RTLS Real Time Locating Systems

Real-time locating systems (RTLS) are a type of local positioning system that allow 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.^[1] 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.

System 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.

Operation

For RTLS to function, the location of tagged items must be determined either by a central processor or by an embedded mobile computing facility. Locating is generally accomplished in one of the following ways

• 1. ID signals from nodes are identifiable to a single reader in a sensory network thus indicating the coincidence of reader and nodes.
• 2. ID signals from nodes are picked up by a multiplicity of readers in a sensory network and a position is estimated using one or more locating algorithms
• 3. Location signals from signposts with identifiers are transmitted to the moving nodes and are then relayed, usually via a second wireless channel, to a location processor.
• 4. Mobile nodes communicate with each other and perform metering distances.

Examples one (1) and three (3) have much of the same characteristics. They typically require that a node be assigned at a time to a single reader/signpost. Separation from overlapping readers/signposts is roughly provided by RSSI or Physical Space Division (walls/floors/ceilings). Readers/signposts are often associated with highly stable location boundaries (i.e. a room or room division). In these examples, locations are listed as "Current Location" or "Last Known Location."
Example two (2) requires that distances between nodes in the sensory network be determined in order to precisely locate a node. In this instance, the determination of the location is called Localization. The location is calculated through Trilateration or Multilateration from the determined distance between the nodes or through Triangulation from the determined angles between nodes. The determination of distances is called Ranging.

Application

RTLS serves in operational areas for logistics and other services,as for example stock grounds or storehouses, and for servicing areas in clinics and industrial plants. Tasks done by a RTLS include:

• to combine identity and location of any type of items or objects
• to combine identity of items with location of lifter placing the items
• to ensure permanent availability of proper information about temporary placement
• to support notification of placing of items
• to prove proper manning of operational areas
• to prove consequent evacuation of endangered areas
• to make marshalling staff dispensable

Standards

ISO/IEC

The basic issues of RTLS are standardized by the International Organization for Standardization and the International Electrotechnical Commission, under the ISO/IEC 24730 series. In this series of standards, the basic standard ISO/IEC 24730-1 identifies the terms describing a form of RTLS used by a set of vendors, but does not encompass the full scope of RTLS technology.
Currently several standards are published or under discussion:

• ISO/IEC FDIS 19762-5 Information technology AIDC techniques — Harmonized vocabulary, Part 5 — Locating systems
• ISO/IEC 24730-1:2006 Information technology real-time locating systems (RTLS) Part 1: Application program interface (published).
• ISO/IEC 24730-2:2006 Information technology real-time locating systems (RTLS) Part 2: 2,4 GHz Air interface protocol (published, WhereNet/Zebra approach).
• ISO/IEC WD 24730-5 Information technology real-time locating systems (RTLS) Part 5: (drafted ISO/IEC standard out for balloting in 2008, Nanotron approach).

The other proposals ISO/IEC 24730-3 and ISO/IEC 24730-4 had never left the stage of intention. For copies of these documents see references.
These standards do not stipulate any special method of computing locations, nor the method of measuring locations. This may be defined in specifications for triangulation or any hybrid approaches to trigonometric computing for planar or spherical models of a terrestrial area.

ANSI standards

• ANS/INCITS 371: Information Technology – Real-Time Locating Systems (RTLS).

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)

Privacy concerns

RTLS may be seen a threat to privacy, if applied to persons, either directly or parasitically. The requirement therefore is to describe the purpose and the conditions of operation to those affected and to advertise for expressed agreement. Recent adjustment of jurisdiction leads to more careful assessment of needs and options. The newly declared human right of informational self-determination de:Informationelle Selbstbestimmung, i.e. to prevent one's identity and personal data from disclosure to others, covers disclosure of locality as well. Base of discussion is very similar to disclosure of personal data for passing immigration at US airports: Balancing threat and burden [4].

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).^[2] Methods include:

• Active radio frequency identification (Active RFID)
• Active radio frequency identification - infrared hybrid (Active RFID-IR)
• Infrared (IR)
• Optical locating,^[3][4]
• Low-frequency signpost identification
• Semi-active radio frequency identification (semi-active RFID)
• Radio beacon,^[5][6]
• Ultrasound Identification (US-ID) ^[7]
• Ultrasonic ranging (US-RTLS) ^[8]
• Ultra-wideband (UWB) ^[9]
• Wide-over-narrow band ^[10]
• Wireless Local Area Network (WLAN, Wi-Fi)^[11]
• Bluetooth,^[12][13]
• Clustering in noisy ambience,^[14][15]
• Bivalent systems ^[16]

A general model for selection of the best solution for a locating problem has been constructed at the Radboud University of Nijmegen.^[17] Many of these references do not comply with the definitions given in international standardization with ISO/IEC 19762-5 ^[18] and ISO/IEC 24730-1.^[19] However, some aspects of real-time performance are served and aspects of locating are addressed in context of absolute coordinates.

Locations for sensors

Locating at choke points

There is class of most simple locating which applies no physical measurement at all, but just communicates at coincidence of transceiver and transponder. Then locating collapses to simple application of RFID technologies according to the equivalent standard.^[20] This is the only option to apply passive RFID tags for locating. Then the reach of the RFID reader determines the choke point. Hence accuracy is defined by the sphere spanned with the reach of the reader.

Locating in relative coordinates

Many references describe locating a relative coordinates. This is a valuable support for many operational needs, whereas the precision of the term RTLS is widely diluted to arbitrary interpretation. Such solutions may be referred as fuzzy locating.

Locating in absolute coordinates

The high precision of satellite navigation systems led to some snugness in setting the requirements for locating of objects. generally the determining of absolute coordinates is the most challenging approach. Such solutions may be referred as crisp locating.The difference to the qualities of relative coordinates may be easily sensed with indoor operations, where satellites are not commonly available for referring to global coordinates.

Locating in contiguity

A newer approach for locating defines a location just as the contiguous ambience of the person looking for something to be located. That is very similar to choke point locating, However, the accuracy may be much better tuned, as the reach is not influenced by the illumination of the tag with the reader, but just by the transmission power level of the active RFID tag as a beacon.
This is the easy option to apply active RFID tags for economised locating. Then the reach of the RFID receiver determines the base point. Hence accuracy is defined by the algorithm for varying the minimum reach of transmission of the beacon. Solutions ara available as very simple electronic leashes or in more complex designs.

Erratic effects in locating systems

Real-time locating is affected by a variety of errors. The major reasons are physical and may not be reduced by improving the technical equipment. The only escape is mathematical intelligence to improve.

None or no direct response

Many RTLS systems have a very mundane requirement: they require direct and clear wireless visibility. For those systems, where there is no visibility on the path from mobile tags to resident nodes there will be no result or a non valid result from locating engine. This applies to satellite locating as well as other RTLS systems such as angle of arrival and time of arrival. Fingerprinting is a way to overcome the visibility issue: If the locations in the tracking area contain distinct measurement fingerprints, line of sight is not necessarily needed. For example, if each location contains a unique combination of signal strength readings from transmitters, the location system will function properly. This is true, for example, with some Wi-Fi based RTLS solutions. However, having distinct signal strength fingerprints in each location typically requires a fairly high saturation of transmitters.

False location

The measured location may appear entirely faulty. This is a generally result of simple operational models to compensate for the plurality of error sources. It proves impossible to serve proper location after ignoring the errors.

Locating backlog

Real time is no registered branding and has no inherent quality. A variety of offers sails under this term. As motion causes location changes, inevitably the latency time to compute a new location may be dominant with regard to motion. Either an RTLS system that requires waiting for new results is not worth the money or the operational concept that asks for faster location updates does not comply with the chosen systems approach.

Temporary location error

Location will never be reported exactly, as the term real-time and the term precision directly contradict in aspects of measurement theory as well as the term precision and the term cost contradict in aspects of economy. That is no exclusion of precision, but the limitations with higher speed are inevitable.

Steady location error

Recognizing a reported location steadily apart from physical presence generally indicates the problem of insufficient over-determination and missing of visibility along at least one link from resident anchors to mobile transponders. Such effect is caused also by insufficient concepts to compensate for calibration needs.

Location jitter

Noise from various sources has an erratic influence on stability of results. The aim to provide a steady appearance increases the latency contradicting to real time requirements.

Location jump

As objects containing mass have limitations to jump, such effects are mostly beyond physical reality. Jumps of reported location not visible with the object itself generally indicate improper modeling with the location engine. Such effect is caused by changing dominance of various secondary responses.

Location creep

Location of residing objects gets reported moving, as soon as the measures taken are biased by secondary path reflections with increasing weight over time. Such effect is caused by simple averaging and the effect indicates insufficient discrimination of first echoes.