tracking. Location information usually does not include speed, direction, or spatial orientation.
Origin
The term RTLS was created (circa 1998) at the ID EXPO trade show by Tim Harrington (WhereNet), Jay Werb (PinPoint), and Bert Moore (Automatic Identification Manufacturers, Inc., AIM). It was created to describe and differentiate an emerging technology that not only provided the automatic identification capabilities of active RFID tags, but also added the ability to view the location on a computer screen. It was at this show that the first examples of a commercial radio based RTLS system were shown by PinPoint and WhereNet. Although this capability had been utilized previously by military and government agencies, the technology had been too expensive for commercial purposes. In the early 1990s, the first commercial RTLS were installed at three healthcare facilities in the United States and were based on the transmission and decoding of infrared light signals from actively transmitting tags. Since then, new technology has emerged that also enables RTLS to be applied to passive tag applications.
Locating concepts
RTLS are generally used in indoor and/or confined areas, such as buildings, and do not provide global coverage like GPS. RTLS tags are affixed to mobile items to be tracked or managed. RTLS reference points, which can be either transmitters or receivers, are spaced throughout a building (or similar area of interest) to provide the desired tag coverage. In most cases, the more RTLS reference points that are installed, the better the location accuracy, until the technology limitations are reached.
A number of disparate system designs are all referred to as "real-time locating systems". Two primary system design elements are locating at choke points and locating in relative coordinates.
Locating at choke points
The simplest form of choke point locating is where short range ID signals from a moving tag are received by a single fixed reader in a sensory network, thus indicating the location coincidence of reader and tag. Alternately, a choke point identifier can be received by the moving tag and then relayed, usually via a second wireless channel, to a location processor. Accuracy is usually defined by the sphere spanned with the reach of the choke point transmitter or receiver. The use of directional antennas, or technologies such as infrared or ultrasound that are blocked by room partitions, can support choke points of various geometries.
Locating in relative coordinates
ID signals from a tag are received by a multiplicity of readers in a sensory network, and a position is estimated using one or more locating algorithms, such as trilateration, multilateration, or triangulation. Equivalently, ID signals from several RTLS reference points can be received by a tag and relayed back to a location processor. Localization with multiple reference points requires that distances between reference points in the sensory network be known in order to precisely locate a tag, and the determination of distances is called ranging.
Another way to calculate relative location is via mobile tags communicating with one another. The tag(s) will then relay this information to a location processor.