RLS / Asset Management / Real-Time Locating Systems (from Wikipedia 2011)

Real-time locating systems (RTLS) are used to track and identify the location of objects in real time. Using simple, inexpensive badges or tags attached to the objects, readers receive wireless signals from these tags to determine their locations. RTLS typically refers to systems that provide passive or active (automatic) collection of location information.

Location information usually does not include speed, direction, or spatial orientation. These additional measurements would be part of a navigation, maneuvering or positioning system.

Origin
The term RTLS was created (circa 1998) to describe an emerging technology that not only provided the Automatic Identification capabilities of active RFID tags, but added the ability to see the physical location of the tagged asset on a computer screen. Although this capability had been utilized previously by military and government agencies, the technology had been too expensive for commercial purposes.

By the early 1990's, commercialization began at two healthcare facilities in the United States (Foote Hospital in Jackson, MI and Broward Children's Hospital in Pompano Beach, FL). These early adoptors are atrributed to real-time locating industry innovator Precision Tracking (Versus Technology, Inc.) and were based on the transmission and decoding of infrared light signals from actively transmitting tags.

Systems Designs
RTLS excludes passive RFID indexing (radio frequency transponder indexers) and Cellnet base station segment locators (location-based services) from the scope of the ISO/IEC approach to RTLS standardization as well as all beacon systems, that ping without request. RTLS systems apply typically in confined areas, where the required reference points would be equipped with wireless anchor nodes.

Ranging
Ranging, as a special term for measuring distance, is the prerequisite for locating. Measuring a bearing angle, i.e. angulating is the other alternative.

Determining the distance may be either a non cooperative scanning process, as with RADAR or LIDAR, or a cooperative direct distance measuring process, as with RTLS. A scanned beam may form an overall image as a model of the whole scene. In all other cases the image of the scene is rather selective.

The following step is extracting the distance information from the scanned image. Direct distance measurement with a single beam targets only the object to be measured, for example, with a laser. This method requires additional information about the direction of the beam. The remaining method is omni-directional transmission with a signal containing an address code. Only the addressed object responds to the request. The time required for the signal to reach the object can be used to calculate the distance. After completing the distance measurement, the location may be computed.

There are two different principles when measuring travel time of radio waves:

- Trilateration derives the travel time of a radio signal from a metering unit, and measures and computes the distance with the relation of light speed in vacuum, the (Time of arrival concept).
- Triangulation derives the travel time of a pair of synchronous radio signals from a metering unit with two transmitters, and measures and computes the difference of distance with the relation of light speed in vacuum as an angle versus the baseline of the two transmitters (TDOA time difference of arrival concept).

All the terms named here just apply to measurement concepts. All information about location is for services applied to mobile or portable or otherwise transportable objects. Location information may be relevant for managing interaction of persons with services as well.

-- Angle of arrival (AoA)
-- Line-of-sight (LoS)
-- Time of arrival (ToA)
-- Multilateration (Time difference of arrival) (TDoA)
-- Time-of-flight (ToF)
-- Two-way ranging (TWR) according to Nanotron’s patents
-- Symmetrical Double Sided – Two Way Ranging (SDS-TWR)
-- Near-field electromagnetic ranging (NFER)

Types of technologies used
There is a wide variety of systems concepts and designs to provide real-time locating. A good choice is listed in RTLS for Dummies by Ajay Malik (Wiley 2009). Methods include:

-- Active radio frequency identification (Active RFID)
-- Active radio frequency identification - infrared hybrid (Active RFID-IR)
-- Infrared (IR)
-- Optical locating
-- Low-frequency signpost identification
-- Semi-active radio frequency identification (semi-active RFID)
-- Radio beacon
-- Ultrasound Identification (US-ID)
-- Ultrasonic ranging (US-RTLS)
-- Ultra-wideband (UWB)
-- Wide-over-narrow band
-- Wireless Local Area Network (WLAN, Wi-Fi)
-- Bluetooth
-- Clustering in noisy ambience
-- Bivalent systems

A general model for selection of the best solution for a locating problem has been constructed at the Radboud University of Nijmegen. Many of these references do not comply with the definitions given in international standardization with ISO/IEC 19762-5 and ISO/IEC 24730-1. However, some aspects of real-time performance are served and aspects of locating are addressed in context of absolute coordinates.