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Real-time locating

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

According to ISO/IEC JTC1 SC31 and the accepted standard ISO/IEC 19762-5 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.

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 mitions. 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.

Here one should consider primarily rigour in physics to avoid ambiguity of terms: Signal travel time is a sound criterion for distance, strength or power level of the received signal is an alternate though much weaker estimation. Secondly the determintation of any location in n dimensions requires n+1 equations for unambiguous determination, all other approaches e.g. via reasoning leads to lesser reliability. Thirdly, a single location does not include direction. Any one direction may be obtained from at least two locations.

Identification however only provides the known data from an identifier and hence identifying concatenates to the location of the receiver and the coinciding receiver marker of the object, not the location of the object itself, nor the orientation of the object as an indicator of actual direction, nor the direction in which the object moves or the destination to which it is moving. Hence, declaring passive RFID as a means of absolute locating is equally not meaningful in absolute or comparative terms (references see e.g. , Hybrid solutions with active RFID however define a different class of locating, yet not compliant with ISO/IEC 19762-5.

Real-time locating systems (RTLS)

The further explanation 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 shall be equipped with wireless anchor nodes. For all applications, where anchor nodes cannot be placed to known locations or positions, GNSS satellite systems may apply that formally count as RTLS, but yet do not appear as international standards with ISO.

Purpose

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.

Requirements

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.

Discrimination of systems designs

The primary segregation of Real Time Locating systems approaches is upon the requirements of the user. The most common discrimination is with the qualities of

Granularity, which means the spatial resolution of alocation and the spatial separation of objects in a location

either to determine presence in an ambiance or confinement or
the best determining of mostly precise coordinates, down to the
precision with which the reports stick to known reference systems.

Actuality, which means the timely delivery with the aspects of

latency (waiting time until the system firstly responses) as well as
delay (time lag between passing a location and report on this event) and
frequency (time resolution between each two consecutive responses) and finally
jitter (local resolution under operational and ambient interference conditions)

Operability, which includes all other aspects and effects that might affect the operational value of the offered support, especially under the aspects of

benefit-to-cost ratio, which includes the direct impact on operation compared to share in total cost of operation from systems definition till annual cost of operation
dynamics of the presentation with reference to the situation that depicts not just the object located but also the ambiance where it resides.
suitability of the equipment on site of operation or with the person that carries the display for presenting the appropriate support to current demand.

In more advanced designs, server function may be allocated to each autonomous mobile node.

The definition given with the ISO/IEC standards postulate no quantities concerning precision. The application will be paid not by accuracy of any solution, but by the adequacy of the operational benefit to the total cost.

Classification RTLS vs. active RFID

Active RFID solutions make use of wireless anchor nodes in cooperation with mobile nodes. In such solutions, the key feature of measuring distances by travel time TOA or angles by differential travel times TDOA is not included. As common with wireless communications, radio signal strength of active transponders is an essential, but delivers no proper discrimination of location or position. However such systems designs work with RFID tags activated by anchor nodes, when physical confinements, as with walls, support the locating. Such approaches work properly, where and while mapping of propagation is not required with the systems design.

The definition given with the ISO/IEC standards, however, does not explicitly address an active RFID-type approach to indexing and metering, which is supported by some qualified suppliers.

Classification RTLS vs. RSSI

RSSI solutions make use of WLAN infrastructure and claim not requiring additional deployment of wireless anchor nodes and in cooperation with mobile nodes. As common with wireless communications, radio signal strength in infrastructures with operational wireless links is an essential, but not a sole solution for precise locating. Such approaches do not work properly, wherever and whenever the required mapping of propagation is unstable.

The definition given with the ISO/IEC standards, however, does not explicitly exclude the sole RSSI-type approach to metering, which is supported by some suppliers and dispraised by others.

Principles of RTLS process designs

RTLS is a wireless concept, as RFID, but RTLS goes beyond (see ISO/IEC FDIS 19762-5 ).

RTLS systems serve for sound approaches to overcome conventional search. With RTLS the operation is to locate known and cooperative objects disregarding the actual location, unless residing or moving in the operational area of the RTLS system. Compared RFID: Locating happens only, when the RFID tag is in the range of a RFID reader and proper separation must be provided to segregate one RFID tagged object from others.

RTLS segregates objects all time by coordinates of location (see ISO/IEC 24730-1 ). Separation of RTLS nodes is not required.

Locating with RTLS vs. search and find

Common behaviour in daily life and in professional life is either systems support or traditional staffed search and find: Missing objects have to be searched and to be found. In case nobody remembers the last occurrence, some marshal's or detective’s skills are required to find traces of occurrence to follow tracks. RTLS systems support shall reduce search times and make staffing for marshalling dispensable.

Referencing with a priori knowledge

Referencing precisely the actual placement is an approach of moderate cost concerning preparation. The special approach guarantees fast success, when a priori knowledge is applied. All objects properly referred are to be found easily, the safe solution monitors the placement and retrieves the known positions. RTLS may serve such approach automatically and with sufficient success. Application systems designers must take several features into account:

Oblique locating through the means of transportation, as e.g. fork lifters
Concatenation of identity information with location information
Tracking capabilities eith subsequent locating results
Latency time to make location information available
Precision of reproducible location information
Battery life time for mobile nodes without vehcle power supply

The single mobile RTLS tag is at least two magnitudes (100 times) more expensive than passive RFID tags. Just precision requirement is very costly, followed by speed requirements. All approaches using a priori knowledge and serving just an instantaneous locating will remain comparatively strong. With other concepts the conditional answers consume search time and raise cost for time, staff and generate solely weak response.

Systematic registering and easy retrieving with RTLS

The more advanced behaviour is register where objects become placed and retrieve the objects in that very place. Mandatory condition is that registering happens and nobody replaces the objects without. For all heavier or larger objects that require transportation means as fork lifters or van carriers unregisterede repositioning is rather improbable. RTLS may serve such approach automatically for pallets, horses, cradles, swap bodies and containers.

Automatic tracking and relocating with RTLS

The most advanced option with RTLS is to track objects and thereby collecting information about the last appearance thereof. That tracking allows for proper detecting actual location from the estimate calculated from the last fixes in conjunction with actual lateration results. Then even in case of weak measurement, other accidental misjudgement or missing or faulty manual registering, the actual location is stated easily. RTLS may serve such approach automatically applying modern filtering concepts.

RTLS for automatic instantaneous locating

This option with RTLS is lesser qualified compared to solutions based on laterating plus tracking. In case of instantaneous attempts to locate an object with no other information but just the identity of the object to be located, the last locations may be unknown. Then search will be performed based on spontaneous communication. Basically the lacking of past location information challenges the technology more than physics will give support. The presence of noise, reflections and ther absence to direct view may lead to not acceptable results, especially results that prove not operationally valid. Therefore this approach may result in disappointment. However, this process may be fully automated and is well responsive. Cooperative targets, the lines of sight of which are not covered, will be located and found rapidly. RTLS may serve such approach automatically.

RTLS with additional sensors

Highly skilled satellite locators work with additional inertial sensors to support the tracking estimates. This is the option for terrestrial RTLS as well. The most recommended sensors are acceleration sensors in three dimensions and an angular rate sensor at least for upright turns. A higher grade of accuracy for these sensors is not required, an improving of the locating process may be yet achieved with MEMS sensors.

RTLS latency times

As wearisome as waiting may be for observing residing processes, real-time locating in transportation processes requires waiting as well. The time between arrival at a new location and obtaining information about that location in coordinates is the typical latency time of a real-time locating system. This temporal specification datum may be qualified

for a single mobile node in an area metrically confined with anchor nodes,
for a couple of such mobile nodes or
for a multiplicity of mobile nodes

all with or even without referencing anchor nodes.

Locating objects, assets, items

There is no doubt: For dead objects of any kind the locating is a common requirement. From daily life we know the relief to know where searched objects locate. This applies to professional needs as well. And there is no advocating to grant individual rights to objects. However, objects carried by persons may be regarded different. When an object is used as the means to locate an individual, the common rules for individuals apply.

Wilful subscription to RTLS services

To avoid irritations: RTLS is not made for clandestine tracking of individuals. It is a means to assist operational requirements in any configuration. This may apply also to staff or even limited public applications. Then, the wilful acceptance shall be expressed basically with a formal and written subscription.

Interested parties must take into account from the very beginning, that there is a tendency to advocate for the individuals involved through advocates not invited to do so. The users of locating equipment may thoroughly demand for such support, whereas the representatives of groups will strictly object against any implementing of such support without reflection. The arguments for securing individuality and the right of self determining the usage of personal data often prevent from a rational discussion of the purpose and of a wilful agreement to well bearable intrusion into the individual sphere.

Buying-in of the individuals to RTLS services

To make staff buy-in to RTLS services, the operator should offer a benefit for his personnel in exchange to a mentally felt burden from being located. This is an easy approach to turn the Big Brother syndrome into Big Mother Support.

Locating with RTLS under labour and service contracts

There is no escape: Individuals bound to a contractor by a qualified written agreement have to bear what they undersigned. However, employers and contractors may not ignore privacy and intimacy. Under all circumstances, validity of contracts is bound to mandatory requirements of balanced burdens and compensation. In case of doubts, written consent on mutual understanding of common aims is a sound approach. However, the locating of persons shall be limited to substantial needs and shall be precluded as a permanent tracking. To support such limiting, several means and methods may be applied: Switching off the locating electronics, which may not be overridden by hidden internal circuitry. GSM mobile phones are of such type. Logging out from services that enable locating, which may be sufficient for cooperative approaches. RTLS is of such type.

Activating a basic service policy that intentionally obviates locating, as for times off and for work breaks.
Activate a special service policy that restricts locating, as for agreed professional needs.
Activate a reciprocal policy by the individual that demands locating of another person just to the persons that are subject of locating by this individual, as for dating purposes.
Limiting the locating to certain conditions of the individual, as for emergency purposes
Limiting the locating to defined time windows, as for agreed times of locability
Limiting the locating to defined areas with the location server, as for agreed location of work
Diluting precision of location information through special procedures with the location server
Obscuring availability of location information through special procedures with the location server

Further methods may be found in the context of location based services (LBS) with mobile phones.

Acceptance of RTLS in the market

Currently (2008-05) acceptance of RTLS in various markets is weak. Some of the early proponents of systems are one of the reasons to extend this period of weakness. Especially the crude locating engines with RSSI approaches contribute to disappointment. A mean reproducible accuracy of >+- 5m is not what customers expect. There is no object in road or rail logistics that is larger than such accuracy, hence segregation of two container like objects is not well supported, not to talk about segregating two cardboard containers or pallet piles. For indoor applications the segregation of two rooms is no artwork. There will be some more seasons waiting for better relief.

But remember earlier statements before technology emerged: ‘I hope none of you Gentlemen is so foolish as to think that aeroplanes will be usefully employed for reconnaissance from the air. There is only one way for a Commander to get information by reconnaissance and that is by the use of Cavalry.’ Sir Douglas Haig – addressing the Staff College, July 1914

The weakness is no surprise. Hype-cycle has not yet started, and many participants in the group of early RFID technology adopters still lick their wounds. However, the requirement for truth and substance of reports in transportation and servicing leads directly to more than just clock time, identity of and some guess about quantity of objects. The subsequent questions are from where?, whereto? and others.

Dissemination of RTLS to markets

The quantity of RTLS design trials is numerous. The identifying is not easy, as the taxonomy varies depending to background of the design team and intent of the application team. Also, the quality of the approaches varies, from strict obedience to physics to bare neglecting of physics and mathematics. Wireless solutions performing with reading through walls do not support reliably nor trust. The market forces will separate the wheat from the chaff.

Currently (2008-05) the deployed solutions cover simply applications without support to moving nodes. Suppliers are rather hesitant to announce next step with locating moving objects, as required qualification of solutions still tries to become mature with LBS.

Economical justification

Many applications of RTLS appear attractive at first glance. But qualified specifying the requirements generally leads to the assessment of current organzational structures and operational practices. Many cases end in the call for improving organization first. This lesson may be learned in trials. or knowledge may be bought for free from reading between the lines.

Taxonomy

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.

Cooperation is basic principle of RTLS

RTLS in the sense of the definition with ISO/IEC FDIS 19762-5 rely on cooperation of participating wireless nodes:

The basic requirement is the contribution minimally to the measurement process and optimally as well to the communication process.
The best economy is achieved, when additionally the locating process is hosted by each of those of the moving nodes, which ask for the result.
The locating process may be reserved also to those nodes, which ask for the result.
The additional advantage then is for those nodes which just disclose its location.

In the sense of the definition with ISO/IEC FDIS 19762-5 also GPS and Galileo might be considered as an RTLS, but the results of former attempts to standardization is clear: GPS will not be standardized in ISO/IEC 20730-4, as intended years ago, and thus none of the systems envisaged for standardization with ISO is a non cooperative system. Galileo will not act different in future.

Basic functions of an RTLS

Each RTLS as a system is composed of wireless nodes. Each of these nodes serves the following functions:

Identify itself
Join a network of similar nodes
Ranging on the basis of RSSI or RCPI guessing
Ranging on the basis of travel time metering (either lateration or angulation or both)
Compute location based on ranging as the kernel function
Tracking composed of subsequent computed locations

Classification of RTLS system concepts

In applications markets, there appears a tendency to compare apples and pears: Both are juicy, but that approach does not lead to common contentment. Careful assessment of the differences is required. Then the results of systems design and careful layout according to the requirements will provide success with sound functioning.

Today technology offers all options required. However, requirements determine cost.

Generally defining, there are largely different approaches in various classes of locating systems designs that may provide locations of objects in real time, e.g. as follows:

The traditional ones for wide range ranging and locating:

Radio Scanner Locators (i.e. RADAR), as a non-cooperative type of scanning and ranging systems
Radio Beacon Locators (i.e. TCAS), as a cooperative type of scanning and ranging systems
Passive Radio Frequent Locating Systems (i.e. LORAN), as a non-cooperative type of systems
Passive Radio Frequent Locating Systems (RTLS earlier proposed for ISO/IEC standards), as satellite based systems for down link asymmetric locating (e.g. NAVSTAR/GPS, GLONASS, GALILEO)

The terrestric ones for short range locating:

Active Radio Frequent Locating Systems (RTLS according or not to a certain ISO/IEC standard), as transponder based systems for a server centred uplink asymmetric locating (e.g. www.WHERENET.com, ISO/IEC 24730-2) with a central facility
Active Radio Frequent Locating Systems (RTLS according or not to a certain ISO/IEC standard), as cooperative distributed systems for symmetric locating (e.g. www.NANOTRON.de, ISO/IEC WD 24730-5) with node wise autonomy in localising for all nodes.
Active Radio Frequent Locating Systems (RTLS not according or not to a certain ISO/IEC standards), as cooperative distributed systems for asymmetric locating (e.g. www.SYMEO.de, not a standard) with node wise autonomy in localising just for the moving nodes.
Cellnet based locating (location based services) of the telecommunications providers (well defined with ETSI breeds like GSM, GPRS and UMTS according to various standards apart from ISO/IEC 24730-1)

Not to forget the other ones using sound or light for scanning and tracking which may include locating:

Acoustical Scanner Locators , as a non-cooperative sound based tracker type of systems, an alternative with the lowest tracking bandwidth
Optical Scanner Locators (i. e. LIDAR), as a non-cooperative optical tracker type of systems (e.g. SICK-IBEO, GOETTING)

Finally there are numerous hybrids that combine features of these approaches. This list may be extended upon desire.

There are other locating systems without any travel time metering, just operating lateration via power level guessing (e.g. in WLAN IEEE 802.11 or WPAN IEEE 802.15 framework), where bearable inaccuracy is large compared to range. Attenuation will generally be heavily influenced with the ambience. Hence the functioning of such approaches is limited in range and requires close co-location of transmitters and receivers to determine more precise than just evidence for presence. These types are generally either one of two:

Beacon based RFID systems (IEEE 802.11, IEEE 802.15, or non-standards-based), as asymmetric and not cooperative locating systems. This approach has to consider the restrictions with regulations to a blink rate of not more the 10/s (FCC USA 2007).
Polling RFID systems (IEEE 802.11, IEEE 802.15, or non-standards-based), as asymmetric and cooperative locating systems.

Standardized RTLS

Each interested party may assess the suitability of one of these low-cost approaches. Please note that the remainder of this page is written from the perspective of the ISO/IEC 24730 definition of RTLS, which excludes many common methods. Other, broader definitions exist and are commonly adopted in the market.

For details on standardization please see Real-time locating standards.

Ranging

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.

Active and passive ranging with RTLS

As indicated, the main difference in communications links is the activity of the locating node:

Passive modes in wireless communications

If the target to be located always remains passive, but just reflects energy to the ranging transmitter, i.e. even is not capable to perform any ranging itself, the method does not refer to any contribution of the target.
If the ranging receiver as a node is working independent from other nodes, i.e. never operating in transmission mode, the method of ranging is entirely passive. Hence, the node may operate in ultimatively low power consuming sleep mode just to receive third party signals for determining its distance just from the received signals.
If the ranging receiver or transponder as a node in a meshed network remains just listening, i.e. is not ranging in transmission mode, the method again is just passive ranging. Then the node receiving a ping may stay unwilling to reply. Hence, the node may operate in ultimatively low power consuming sleep mode just to wake up in case that someone pings for its distance.

Active modes in wireless communications

If the ranging transponder as a node in a meshed network is willing to contribute to the ranging process of other nodes, i.e. sometimes starts operating in transmission mode, the method appears as cooperative ranging. Hence, the node may operate in moderate low power consuming sleep mode just to wake up in case that someone asks for its distance. During ranging, the node may determine the distance to the requesting nodes, thus requiring some energy for such responding and even corresponding ranging procedure and on completion again requiring some energy for communicating the results. In this manner each cooperative node may determine its own location.
If the ranging transponder as a node in a meshed network is itself moving whilst performing ranging, i.e. updating its own distance information operating in repetitive transmission mode, the method is cooperative ranging in a mobile network. Hence, the node may operate in moderate power consuming mode just to locate itself according to a certain schedule and/or its own motion characteristics. Whilst ranging repetitively, the node may consume significantly higher quantities of energy to determine its distances and finally compute its location repeatedly. Thus such operation requires sufficient local battery or on-board power line for such ranging and locating procedure as for communicating the information with the other nodes involved.

Segregating RTLS from further methods of locating

There are many options to obtain information about a distance to or a location or position of an object. The easiest approach is to identify the generally known location where an object temporarily resides from a database.

However, international standardization for RTLS addresses the determining of absolute or relative coordinates of non predetermined locations or of the relationship of such:

Real Time Locating Systems (RTLS) according to ISO 24730-1 currently are nothing but solutions in the class of active ranging systems. This may pertain unless the initially envisaged standardization of passive ranging satellite based systems as GPS (and GALILEO after full deployment) may happen.

Real Time Locating Systems (RTLS) according to ISO 24730-1 are primarily terrestrial solutions and thus do not use any satellite communication elements in the sense of satellite navigation. However, to obtain a position fixing with a terrestrial system of coordinates, hybrid solutions may include such equipment.

Real Time Locating Systems (RTLS) according to ISO 24730-1 are primarily solutions working apart from any global or regional radio illumination and thus do not use any common network communication elements in the sense of location awareness. However, to obtain a rough topographic or geodetic fixing with a common terrestrial communications network. Hybrid solutions may include such equipment.

Real Time Locating Systems (RTLS) according to ISO 24730-1 in the scope of current standardization are generally cooperative systems, whereas RADAR like system concepts were originally designed using non-cooperative scanning methods. Today, advanced RADAR systems designs, as IFF and TCAS show aspects of cooperativeness. But it makes no sense, to mix the definitions.

Real Time Locating Systems (RTLS) according to ISO 24730-1 are operated on the basis of "travel time" ("time of arrival" or "time of flight") metering according to connatural methods (TOA, AOA or TDOA).

Real Time Locating Systems (RTLS) according to ISO 24730-1 are primarily not equipped with any inertial sensing elements and thus do not provide any motion detection in the sense of inertial navigation. However, to smooth a trajectory, hybrid solutions may include such equipment.

According to ISO 24730, power level guessing as RSSI (Received Strength Signal Indication) is just applied for building the inter-node communications but not for properly determining inter-node distances. The affecting with the phenomena of reflection, deflection, diffraction, attenuation heavily disturb power level metering. Only in environments where these phenomena do not affect metering, the RSSI guessing may serve operational needs. Many vendors and industry analysts take a broader view than ISO 24730 on this point, encompassing RSSI.

Precision of ranging with RTLS

The operation of the wireless functions requires well defined approaches to usability. The basis for success is a sound concept for metering. Currently the approaches in IEEE 802.15.4a show various options. Additionally the concepts of mathematical calculation and for operational networking with or without administering are crucial for performance. Early question of interested parties always demands for specifying accuracy.

Generally resolution with RTLS defines accuracy basically, but not as a guaranteed datum. Accuracy is heavily influenced by time variation, stochastic errors and dynamic behavior. A moving node may be metered as precise with the time consumption for the disambiguation multiplied by speed. This limits the capability to provide accuracy for spontaneous locating. Calculating the track enables a better operational precision, but requires steady behavior in motion.

The first useful answer refers to operational metrics and statistics:

The instantaneous error of single measurement is mostly awful.
The average operational error may be reduced to reasonable levels, but this issue is irrelevant for rapid decision making in operation.
The error after computing with secondary information especially from motion may serve all operational demands.

The second useful answer is even simpler: It depends on the purpose, challenging demand served by tailored approach may end contrary to economic reasons. Generally RTLS may provide resolution to the single frequency cycle, as defined with the carrier or modulation frequency. With phase sensitive systems, the resolution may go beyond. But only with context computing the mission may be accomplished. It is the challenge for the customer to tailor his desire to what technology delivers. Elso no solution would become operational.

Generally the key threat to precision is multi-path propagation out of a bunch of error sources. The maturity of any RTLS system may be proven in a heavily reflective environment, where echoes arrive at the receivers in numbers, only the first, not the strongest representing the direct response on line-of-sight, and only if such exists.

Standard measurement procedures

Beyond power level guessing as with RSSI, several descriptions are given for the metering process. The names vary from AOA via TOA, TDOA, TOF finally to SDS-TWR. For details see the references below.

For locating any person or object, some standard procedures are defined as mathematically sound:

Travel time trilateration against reference points (TOA, time of arrival or TOF, time of flight) using one frequency for metering
Differential travel time triangulation against reference points (TDOA, time difference of arrivals) using more than one frequency for metering
Travel time triangulation against reference points (AOA, angle of arrival) using more than one receiver for metering
Mixed configurations of unilateral or bilateral, asymmetric or symmetric lateration or angulation
Other ranging and/or bearing in a covered or otherwise confined area, e.g. with light or sound in transmission and reception.

The terms of trilateration and triangulation result from planary (2D) models of the surface of Earth. The more correct term with spatial (3D) models hence is multilateration and similarly multiangulation.

Generally the calculating of measurements to obtain positions will be performed in iterations and as a process inverse to surveying. The process, however, may use a well defined grid to refer to known coordinates or just uses the distances disregarding the orientation of the triangle of coordinates. Despite any stepwise approach of considerations, any system not finally providing distances in a standardized metrics or without good reproducibility is not an RTLS. Details are explained in the following sections with reference to the diction in the named standards.

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).

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 (time of arrival, TOA 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 (time difference of arrival, TDOA concept).

Various principles of lateral measurement

The concept of travel time measurement is various according to precision availability and precision requirements:

The measuring nodes transmit a message to the corresponding neighbors and receive a message with the travel time included (singular measurement).
The measuring nodes transmit a message to the corresponding neighbors and receive a mirrored answer and thus perform the travel time measurement themselves (asymmetric measurement).
The better reliability is obtained, when the corresponding neighbors perform the reciprocal process and serve a second result for comparison (symmetric measurement).

Only the last concept compensates clock rate differences between the nodes involved by symmetry instead of by synchronization.

Various principles of angular measurement

For concepts of angular measurement by time difference, variation according to precision availability and precision requirements is different:

The measuring nodes transmit a bi-frequential transmission to eliminate frequency and thus clock rate differences (correlative measurement).
The measuring nodes are synchronized when transmitting messages to the corresponding neighbors and thus enable angular measurement (asymmetric measurement).
The measuring nodes transmit a signal and receive a mirrored response with dual receiving antennas (phase measurement).

In comparison, TDOA generally requires higher precision of time measurement compared to TOA concept.

Dimensions of the operational task for RTLS

The operational challenge for Real Time Locating beyond the technical task is not just to know, where an object resides or currently is, but in which direction and to which destination it currently moves. In addition, the ambiance, in which the object resides or moves may be of equal interest. Thus Real Time Locating copes primarily with motion from point to point or within sets of populated locations and not too much with single locations only. The result is not merely a momentary indication, but more the basis for tracking and tracing. The result may also be the coincidence of objects in a neighborhood to pursue in some cooperation.

The operating of an RTLS requires some preparations. This is not only the build of a certain infrastructure. It includes primarily the balanced estimate for benefit and cost. Thus, the implementing asks for a sound specifying of the technical layout. This is not a question of technology, but just mandates for proper and skilful engineering.

Proponents and Inhibitors

Basically technically driven euphoria will boost demand for RTLS. But, indeed, this does not describe a path to success. The key challenge is to convert expressed inhibitors to proponents.

Threat to privacy vs. supportive advantages

RTLS will 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 , 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

Simply looking at locating as a threat is the first impulse, and emotions will drive this attitude to steadiness. However, getting located in daily business life may be regarded as a chance to balance burdens. The work share in organizations always needs re-balancing, and the individual does not serve this need by hiding oneself. The learning about threats in modern organizations asks for experience with overload after staff reduction in quantity and qualification. This is the real threat to health and contentment at work.

Coping for automated better acknowledgment of work overload

For truck drivers the tracking of the schedules and route map is standard, though not regulated. The equivalent tracking of the schedules and walk patterns in staffed indoor service operations may become a standard as well, and this assessment is not mandating for regulation. The legal stipulations are clear and simple: Agreement is required.

Then with RTLS there is a new escape to exhaustion, ending up running around without relief, by simply proving the overload conditions through tracks from the worker to the scheduler. And obviously the threat to exhaust the right of privacy is a much smaller threat to health than physically breaking down under scheduled work, as well as for the employer, who has to face the threat of higher rates of failure, injury and damage caused by exhausted staff.

Locating on the spot

To locate an object is not defined for an immediate answer to a spontaneous question. Precision costs money and patience in a balance. Remember, even in real time mode (GPS) there is a need to synchronize operation of GPS locator to current availability of received satellites. RTLS is comparable. To locate an object needs line of sight first and some stable availability of received signals second. After some signals received and processed, location will be available readily computed and updated with motion. If the used system is not capable to deal with motion, then updates will be missed.

Infrastructure requirements

Generally, for locating an object or a person, some reference requirements must be fulfilled. The linear, the planar and the spatial locating require at least 1, 2 or 3 reference points for a twofold equivocal or dual solution and 2, 3 or 4 reference points for any unambiguous or univocal (unequivocal) solution. Such dimensional layout has to be chosen for an appropriate setup. The respective deployment of such reference points is a prerequisite for operational preparedness. Some examples of RTLS infrastructure requirements may be:

a) with any RTLS:

fixed reference points with known positions
moving reference points with certain validity of position information

b) with an RTLS in any confinement:

e.g. wall-mounted reference points
e.g. lamp-post mounted reference points
other qualities of reference points

c) with an RTLS operating centralized functions:

central time synchronization: RTLS systems may need some central synchronization of time to prevent from travel time measurement errors.
central computing facility: RTLS systems that rely on central computing facilities must distribute the results of locating computation to the located nodes.
central administration facility: RTLS with conventional networking are configured with traditional administering.

d) with an advanced RTLS requiring very few infrastructural support:

RTLS in all known breeds of system layout require sound time synchronization, alternatively at least some intelligent concept of balancing the timer errors resulting from drifts and stochastic errors in quartz crystals.
Normally, an RTLS may be a stand-alone solution that does primarily not require any link to resident data sets. Thus, an RTLS based on an ad hoc networking concept does not require central facilities for administration.
More advanced RTLS systems distribute an economized computation capability and thus enable each moving node to compute its location on its own facility.
For security reasons, a non-recurrent distribution of enrollment information for authorized nodes is recommended as a support means.

Benefit and cost of operating RTLS

The benefit of any RTLS deployment is the key to success. Benefit must be determined before arguing the cost of implementing and operating such systems.

RTLS nodes produced in large quantities appear as low-cost personal equipment. The lesser the production quantities and the more complex the system layout, the higher the price of RTLS nodes will rise. However, cost of nodes may be balanced with a strategy to allocate functional cost primarily either to fixed nodes or to mobile nodes and not equally to both. The respective population tells what is the advantageous approach.

Basically, any RTLS node may not be operated without at least a battery like energy source. This defines the segregation in cost to passive RFID tags. Disregarding economic interests of the vendor and the purchaser, an RTLS node may cost in the range of tenfold the price of any active RFID tag.

The key economic advantage with a well designed RTLS is the fact that few or better no central facilities, no general illumination and no fixed cabling installations should be required to enable proper operation. Older designs that do not anticipate the principles of wireless modern adhoc networking, as defined in proposals with the IETF MANET concepts, do no longer comply with expected economic advantage.

The interested party should carefully assess the stability of locating functionality.

Decision and choice when thriving for an RTLS

As with all complex technical systems, RTLS is a solution exposed to failure and RTLS is operated under strict constraints: There is no 100% success guarantee and there are conditions for unexpected spontaneous failure. And there is no escape, no evolution of technologies will ever overcome this limitation.

However, RTLS is a great and highly innovative approach to intensify the feedback from industrial processes to the people involved, from the worker to the CEO. Besides, the included wireless and networking technologies are all mature, but the combination applied to RTLS deserves hardening with the challenge of implementing it anytime anew.

There is an initial recommendation for all potential implementers of an RTLS solution: There is no balance of effort and benefit, when RTLS shall come before automatic identification of moved objects. When already e.g. RFID is too costly, then RTLS is economically unattainable. This is a problem for the suppliers as well as for the purchasers. As a purchaser, do your homework first, before you start to travel to the stars. The first and decisive questions are:

Is the aim security (CCTV, EAS) or productivity (RFID, RTLS) and are the security challenges yet solved?
Is the starting application of automatic identification for moved and dead objects already installed (RFID or 2D-code reading)?
Is an RTLS the necessary and inevitable approach to the operational task or has the benefit to be justified versus cost of installation and cost of operation?

If no, simply return to start. These questions may be neglected only, when all moving entities have a brain. Then, first, the identity problem may be solved in traditional communications. And, secondly, these brains may end in desperation, when there is only weak support to cope with repetitively occurring and mentally strenuous challenges. Otherwise you may continue as follows. The purchaser of an RTLS shall consider the maturity of the RTLS solution with several aspects:

Do you look for a solution operating in any confinement (with a set infrastructure) or on an open plane?
Are there restrictions to common wireless illumination? (e.g. inside reinforced concrete buildings or just under a roof)?
Which systems are in operation as a reference (state of the art in 2007 is: not too much)?
Which platform is the basis for the design of this reference (is it a homogeneous platform or just a desperate hybridisation)?
Do standards apply and where are the resources in the chain of integrators (do you rely on some stable availability on mid term)?

The choice should lead preferably to standardized approaches, in minimum to published technology (applied patents) or at least to comparable success stories.

Selection of RTLS node types

RTLS deployment makes use of wireless nodes. The following node types are available with various vendors in different configurations:

concerning communicativity

gateway nodes for access to other LAN or WLAN networks installed to PC or router like equipment
integrated units where the node resides embedded in a PDA, handy or laptop

concerning locability

anchor nodes for fixed mount with known location and with battery or power line attachment
autonomous nodes with locally implemented locating engine
ordinary mobile nodes with portable energy source from batteries

concerning modes of operation

beacon nodes for permanent or intermittent transmission
hybrid nodes with different frequencies for receive and transmit, as e.g. infrared or ultrasound
dual nodes with RTLS and separate RFID functions
silent nodes that just serve sniffer functions

The choice is subject of sound consulting with experienced vendors. Details on how to select are referred in RTLS implementing.

Deployment of RTLS nodes

Any indoor deployment is subject of sound planning with experienced vendors and requires reasonable tools. Details on deployment are referred in RTLS implementing.

Communicating for wireless measurement with RTLS

This section was taken out to form a new item RTLS communications.

Implementing an RTLS into operational environment

This section was taken out to form a new item RTLS implementing.