Real-time locating

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This page specifically concerns operational aspects of RTLS. For methodology issues see locating engine. For technology issues see wireless.

Real Time Locating Systems (RTLS), according to international standards[1], are used to track and identify the location of objects in real time using simple, inexpensive nodes (badges/tags) attached to or embedded in objects and devices (readers) that receive the wireless signals from these tags to determine their locations. RTLS typically refers to systems that provide passive (automatic) collection of location information.

According to ISO/IEC JTC1 SC31 and the accepted standard ISO/IEC 19762-5 [2] an RTLS (real-time locating system) is a combination of wireless hardware and real-time software that is used to continuously determine and provide the real time position of assets and resources equipped with devices designed to operate with the system.

Disambiguation of terms

Information with products for locating often uses the term "positioning". This may be understood as equivalent, but positioning normally includes the effecting of a new position. Locating never effects anything to the object observed, but just observes. Locating is indeed sensing, and therefore not positioning. Moreover, in terms of topography, a position includes not just the location, but also orientation and in dynamics also the other determinants describing motions. These sets of data altogether describe the position respectively in the Hamiltonian sense, the state determinants in phased space. When searching for comparable technical approaches make sure to cover both variants "locating" and "positioning" in wording.

Disambiguation of systems designs

RTLS as standardized explicitly excludes passive RFID indexing (radio frequent 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. So shall this text.

RTLS systems apply typically in confined areas, where the required reference points would be equipped with wireless anchor nodes.


RTLS serves in operational areas for logistics and other services, as e.g. stock grounds or storehouses, and for servicing areas in clinics and industrial plants

  • 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
  • etc.


RTLS is operated by a server function and by means of mobile nodes fixed to items or carried by persons or vehicles, and where wireless anchor nodes encircle the operational space. RTLS requires a common illumination of the surveyed areas and direct view between the anchor nodes and the mobile nodes.

The challenging task of real-time locating involves determination or best guessing real time locations without hampering delays (latency time) and a minimum of subsequent computations (iteration). Measurement may be performed with distances only, angles only or both. However, location information is never obtained in a single step.


The RTLS capability is elsewhere referred as Real Time Location Systems, Local Positioning Systems or simply Positioning Systems. Generally speaking, locating or localizing is the determination of the locality of an object. Description of locality is the location. Ranging is the prerequisite for locating, hence delivering angles or distances between locations. Any current location of any existing object is always real. The task is to obtain such information as long as it is valid, i.e. performing in real time. Hence the current or momentary location may be seen as a real time location.

Effecting a change in location in common terms is positioning. Determining a position may be not just locating but also determining bearing. Hence, positioning may include just altering location only, or bearing only or both at the same time. For other variation of wording see the glossary below.

Traditional nomenclature and RTLS

The usage of a common wording for the operational tasks with locating systems is somewhat hampered by tradition of scouts, seafarers, artillerymen and other military and nautical disciplines:

  • Locating is the new and precise term for a cooperative approach to determine the location of an object with reference e.g. to locations of other yet known objects.
  • Fixing a position is just the very same with the additional requirement to determine such location with reference to absolute coordinates either on the surface of the globe or in any confinement.
  • Detecting and Ranging, as with RADAR or LIDAR, is the term that since WW II describes the non-cooperative locating of any object that might be friend or foe.
  • IFF, identification of friend or foe, is just the identifying of such object after detection to segregate friend or foe and thus requires detection as a prerequisite, but does not necessarily include detection. Possibly IFF may be a beacon based function not requiring cooperativity.
  • Fixing a bearing by angular determining is some part of navigating that helps to plan the next course towards the next waypoint, thus neglecting the absolutely expressed distance to such waypoints, if not required.
  • Dead reckoning is navigating on a halt, providing a relative position fix, a set of bearings and a speed log and thus neglecting the absolutely expressed coordinates of such waypoints, if not required.
  • Navigating on the drive requires some starting information from where a vehicle drives to a determined destination and thus requires locating the starting point.
  • Navigating, in generalising this term, may be just planning the next distance towards the next waypoint, thus neglecting the absolutely expressed coordinates of such waypoints, if not required.
  • Surveying is some type of locating of points of interest on the surface of the globe, but generally addresses such points that do not move.

Standardized RTLS

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 (see Methodologies Used in Real Time Locating Systems listed below)
  • 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. For details see Real Time Locating Systems.

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.

Each interested party may deliberately assess the suitability of one of these options. This entire page is written from the perspective of the ISO/IEC 24730 definition of RTLS, which excludes many common methods of unilateration and other guessing approaches. Other, broader definitions for location aware systems exist and are commonly adopted in the market.

Other types of technologies used in location aware systems design

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). Types of technologies apart from the definitions in ISO/IEC 24730-1 used in the context of location aware systems include:

  • Active Radio Frequency Identification (Active RFID)
  • Active Radio Frequency Identification - Infrared Hybrid (Active RFID-IR)
  • Infrared (IR)
  • Optical Locating[verification needed]
  • Low-frequency Signpost Identification
  • Semi-Active Radio Frequency Identification (semi-active RFID)
  • Radio Beacon
  • Ultrasound Identification (USID)[verification needed]
  • Ultrasonic Ranging
  • Ultrawide Band (UWB)
  • Wireless Local Area Network (WLAN, Wi-Fi with RSSI capability)

But which technique provides the best solution for a specific (location) problem? A general model for this problem has been constructed at the Radboud University of Nijmegen [3].


Ranging, as a special term for metering a spatial distance, is the prerequisite for locating. Metering a bearing angle, i.e. angulating is the other alternative as a prerequisite for locating. For either of both options the term metering is used instead of the generinc term measuring.

Determining the distance may be either a non cooperative scanning process, as with RADAR or LIDAR, or an either uncooperative distance metering process, as with laser, or a cooperative direct distance metering process, as with RTLS. In case of scanning with a rotatory beam sweep, the sweeping system may obtain an overall image as a model of the whole scene with repetitive sweeping. 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 metering with a single beam targets only the object to be metered, by targeting it e.g. with a laser. This method requires additional information about the direction to which the beam points.

The remaining method is omni-directional transmission with a telegram containing an address code. Then the addressed object only responds cooperatively to the request. Hence the correspondent target delivers either a measurement travel time based on synchronized clock time. Or the addressed receiver enables the measurement of travel time from the transmitter position by reflecting the received signal to the transmitter and delivers the travel time for the double distance.

After completing metering e.g. by ranging, the location may be computed.

Principles of ranging

To ease understanding, a simple segregation is between TOA and TDOA. Generally, these two different principles of metering apply when metering travel time of radio waves in the atmosphere:

  • Multi-Lateration 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

(TOA time of arrival concept).

  • Multi-Angulation 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).

Methodologies used in location aware systems designs

The following methodologies used in location aware systems designs include a variety that does not comply with definitions given in ISO/IEC 24730-1:

  • Angle of Arrival (AoA)
  • Line-of-sight (LoS)
  • Time of Arrival (ToA)
  • Time Difference of Arrival (TDoA)
  • Received Channel Power Indicator (RCPI)
  • Received Signal Strength Indication (RSSI)
  • 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 [1], 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 [2].

RTLS performance measurement terminology

A wide variety of concerns applies for RTLS. For example this segregation applies:

  • Granularity - the degree to which reported location information is accurate
  • Latency - the degree to which reported location information is timely
  • Integrity - the degree to which reported location information is repeatable over time

The preferred choice of criteria strongly depends to the chosen application.


See also

External links


Malik, Ajay (2009). RTLS For Dummies. Wiley. 384. ISBN 978-0-470-39868-5. 

  • Indoor Geolocation Using Wireless Local Area Networks (Berichte Aus Der Informatik), Michael Wallbaum (2006)
  • Local Positioning Systems: LBS applications and services, Krzysztof Kolodziej & Hjelm Johan, CRC Press Inc (2006)