Real-time locating: Difference between revisions

According to ISO/IEC 19762-5 an RTLS (real time locating system) is a combination of hardware and 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.

Location information is never obtained in a single step. To determine or best guess the location in real time means to achieve sound results for the required metering without hampering delay and subsequent computing the location. This is the challenging task of Real Time Locating. Metering may be performed with just distances, or just angles or both. A location may be described through relative position data, absolute position data or any intermediate information to obtain such position data. Such descriptions are topographic ones, mostly referring to a map of a terrain or a plan of a building. RTLS does not suffice with the topologic descriptions, that e.g. just include neighbourhoods and hop counts, as with communications networks. However, this topologic description is a prerequisite for operating some types of RTLS to obtain the topographic thereafter.

Please consider the power of physics and the span of terms: Travel time is a sound criterion for distance, strength or power level of received signal is an alternate but much weaker estimation. Locating is sensing and therefore not positioning, which is effecting. Identifying provides just location of the reader and the read marking.

The further explanation explicitly excludes RFID indexing (radio frequent transponder indexers) and Cellnet base station segment locators (location based services) from the scope of the ISO/IEC approach to standardization as well as all beacon systems, that ping without request. So shall this text.

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
  • 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.

The definition, however, does not explicitly exclude the sole RSSI-type approach to metering, which is supported by some suppliers and dispraised by others. As common with wireless communications, radio signal strength is an essential, but not a sole solution.

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.

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

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.

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, as a special term for metering the distance, is the prerequisite for locating. Sweeping is determining the distance may be either a scanning process, as with RADAR, or a direct distance metering process. In case of scanning with a rotytory beam sweep, the system obtains an image as a model of the whole scene. 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 to the request and hence 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. After completing ranging, the location may be computed.

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.

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 are currently only 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.

The usage of a common wording for the operational tasks with locating systems is somewhat hampered by tradition of seafarers, artillerymen and other military 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.
• Fixing a bearing is some part of navigating that plans 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.
• 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.

To ease the understanding, some basing principles are outlined:

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.

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.

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.

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 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?

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.

The implementing of any RTLS must be tailored to the specific needs in the operational area and for the support of users of the obtained information. There is no standard solution to serve special requirements without tailoring. That does not require entirely new development for each implementing, but at least some sound parametrising for such implementing.

For the appropriate set of parameters for the implementation the purchaser must take into account the following:

• General operational conditions, especially
• Power supply for portable units
• Ambient conditions, especially
• Networking conditions
• Features of systems dynamics
• Features of operational populations
• Requirements for communications of operational data

etc.

This bouquet of considerations will lead to a careful phasing of the implementing process. Above all steps the specifying of the various layers and for the diverse aspects of system layout is the key to success.

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.

The nodes in a wireless network are the vertices (pl. of vertex) in a mesh. The edges of this mesh are the distances between the nodes to be measured. To enable adequate wireless communication between these nodes, the properties of these nodes must be adjusted to become members in the network and then published to support recognition though the other nodes. However, not all of the nodes will see each other node and not all nodes must know all other nodes. But to disseminate information at least about direct neighbors and some of other next nodes in the vicinity, there must be a concept of administering the nodes and their membership data.

Compared to traditional administration, ad hoc networks work out this task by some concepts of autonomous pushing and pulling with subsequent memorizing to keep the network largely free of this administrative load. The degree of autonomy may be regarded as a feature of qualification of the networking concept.

In mobile ad hoc networking (IETF MANET), the main categories of routing are proactive and reactive. Grouping vertices in an RTLS shall take these classes into account:

• There are residing vertices with known positions in most RTLS set ups. For these exploration will be very effective being performed advance to any locating of moving nodes. Such is proactive routing.

• There are temporarily moving nodes that might contribute to networking the RTLS set up. For theses nodes there should be a group membership determined by motion.

• There are simply moving nodes knowing none about there own location. For such condition, reactive routing is the best choice.

As with any communications system the coexistence of various wireless systems is affected by any new participant in the same band. Many systems the same licensed ISM bands and therefore interfere with each other. There was an understanding, that deterministic continuous wave communications are more harmful to each other. That led to the quasi-stochastic pulse coded wide band communications, that macroscopically just add some noise in the used bands thus pretending better tolerance and coexistence.

With increase of such population the noise level rises also significantly, hence producing a threat of ubiquitous stressy burden for yet existing systems. New strategies will enable sound balancing of stress to bearable levels. Such strategies may apply

• neglecting any problem (UWB solutions in the 2.45 GHz range)
• conformance with existing frequency and channel schemes (e.g. Nanotron, 2.45 GHz WLAN scheme)
• using fewer populated frequency bands (e.g. SYMEO, 5.8 GHz)
• escaping to newly released frequency bands (e.g. UBISENSE, 5.9 GHz, released in UK)
• escaping to a wider range of UWB bandwidth (3.1 GHz to 7.0 GHz in US)

Increasing usage of such approaches will result in new findings about real interference.

Despite the fact that UWB and CSS modulation concepts operate transmission with short pulses, the group of nodes operating under this paradigm still require quiet times to transmit and receive between pairs of vertices. At least the two normally do not operate in full duplex mode without colliding. This defines the demand for anti collision:

• The easiest but most expensive approach is the use of separate channels in a crowded ambience
• The other approach is the usage of statically interleaving pauses, which is contra dictionary to real time
• The skilled approach is some proven anti collision concept as TDMA, CSMA, CSMA/CD, CSMA/CARP,

OFDM, OFDMA, and others, chosen coherent with the modulation concept. However, anti collision concepts as Aloha, slotted Aloha and CSMA/CA are not supportive for real time requirements, as known with older RFID systems designs. If there is no concept applied, the systems simply fails, latest under heavy load in dense populations. Do not believe in demos with single moving nodes.

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.

Beyond power level guessing, 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.

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 knows 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.

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 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 concept).

Basically for time measurement between two points either the clocks at these points shall be synchronous or the difference from synchronism must be known.

Regarding TOA and TDOA, synchronization of time is conditionally required. This may be obtained in different ways

• explicitly with synchronized clocks in the metering locations
• with measuring supported from third reference points

For details please refer to respective system design manuals or patent applications.

Hence, when pairs of radio signals for angulation must be precisely synchronized between distant units, the better results may be obtained with lesser precise lateration compared to angulation.

To overcome the synchronization problem, already GPS applies a multi frequency transmission and as well makes use of precisely synchronized clocks in conjunction with relativistic correction calculus.

As for terrestrial and cooperative locating systems the distances are very short compared to satellite metering, such sophistication is not required in hardware. With sufficient accuracy, the metering may be based on the concepts of non synchronised metering with just stable generators for transmitted frequencies.

The easier approach is:

• either to reflect a signal from the receiver to the first transmitter and save the synchronizing at all - or
• to measure angles with two close positioned receivers on the very same basis - or
• to measure phase differences for at least two frequencies traveling the very same path.

Hence the synchronization problem is solved by different method to implicitly compensate for asynchronism

• with two propagation concepts and highly different travel speed measured with different equipment
• with pulsed multi-frequency propagation and metering the pulse phase difference in one channel
• with phased single-frequency propagation and metering the phase time difference between two channels

The latter two solutions are state of the art and basis for commonly available products.

The first concept is known from roughly determining the distance to a thunderstorm. Applications are known not for better precision of distance metering, but for better segregation of residence in neighbored confinements, as with infrared transponders combined with radio transponders or ultrasound transponders in such combination.

A completely different calculus may be obtained from application of two or more different frequencies transmitted from one position. The travel time varies with frequency and the distance from transmitter to receiver again is a linear function of time difference of arrival, after the two signal were transmitted at the same time. For this method the synchronization task is inherently performed with the coincidence of sending the two signals and some reliable stability of the used frequencies, especially the frequency ratios of the two carriers.

Many other approaches are considered by the market at large to fall under the category of RTLS, though they are not included in the ISO 24730 standard. These include:

• RSSI Received Signal Strength Indication is a procedure for utilizing the strength of a signal to determine a best estimate for current location. This procedure can either use the strength of a single signal arriving at multiple readers, or the strength of multiple signals arriving at a single reader. As with any RTLS solution, accuracy can be variable based on the environment and number of RF reading devices. This method is most common in systems based on the IEEE 802.11 standard for wireless networks, and is most frequently used in tightly enclosed indoor areas (hospitals, office buildings, etc.)

• Index relations with coarse fixed location incrementing, e.g. using RFtag indexing.

The index relations alternative makes use of components as with RFID. However the RF tags are fixed and the RF readers are moving. That reverse configuration rules why this systems approach shall not be understood as a trade off from any RFID implementing, where the RF tag identifies an object and the RF reader reads the identity en passant.

RSSI is not a metering solutions with RTLS but a necessary and an auxiliary means. Those who had set this notion RSSI where closer to physics than those who market this as an RTLS concept. Any indicator may not become a proper metrics with any defined accuracy. This does not affect the basic importance of RSSI to build an ad hoc network.

If there are any contradictions: Test it, once is enough. Discussion with suppliers will show: The limitations are not to be neglected. Many approaches take this into account with hybrid solutions including LASER ranging (LIDAR. But remember, light has just another frequency, but generally adhere to the rules applicable of all types of radio waves.

Travel time of radio waves between transmitters and receivers can be measured disregarding the type of propagation. But, generally, travel time only then represents the distance between transmitter and receiver, when line of sight (LoS) propagation is the basis for the measurement. This applies as well to RADAR, to Real Time Locating and to LIDAR. The term line of sight not necessarily refers to visual sight, but to chosen transmission and for the used frequency. This rules: Travel time measurements for determining the distance between pairs of transmitters and receivers generally require line of sight propagation for the applied transmission to obtain proper results.

Whereas for communicating the desire to have just any type of propagation to enable communication may suffice, this does never coincide with the requirement to have strictly line of sight at least temporarily as the means to obtain properly measured distances.

The travel time measurement may be always biased by multi-path propagation including line of sight propagation as well as non line of sight propagation with single or multiple reflections in any random share.

A qualified system for measuring the distance between transmitters and receivers must take this phenomenon into account. Thus filtering signals traveling along various paths makes the approach either operationally sound or just tediously irritating.

Any of the known locating approaches, either RTLS, GNSS or other system require

• either direct line of sight between the nodes forming the network, or
• common illumination of the operational area.

If neither one of the two conditions applies, locating will not work. Especially RTLS is bound to the line-of-sight (LoS) condition. To anybody who is in doubt about this message, feel invited to prove the contrary. However, in terms of wireless transmission, LoS does not require visual sight for humans to enable proper links in radio frequent communication bands. The longer waves of radio transmission may pass visually dense obstacles. But, there is no escape: Electromagnetic waves do not travel around corners or through electromagnetically interfering material.

Generally the wireless link with any non-line-of-sight propagation (nLoS) enables some propagation between two vertices via a couple of unknown paths. Hence, only the straight wireless propagation path along the line of sight (LoS) between pairs of vertices delivers the appropriate measurement, despite possibly missing optical visibility along this line. Practically such through-the-wall propagation is again LoS metering and may contribute to locating as far as sufficient signal discrimination and filtering is performed. This requires sufficient power to transmit and receive between pairs of vertices through walls. Hence any single metering through walls serves results, but never the correct geometric distance. Errors add to the inherent metering errors in the systems performing the metering.

Consequently, the result of any wireless nLoS between pairs of vertices will contribute to build an ad hoc network, but only such visual nLoS but wireless LoS may contribute to distance metering. The escape is not as easy: It would require the metering with composite straight line LoS segments between a set of pairs of vertices to form the entire distance. Hence the well defined speed c= 299,792,458 m/s of light in vacuum does not apply for transmission through materials. The error on short distances may easily exceed the resolution.

However, despite all statements on possibilities crossing walls with appropriate transmission power and suitable choice of frequencies, the travel time or time of flight (TOF) of radio waves is biased for materials, because any material will affect the speed of travel.

The proof is easy: Test it. Is the offered solution simply operating and makes it expedient use of nLoS e.g. in buildings for distance metering or not??

The signal flow from transmitter to receiver is much better performing, when with digital modulation schemes the generating of one or two signals may use special pulse forming to improve the determining of direct wave propagation signals more easily. Time of arrival of any single sharp pulses is always better determined even in noisy environments than any continuous wave carrier modulated with switches signals. For reconstructing the received signals from ambient noise overlays, such signal types like sinc pulses (sinus cardinalis) offer special characteristics.

Various systems are offered that make used of advanced signal procession technologies. However, even sharp pulses are not immune against multi path propagation and will come of several paths. Publications that neglect this fact might refer to systems not capable to filter properly the direct path signals from clutter.

To obtain an appropriate result with locating, not only precision is required, but primarily the unambiguousness of data for processing is required.

Then, fulfilling the condition for unambiguousness, metering shall be performed from or towards various reference points to calculate the unknown location as the unknown position inside a plane circle triangle (3 reference points in a 2D space with three distance circles) or inside a spherical tetrahedron (4 reference points in a 3D space with four spherical shell surfaces).

That is quite simple to understand:

• On a trajectory, it is sufficient to know two way points to determine a third location. Any leg defined with two known way points gives a unique set of coordinates for any third way point on the very same trajectory, disregarding the orientation of the trajectory on a surface or in a space. However, the trajectory must be well defined. In the case of a straight line, the two way points with coordinates and a Euclidean norm for the trajectory fulfil this condition.
• On a surface, it is sufficient to know three way points to determine a fourth location. Any triangle defined with three known way points gives a unique set of coordinates for any fourth way point on the very same surface, spanned with the triangle and disregarding the orientation of the triangle in a space and the planarity of the surface. However, the surface must be well defined. In the case of a plane, the three way points with coordinates and a Euclidean norm for the plane fulfil this condition.
• In a space, it is sufficient to know four waypoints to determine a fifth location. Any tetrahedron defined with four known waypoints gives a unique set of coordinates for any fifth waypoint in the very same space, spanned with the tetrahedron and disregarding the orientation of the tetrahedron in space and the orthogonality of the space. However, the space must be well defined. In the case of a plane, the four waypoints with coordinates and a Euclidean norm for the space fulfil this condition.

To measure a distance via the travel time of a wave requires an exact conversion of time to distance. Hence stability of the time reference is an asset requiring extensive effort. There are three concepts in the market, which differ in cost and precision:

• local oscillators with phase locked loop control but without synchronization

This solution may be found with the ISO/IEC 24730-5 standard proposal. It appears strange, that such approach will provide sound metering, but it works at a very interesting balancing of benefit and cost.

• local oscillators with central synchronization and local computing

This solution is the classical approach with GNSS systems, as with GPS and GALILEO. It requires a very expensive infrastructure, which was initially paid just with military requirements. The scope of applications is vastly beyond military, however private equity funding did not discover this operation as too much interesting. Due to military non disclosure requirements, such solutions are yet not set out for publishing details as standards, as earlier foreseen with ISO/IEC 24730-4.

• centralized systems with broadcast time information

This solution defines the lowest requirements in hardware for the nodes, but either puts a heavy burden on the wireless communication or invests largely in cabling. Such solutions are not set out for standardization. Each party deciding on implementing RTLS must tale all the constraints into account.

Normally, an RTLS according to the definitions given with ISO 24730-1 is composed of largely autonomous wireless nodes that communicate to each other. For an RTLS solution the interface to other networks is not mandatory, but for advantage of the using application, any RTLS has a link to another network to enable access to data bases. However, for locating an object or a person, some reference requirements must be fulfilled, where it does not require any further infrastructures.

Some common features of RTLS infrastructure requirements may be:

• Database with the coordinates of the deployed reference points
• Database with the addresses of the reference points and other nodes
• Server function for distributing and collecting operational data
• Server function to supply display functions in other than the moving nodes

Some RTLS systems in the market provide excellent accuracy for locating moving nodes, i.e. the objects or individuals bearing an RTLS locator. However, a fair comparison of systems should take into account, at which expense such accuracy is obtained. Digging holes in the floor, wiring walls with copper or spinning densely meshed nets of reference nodes under ceilings may appear as reasonable for steady implementation, but this is contradictory to any rapid deployment requirements.

RTLS is a mature technology. However, the quantities in production are still on he rise. This rules also the integrating of functions with chips. Most of the solutions offered are well contained hardware developments, however the degree of integration varies. This is at stake for the cost of any deployment. The better the integration is elaborated, the lower the cost for the transponders will be. This applies as well to all types of transponders: For fixmount operation, for personal carriage and for application to dead property. On the other hand, price is not the absolute measure, the balance between benefit and cost rules whether transponders are economised. However, chip integration as well rules the power consumption in operation. This again has to be balanced, as functions like active ranging always consume more power than just passive receiver operation.

Normally in advanced wireless communications systems, the communicating units on air are autonomous wireless nodes. That feature of autonomous operation describes the advantage that basically and beyond unconditionally required knowledge for direct addressing no administering is required.

An easy approach is unilateral broadcasting to start a polling process, but this invites unwanted parties. However, for exclusion of unauthorized third party nodes and inclusion of nodes for membership in a local network, at least some identification and some exchange on encryption is recommended to keep unauthorized intruders apart. Such enrollment procedures may be performed with any node that has the knowledge of accepted addresses, additional identification and passwords along with required encrypting. Otherwise any node would be able to interfere with the members of the network.

Lesser advanced systems may have limitations in the following aspects:

• Communications may be limited just to process locating and to exchange of location data sets
• Addressing is limited to exchange of any MAC addresses or even proprietary addressing concepts
• Identification may possibly be overwritten so that any node may change its system identity beyond MAC addressing
• Encryption may not be a feature foreseen for securing the communications

Such RTLS implementations do not comply with the referred standards according to ISO/IEC 24730.

Normally, any RTLS may use its specially designed set-up of channels in any available ISM wireless communications band. However, some aspects recommend the choice of a band and bandwidth that may support just the locating and some communications functionality beyond and without impact to other already existing infrastructures. There is an approach to an RTLS which is compatible with other users in the same ISM band and which is currently in the process of standardization. This approach is capable of locating and communicating in dense populations and with reasonable channel capacity: The proposal is under discussion in ISO/IEC DIS 24730-5 Information technology real-time locating systems (RTLS) Part 5. This standard proposal is taking the basing IEEE 802.15.4aCSS standard as a specification for communications (promoted through Nanotron Berlin, Germany, www.nanotron.de, and Orthotron, Seoul, Korea, www.orthotron.com) as a platform. This communications approach led with the manufacturing of Nanotron to an industrially designed chip-set (nanoNET NA5TR1) including a metering engine on the basis of symmetric TOA on a homogenous link. These chips are ready for implementing in general wireless systems as well as for RTLS implementation with making use of the WLAN channel scheme according to IEEE 802.11 and thus enabling proper coexistence of WLAN networking with RTLS locating in the very same area.

The competing requests from any nodes in any ad hoc advanced network or any administered legacy network create collisions. The mandatory requirement is a proper strategy for avoiding collision or at least live with the competition and be effective. The basic issue for any RTLS implementation is the capability to provide metering beyond anti collision procedures. The secondary issue is to enable communication beyond metering. Both are tasks before coping with the population challenge.

The solution may not be just a deterministic approach.

Granularity of the RTLS deployment is not just a question of resolution and accuracy, but also a parameter for coexistence of adjacent vertices. The capability of an RTLS to cope with high density of nodes supersedes the accuracy requirement. In case of two RTLS node bearers adjacent to each other, the accuracy of locating each of both might degrade somewhat. But an effect that the locating function might fail due to close vicinity of two or more bearers should be taken as a hard disqualification criterion.

The option to locate an object implies the need to power the unit to be located on the move. This limits the available functions with the moving node. Despite the locating of vehicles there is no power line with moving persons or dead objects.

However, the production lots and the preparations to integrate functions with a processor platform limits the reach of battery. Consider the following nexus of constraints:

• Versatility of application defines communality of requirements
• Communality of requirements controls the quantity of sales
• Quantity of sales in units is proportional lot size in production
• Lot size in production defines the affordable degree of integration
• Degree of integration is somewhat proportional to power consumption
• Power consumption contradicts to battery weight

Generally, taking any actual design of cell net phones with an integrated GPS function as a comparable reference, the locating should be comparable in operational availability. But, this is the target for product development on mid term. Do not forget the design history in the last two decades to arrive at the functional complexity of cell net phones of today.

This section has been moved to Locating Engine.

Generally with moving nodes the validity of location data is limited to the motion characteristics of the nodes located. Hence the quality of the systems approach is related to the dynamics behaviour of the respective systems layout.

Users of wireless systems and of navigational aids will know in detail the effects of any obstacles to direct sight and any interference of more than two transmitters etc. The cardinal quality measure for any such condition is the latency time to cope with relocation of any object. Latency is the sum of times from request through reaction till availability of an answer with the requester. The first proper locating with an RTLS should be ready for display in much less than a minutes time. However, such quality metrics should be independent of any supporting system, as inertial measurements, track extrapolations or optical system aids. All of such options are welcome to improve an RTLS system performance, but they should not be taken into account when comparing different RTLS solutions.

Mobile nodes on the move may change their location without prior notice. They even may change their direction without prior notice. Hence the quality of the location data has to take into account the dynamics of the motion in terms of covered distance and of current angular velocity.

An excellent RTLS will serve the user with an accuracy in the range of his very own dimensions. That may be in the case of individuals an accuracy of about .5m or 2ft (area) down to 2m or 6ft (height). A reasonably good RTLS will serve the user with an accuracy not lesser than a multiplicity of his very own dimensions in the range of about 1.5m or 5ft (area) down to 4m or 12ft (height).

These error margins provide proper support for typical operational requirements:

• Concerning an application in logistics, the size of a pallet will be understood as a reasonable measure for accuracy, then the locating may distinguish between the left, middle or right pallet in a row of three.
• Concerning in an application in conferencing, the distance between persons in conversation will be understood as a reasonable measure for accuracy, thus the distinguishing between the searched person and his conversational partner (interlocutor) should be possible using straight vision.
• Concerning an application with vehicles, a sound RTLS will be capable to determine the position of one or the other vehicle. However, for manned vehicles, the straight vision of the driver may compensate for momentary degradation in locating service.

To obtain benefit with any RTLS implementation and with full deployment, the demand for high accuracy should not encumber the balance of cost and benefit. An RTLS will not replace a measurement subsystem for dynamic positioning or any topographic or geodesic metering.

The essence of RTLS design beyond metering is filtering. As in all measuring systems, erroneous results superpose the wanted correct metering. This starts with noise, followed by statistical errors and ends in heavily threatening multi path propagation. There are several approaches possible to defeat these errors. Thriving for accuracy may be performed at the expense of resolution, of latency times till availability or of stability of the results.

As outlined: Multipath propagation is ubiquitous with wireless transmission. There is no escape to prevent from multiple reception and resulting response to any transmission. Thus, for an RTLS the multipath propagation is a burden in the metering process. Therefore strategies to assess the response is indispensable. Such strategies apply in subsequent steps:

• Balancing metering and routing to ensure stable networking
• Multiple frequency transmission to share the problem
• Repetitive transmission to establish a sufficient statistics
• Reception and computing of more than just the strongest signals
• Well defined filtering to exclude erratically unbalanced measurement
• Strategic filtering to eliminate the longer paths via reflecting surfaces
• Estimating locations based on a well designed and consistent estimator
• Over-determined modeling to compensate for persistent false path returns
• Taking inherent dynamics of moving objects in to account
• Building tracks to compensate for singular false estimates
• Low Pass filtering the presentation to prevent from irritating the observer
• Presenting the results as a continuous flow of information to stimulate trust
• Assessing the flow of results and publishing a trust parameter

This list may be extended for the purpose of the operational task. Another escape is hybridization of the system to enable stronger start-up conditions and to provide supporting external sample points. Finally, the solution may not be just a deterministic approach.

This section has been moved to Locating Engine.

To apply any RTLS system must address some requirement to make information available on the spot. It makes no sense to deploy any RTLS, when the Real-Time aspect does not drive the choice. In function, RTLS shows many resemblance to location based services of mobile phone networks. Such typical application may be

• Resource tracking with local distribution: Fork-lifts, pallets, cardboard-boxes, tools, machines
• Resource tracking in a protective manner: Objects without other privacy controls,
• Finding someone or something: Look-up a person by skill (doctor), by capability (disabilities), by quality (tool sets, handy machines etc.)
• Proximity-based actuation (push or pull) buddy finding (grouping), common profile matching (dating), staff management (convening) and resulting approaches
• Proximity-based notification (push or pull) fulfillment based upon proximity (sports, pass, toll), threat based upon track (perimeter protection, asset protection, service performance).

This list may be extended upon relevant requirements.

An RTLS is not a radio detection and ranging system, but an autonomously operating and functionally cooperative system of basically equal nodes. Therefore there is no central scanner function nor any central processing instance nor any otherwise specialized RTLS node to receive, collect and compute data in the locating process for any other node beyond itself. As well there is no specialized higher qualified reader function nor any lower qualified and dumb beacon function.

Basically any RTLS node shall obtain its own location in a cooperative measurement process and hence shall be capable to compute its own location with its own local resources. Additionally any other RTLS node shall be capable to receive the readily computed information from other nodes on location of these and further nodes. However, it may depend on the user requirements, which nodes will be excelled with signaling and display capabilities to support the defined operational tasks with the RTLS usage.

For the sake of an economized systems operation, the qualification of the wireless nodes in an RTLS implementation may differ to serve for distinct roles:

• a mesh of fix-mount RTLS reference nodes that just tells its identity and its location
• one each personally carried RTLS nodes to locate just the own location in a deployment of a multiplicity of mobile bearers
• a multiplicity of mobile objects tagged object wise with RTLS nodes to locate just any such unmanned and tagged object
• auxiliary option with the mobile RTLS nodes may be a local gateway function to serve other communications or further purpose processing units with location data
• application option with the fix-mount RTLS reference nodes is some wired power supply as well as any additional gateway function to forward any user information to and fro other network types transparently

Special fine tuning of network configurations may be possible to reduce the effort in resources tailored to the defined operational task. This may apply to permanent or temporal deployment as well as to the spatial distribution aiming at the line of sight conditions.

A basic application for RTLS is the locating of nodes that normally should not move. Recurrently locating such nodes then simply confirms each node being in the determined place and in function. Such control may be achieved with active RFID as well as far as the resolution and stability for the actual location were of no importance. In any other case the 3D locating capability of RTLS with distance metering and locating engine trumps any RFID with bare RSSI capability.

The RTLS systems is just designed to provide a locating capability that may be:

• Firstly below the performance levels of satellite supported navigation (GNSS) regarding the reach of the covered area,
• Secondly right above the performance levels of satellite supported navigation (GNSS) regarding the latency times for locating and,
• Finally below the performance levels of satellite supported navigation (GNSS) regarding the accuracy of locating at low speed.

Normally, a user of an RTLS will be happy just to make use of the special functions of such system under any roofing or other confinement of operation. Additionally for any temporal deployment and for various operational conditions in open air, a hybrid solution may offer new versatility that will not be addressed by any purely satellite based function.

For the user of any RFID labeling solution, there may be full satisfaction with identifying any object right in the scope of a manually held reader or with a fix-mount reader.

But in general, the complex event of identifying objects by means of any tool requires five, at least three data:

• What is the identity (serial number, type number, order number)
• What time is it at show-up of the object (clock time, calendar date)
• What is the direction of transport (where to in / out)
• Who identified the object (identity of reader and identity of operator, optionally)
• Where is the location to identify the object (location of the operator or just location of the reader, optionally)

The latter term of location is mostly neglected, when RFID readers are operated in confined environment. But, to satisfy the perpetuate desire for maximum reach, the question is important, where the object moved or stood in the moment of identifying it. However, this aspect will gain momentum in mid-term, as RFID is still mostly competing with bar-coding or 2D-coding. Besides the basic issues, the RTLS function generally requires power source on the object, so RTLS is not low-cost approach compared to labeling.

Tracking and tracing as concurrent or posterior computing of a trajectory covers the dynamic aspects of RTLS processing. The aim of ranging and locating is somewhat spontaneous, whereas the track or trace serves protracted interests of motion control.

Any information about momentary or temporary location of an object or a person may be collected to form a track or just to trace the instance or the individual. This in effect simply indicates the need to prove willful acceptance of services through the client under any RTLS control as deemed mandatory with any other location based service focusing individuals in wired or wireless networks. Unauthorised or otherwise impermissible tracking or tracing people with any technical means without such willful agreement will surely lead to dismissal of such services with the community affected.

In industrial environment ordinary tasking covers fulfilment of planned and scheduled operations. Thus in industry any spontaneous locating will mostly not be the key interest. This coincides with the systematic weakness of real time locating, where knowledge about a steady motion may support well the computing of actual locus.

In any emergency or military environment however, there is a tendency to ask for spontaneous determining of an unknown locus, added with the requirement to perform as well on unknown territories or in confinements without any available maps. This appears as a completely different task concerning a supporting infrastructure.

Any RTLS system implementation will make use of a priori knowledge about locations of nodes, i.e. all available information about residing nodes or other nodes that disclose their positions. Such information may be just the address for networking and may comprise properly surveyed locations of reference nodes. This a priori knowledge is complimentary to all knowledge yet obtained in operation and compiled in tracks.

Tracing is understood as elaboration off-line from recorded data and therefore never operated in real time. However, tracing makes use of data collected by means of tracking procedures. The actual tracking of nodes may contribute to collecting data for a trajectory in a data base and may then be presented in plots with underlaid maps or drawings. The anchor nodes in such tracking data sets may be

• model of network topology with topologic locations or a terrestrial or geodesic depiction with topographic locations (geo-locations) of residing nodes
• yet surveyed, metered or otherwise described locations

The trajectories of the traveling nodes may be recorded with a composite data set of locations of touched nodes and intermediate metering results.

To be prepared to segregate corn and husk, there must be an appropriate specification form the very beginning. The interested operating party must set out its own demand. This may be answered by the offering manufacturing party with an appropriate test scenario, that will show the capabilities of the RTLS under test. However, the potential purchaser should stress his brain to identify the critical conditions, which he deems important for operational success, as there are:

• proper co-locating of directly adjacent objects, which is a very hard test to assess as well resolution and accuracy in conjunction with processing that prevents from interchange of positions
• sufficient stability, i.e. the track shall not make jumps that the tracked object would never perform
• sufficient precision, i.e. the reported position should absolutely and relatively be within the locating and ranging accuracy
• sufficient "hunting" quality, i.e. the track of the target should resemble the actual motion of the target
• sufficient speed, i.e.the tracking should show a delay that remains steady
• sufficient compatibility, i.e. systems operation shall not interfere with other systems nor crash in active environments
• sufficient robustness, i.e. systems operation shall not be limited to just and handful of moving nodes and shall be capable to let migrate additional moving nodes in as well as reference nodes fade out

To avoid irritations: RTLS is not made for clandestine tracking. It is a means to assist operational requirements in any configuration. This may apply also to public applications. Then, the willful acceptance shall be expressed with a formal subscription.

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.

RTLS shall be capable to derive locating on the fly, en passant, on the move and during performing. This results in some local display of vicinity and adjacencies. Presentation of results is the minimum output of any RTLS. This may be provided for moving entities or for any observer in the area of operations or apart from there.

The RTLS user gets the location of himself or of the objects under control

• either relatively with indication of distances, or
• dynamically with indication of distance changes, or
• absolutely with some accuracy in any defined grid of coordinates

The displaying of such information is performed visually on a map or in other graph. Alternatively the change of location may be indicated aurally with sound signals.

The applicability of RTLS for operational tasks is bound to the stability of the location depicted. It is neither an advantage to get a false information about location, which may appear in real time but with no real quality, nor may an unstable result support operations, i.e. spontaneous erratic jumps with every update, when the user cannot recognize the computed location as steady.

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Beyond local presentation, the common requirement with locating RTLS nodes is some feedback to those monitoring systems and persons in charge that control the work orders. The RTLS system shall be capable to report locations of moving objects in conjunction with some data transfer to the destination of this reports. However, many of the systems designed in the past do not support such data transfer in the very same communications link. As with GPS operating just in downlink, normally data reporting is transmitted via Cellnet. This leads to the following alternatives for RTLS nodes:

• Locating is central and reporting is not defined
• Locating is just a downlink process, reporting is not included
• Locating is operated on one link and reporting requires an independent data link
• Locating and reporting are symmetric and integrated communications on one link
• Locating and reporting are symmetric and enable user defined communications on one link

The respective variety applies for the feedback with any type of moving RTLS nodes:

• Locating is central and no feedback is defined
• Locating is just local and feedback is not defined
• Locating and reporting are fully automatic, but feedback cannot be exploited
• Locating and subsequent reporting stimulate qualified feedback to the moving RTLS node
• Locating feeds back to order processing and order list is fed back in return and actually adapted
• Locating and subsequent reporting stimulate qualified feedback to the operator with the moving RTLS node

The scope and complexity of possible applications determine an unlimited plurality of options. The RTLS concept does not constrain respective operational designs.

• Real Time Location Systems (RTLS)
• Real-time location services (RTLS)
• Real Time Locating in Nederlands
• Real Time Locating en Espanol

The following page addresses the aspects of standardization:

• Real time locating standards
• Local Positioning Systems

For common terms please refer in Wikipedia to the following

• Bearing
• Detection, Ranging
• Locating, Location, Location awareness
• Localization
• Reckoning, Dead Reckoning
• Reconnaissance
• Middleware
• Navigation
• RFID, Radio Frequency Identification
• Telematics
• Topology, Topography
• Geography, Geolocation
• Tracing, Tracking, Tracking and tracing
• Transportation, Logistics
• Vertex, Node, Edge, Hop

The following links refer to this subject under various aspects:

• Angle of Arrival (AoA)
• Dedicated Short Range Communications (DSRC)
• Mobile ad-hoc network
• Multilateration
• Near-field electromagnetic ranging (NFER)
• Received Signal Strength Indication (RSSI)
• Symmetrical Double Sided - Two Way Ranging (SDS-TWR)
• Time of Arrival (ToA)
• Time Difference of Arrival (TDoA)
• Time of Flight (ToF)
• Ultrawide Band (UWB)
• Wi-Fi

• Galileo
• Global Navigation Satellite System (GNSS)
• Local Positioning Systems
• Location Based Services

The variety of terms concerning RTLS is large. Please extend your inquiries for technical and operational information to other terms, standards and to more organizations involved. But, be aware that "location" may address just network topology only and not any quality of geolocation.

• Active RFID
• asset tracking
• auto locating
• dynamic locating
• IEEE Technical Committee on Real-Time Systems
• IEEE 802.15.3
• IEEE 802.15.4
• location based services
• locating engine
• MANET
• MIIM
• Mobile Item Identification and Management
• optical locating
• tracing solutions
• tracking systems
• VAN
• VANET
• Vehicle Area Network
• wireless locating
• wireless positioning
• wireless ranging
• Wireless Body Area Network
• WBAN
• Wireless Local Area Network
• WLAN
• Wireless Personal Area Network
• WPAN
• TOF = time of flight and DTOF = difference of time of flight might coincide with numerous other usage of these acronyms.

• International Standardization in Engineering
• International Standardisation in Industry and Science
• Search Page for ISO standards concerning RTLS
• International Standardisation in Electrical Engineering
• International Standardisation in Communications
• International Lobbying in Automatic Identification
• US American Lobbying in Automatic Identification
• Statement of International Lobbying on Real Time Locating
• ISO JTC1 (TC122) SC31 WG5 Homepage on RTLS
• ISO JTC1 (TC122) SC31 WG5 Homepage on MIIM
• Company driving international standardisation on e.g. RFID and RTLS in US
• Schedule and actual scope of RTLS standardisation in ISO JTC1, TC122 and SC31
• Car-to_Car communications programme funding of the European Union

• 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)

• Nanotron Technologies GmbH, Berlin, Germany, EU; (SDS-TWR TOA, ISO/IEC 24730-5, 2.45 GHz)
• Symeo GmbH, Munich, Germany, EU; (TOA, 5.8 GHz)
• Ubisense Ltd., Cambridge, UK, EU; (TDOA, 5.9 GHz)
• Time Domain Inc., Huntsville, AL, USA; (UWB TOA, 5.7-7.0 GHz)
• AeroScout, Inc., Redwood City, CA, USA; (WLAN TDOA, WLAN RSSI, 2.45 GHz]
• WhereNet, Santa Clara, CA, USA; [TOA, ISO/IEC 24730-2, 2.45 GHz]
• Multispectral Solutions, Germantown, MD, USA (UWB TOA, 5.7-7.0 GHz)
• Parco Merged Media Corporation, Portland, MA, USA (UWB TOA, 5.7-7.0 GHz)

• AeroScout, Inc., Redwood City, CA, USA; (WLAN TDOA, WLAN RSSI, 2.45 GHz]
• Ekahau, Inc., Saratoga, CA, USA; (WLAN RSSI 2.45 GHz - all Wi-Fi vendors)
• GuardRFID, Richmond, BC, Canada;(Active RFID, 433 MHz/ RSSI - small tags)
• Identec Solutions AG, Lustenau, Austria, EU; (semi-passive RFID, 868 MHz)
• Pinc Solutions Inc., Berkeley, CA, USA; (plain RFID indexing)
• RFCode, Austin, TX, USA; (proprietary WLAN compatible RSSI 2.45 GHz)
• RFTechnologies, Brookfield, WI USA; (WLAN TOF, WLAN RSSI, Peer-to-Peer WiFi, Designed for All WiFi Vendors, 433 MHz, 262 kHz, Passive RFID Gen2, Sensors, Multimode tags, plain RFID indexing)
• Real Time Location Systems Inc., Sandy, Utah; (plain RFID indexing)
• RFind Systems, Inc., Canada; (Active RFID, 915 MHz / 868 MHz, RSSI)
• Intelleflex Corporation, Santa Clara, CA, USA; (Extended Capability RFID, Battery-assisted passive RFID tags)

Remark on actuality: Please refer to International Organisation for Standardisation to check whether the scope covers real industry standards or just intended work schedules. The entries in the list were last updated November 2007. Crosscheck on actuality may be performed with reference to published standards of ISO and ANSI.

• ABI Research market survey on RFID and RSSI (includes RFID, no explicit reference to ISO RTLS standards, offered edition 2006Q4, no time tag for updates)
• ABI Research market survey on RTLS (includes RFID, no explicit reference to ISO RTLS standards, offered edition 2007Q3, no time tag for updates)
• ABI Research market survey on Location Aware Services Research Service (includes RTLS, ie. 1 of 7 technologies in 1 of 6 chapters, edition date not specified, no time tag for updates)
• Electronics.CA report on Real Time Locating Systems (editor identical to IDTechEx study, see below)
• GII Express market survey on RTLS (mixed RFID and RTLS topics, offered edition 2006Q3, , no time tag for updates, outdated references to planned but non existing standards)
• IDTechEx market survey on RTLS (mixed profile in technology segregation, intentionally reviewed edition announced for 2007Q4)
• Frost&Sullivan on RTLS Markets for Healthcare Providers (mixed profile in technologies, RTLS 1 of 7 chapters, no reference to standards, edition date not specified, no time tag for updates]
• Research and Markets survey on RTLS (mostly RFID topics, outdated reference to standards, mixed edition 2005 + 2006, no time tag for updates)
• The Infoshop report on Real Time Locating Systems (not segregating RFID, RSSI and RTLS in the case study part, offered edition 2007Q3, no time tag for updates)
• Monitoring the Convergence of RTLS Technologies (no details specified, offered edition 2005Q4, no time tag for updates)
• Yankee Group market survey on RTLS (no details specified, edition 2005Q3, update announced, but no time tag published)