Today, there is a vast array of location technologies that are involved in the calculation of a user’s or object’s position in a space or grid, based on some mathematical model.

REAL TIME LOCATION SYSTEMS

The need to locate people and objects as soon as possible has always been an important part of any organization or industry, especially in manufacturing, health- care and logistics.

Today, there is a vast array of location technologies that are involved in the calculation of a user’s or object’s position in a space or grid, based on some mathematical model.

The technology required for automated location information provisioning has been in continual development for several decades; while the majority has its roots in the military, modern consumer technology is also rising to meet the challenges, specifically in metropolitan areas.

A wide range of problems can be solved with RTLS, therefore a multitude of applications and services can benefit from positioning such as logistics, routing, sales, asset tracking, personal safety and emergency response.

Fortunately, over the past decade, advances in location positioning technology have made it possible to locate users and objects using radio waves, ultrasounds, infrareds or laser.

Real time location systems present opportunities for a rich set of applications such as

navigation tools for humans and robots, interactive virtual games, resource discovery,

asset tracking, location-aware sensor networking and other industrial, commercial,

office, security and military applications.

3.1 Ranging methods

Several methods for performing ranging calculations are possible:

• AOA – angle of arrival

• TOA – time of arrival

• TDOA – time difference of arrival

• RSSI – received signal strength indication

• TOF – time of flight

• TWR – two way ranging

• SDS-TWR – symmetrical double sided two way ranging

• NFER – near field electromagnetic ranging

These methods employ radio waves, ultrasounds, infrareds or laser; as for radio waves they are or can be used, in combination or individually, in GPS technology, RFID systems, cellular networks and wireless technologies as WLAN, Bluetooth, UWB, ZigBee and WiMax.

Ultrasound systems can be accurate to around 1 cm in 3D but they are affected by loud noises and they require a very dense network of receivers; this makes them expensive and difficult to install in rooms with high ceilings or outside.

Optical systems can be very accurate, to around 1mm in 3D, but are affected by smoke, can interfere with vision enhancement systems in night exercises and depend on line of sight, so require a dense network of scanners to avoid occlusions.

GPS uses satellite signals and so requires no special infrastructure, but it will not work reliably indoors or in “urban canyon” situations due to signal attenuation.

Cellular networks generally fail to provide a satisfactory degree of accuracy.

With regard to radio waves, the higher is the frequency, the lower is receivers’

density, but the more ranging is prone to multipath interference.

Multipath cancellation occurs when a strong reflected wave off walls or other objects

arrives partially or totally out of phase with the direct path signal, causing a reduced

amplitude response at the receiver; this lead to positioning errors, so location is

provided with less accuracy.

For example, conventional spread spectrum signals like Wi-Fi experience significant waveform distortion and loss of signal strength providing an accuracy of up to 20 m indoor and up to 40 m outdoor.

If a time-based ranging method is used, the precise moment of signal reception is estimated detecting the leading edge of the cross-correlation function.

It is known that the wider is the signal bandwidth, the narrower are the correlation peaks and the higher is the immunity to multipath effects: the direct path has essentially come and gone before the reflected path arrives and no cancellation occurs.

Hence making the correlation peaks as narrow as possible increases the time resolution of the method and therefore the precision of location.

Ultra-Wideband emerging technology employs a chirped signal or an impulse with an extremely short duration, typically ranging from a few hundred picoseconds to a few nanoseconds, thus resulting in a directly modulated waveform (it does not using a carrier to move signals from baseband to passband).

As a consequence of these extremely short time durations, the average transmit power is extremely small, resulting in huge savings in battery consumption.

Moreover, since the speed of light is approximately 0.3 meters per nanosecond, UWB pulses have spatial extents measured in centimeters, enabling extremely fine range resolution in RTLS.

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3.1 – An UWB pulse

UWB’s short time bursts correspond to a bandwidth of several GHz, thus the energy

is spread over a broad range of frequencies resulting in a very low energy density that

translates into a low probability of interference to proximity systems and minimal RF health hazards.

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3.2 – UWB spectrum

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3.3 – Multipath cancellation effects on Wi-Fi and UWB signals

Another important feature of UWB is its circuit simplicity and size, as can be seen in the following figure:

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3.4 – A comparison of Wi-Fi and UWB integrated circuits’ size

On the basis of such considerations Ultra-Wideband is commonly believed to be the technology of choice for ranging systems: it offers a good compromise between high accuracy (about 15 cm) inside buildings, reasonably low levels of infrastructure (receiver density is a bit higher than in conventional RF systems), outdoor performances, small tag size and fairly low power consumption.

A real time location system consist of only two key parts: a set of transmitters and receivers nodes, used to compute the range between various nodes in the system and a location engine, that is used to determine the position of one of the nodes; the location engine collects the estimated distance measurements from the network of transmitters and receivers and then provides these measurements as input data to an algorithm that determines the position of the target node or set of nodes.

The above mentioned ranging methods will be described in the following paragraphs.

3.1.1 Angle of arrival

AOA is a method for determining the direction of propagation of a received signal:

using direction sensitive antennas on a receiver, the direction to the transmitter can be obtained.

The angle of arrival is determined by measuring the angle between a line that runs from the receiver to the transmitter and a line from the receiver with a predefined direction, for instance the north.

This method can be illustrated as follows, where R

is the receiver and T denotes the transmitter.

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3.5 – Angle of arrival method

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3.6 – Determining position with angle of arrival

Using the positions of two receivers at known locations, the position of a transmitter

can be determined using simple triangulation: for each receiver, the angle of arrival of

the signal received from the same transmitter is calculated and then an algorithm is

used by the location engine to determine the position of the transmitter.

In figure 3.6 is illustrated the way in which position is determined with AOA; the transmitter is denoted as T and the receivers as R

, R

and R

, respectively.

Taking measurements using this method often requires a complex set of between 4 and 12 antenna arrays situated in a horizontal line at several cell site locations: the accuracy increases with the number of antenna arrays used.

In addition to the cost, the resulting angle measurements are rather sensitive against multipath propagation common in building environments: this method is best suited for direct line of sight measurements between transmitters and receivers.

Furthermore, the angle of arrival method is also susceptible to security threats as attackers can easily reflect or retransmit from a different location.

3.1.2 Time of arrival

TOA is a method based on the measurement of the propagation delay of the radio signal (as opposed to a data packet) between a transmitter and one or more receivers.

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3.7 – Time of arrival method

Propagation delay, which can be calculated as t

– t

, is the time lag of the signal departure from a source station T to a destination station R; in other words, it is the amount of time required for a signal to travel from the transmitter to the receiver, as shown in the above figure.

The distance between the transmitter and the receiver is obtained multiplying the propagation delay by the propagation speed of the signal.

In a 2D plane the location of a transmitter can be seen as an intersection of circles and in 3D space it can be seen as an intersection of spheres.

To determine the transmitter position, in a 2D plane at least three receivers and three

circles are required, while in 3D space at least four receivers and four spheres are

required to take TOA measurements.

The time of arrival method for 2D range calculations can be illustrated as follows, where the transmitter is denoted as T and the receivers are R

, R

and R

; the signal is transmitted at t

and received by receivers at t

, t

and t

respectively.

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3.8 – Determining position in a 2D plane with time of arrival

To have any reasonable confidence in the measurement of the propagation delay, the clocks of the transmitter and the receiver must be synchronized.

A distance can be determined by this method only with a considerable cost: to achieve precision up to the nanosecond scale, which results in a more precise distance measurement, an elaborate clock synchronization system must be developed which has high costs in terms of development time and effort.

Furthermore, at least three/four receivers are required, which also adds to the cost

and complexity of the system.

3.1.3 Time difference of arrival

TDOA method is similar to TOA: while the time of arrival method can be seen as an intersection of circles or spheres with center points of known locations, this method can be seen as an intersection of hyperbolas or hyperboloids.

Time difference of arrival measures the difference in the time the signal arrived at different receivers: while TOA uses the time that a transmitter sends a signal to the receivers, TDOA requires that the receivers record when the signals were received.

In a 2D plane the location of a transmitter can be seen as an intersection of hyperbolas and in 3D space it can be seen as an intersection of hyperboloids: that is why this method is also known as three dimensional hyperbolic positioning.

To determine the transmitter position, in a 2D plane at least three receivers and any two of the three hyperbolas are required, while in 3D space at least four receivers and any three of the four hyperboloids are required to take TDOA measurements.

With TDOA each of the receivers receive the signal from the transmitter and record when it was received; this information is forwarded to a location engine which calculates the difference in time of arrival between pairs of receivers and provides an estimated position of the transmitter through an algorithm.

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3.9 – Determining position in a 2D plane with time difference of arrival

Like TOA, TDOA requires the clocks of each of the receivers to be synchronized;

the precision of the location engine is correlated to the accuracy of the clocks, with

more accurate clocks providing more accuracy, but also a higher cost: in most cases, therefore, the clocks run asynchronously with its related affect on precision.

Furthermore, TDOA is also affected by multipath propagation, noise and interferences, thus direct line of site is preferable.

3.1.4 Received signal strength indication

RSSI is based on the strength of the signals sent by at least three reference nodes and received by the node to be tracked; to increase accuracy, more sophisticated RSSI methods use a map, called RF fingerprint, which uses calibrations of the strength of signals at various points in a predefined area.

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3.10 – Determining position with receiver signal strength indication

The RSSI method is illustrated in the figure above, where reference nodes are denoted as R

, R

and R

and signal strengths as S

, S

and S

respectively.

The distance between a reference node R and the node T to be tracked, is

determined by converting the value of the signal strength at R (before attenuation

due to the medium) into a distance measurement based on the known signal strength

at T and on a particular path-loss model.

The location engine algorithm estimates T position using the computed distances between T and several receivers.

Although the determination of the distance differs substantially from the TOA method, the location calculation relies on similar algorithms.

To be effective, this method requires a dense deployment of reference nodes, which adds considerably to the cost of the system; however, the key problem related to RSSI is that an adequate underlying path-loss model must be found for both non- line-of-sight and non-stationary environments; consequently, in practice, estimated distances are somewhat unreliable.

Furthermore, systems using RSSI can be disqualified from security applications as attackers can easily alter the strength of received signals by either amplifying or attenuating a signal or by other methods.

Finally, the issue of using a network in mission critical purposes, while RTLS burdens it with additional tasks, is yet to be resolved.

3.1.5 Time of flight

TOF method uses measured elapsed time for a transmission between a transmitter and a receiver based on the estimated propagation speed of a typical signal through a medium.

Since a time value is used, clock accuracy becomes significantly more important than in previous methods.

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3.11 – Determining position with time of flight

A signal with known departure time is sent from a transmitter with a known location, to the receiver, the departure time is compared with the arrival time and the distance is determined, with an accuracy of about 2 meters, using an estimate of the signal’s propagation speed.

Using three transmitters, each one associated with a coordinate, an algorithm can determine the location of the receiver in 3D space.

The TOF method is illustrated in figure 3.11, where the receiving node to be tracked is denoted as T, the transmitting reference nodes as R

, R

and R

, the departure times as t

, t

and t

, the arrival times as t

, t

and t

and the distances as D

, D

and D

, respectively.

This method does not add additional hardware overhead to the system as it can use the same hardware that would be used for data communication and signal processing.

An ideal TOF system requires costly accurate clocks: clocks’ offset and drift corrupt ranging accuracy; moreover the signal can be affected by interference from other signals, noise and multipath propagation.

Time of flight is reasonably successful in indoor environments, such as with concrete walls and floors, has a relatively high accuracy and is considered to be a secure method for RTLS.

3.1.6 Two way ranging

TWR method overcomes the inherent difficulties of the TOF method making the clock synchronization requirement irrelevant to the measurement.

Two way ranging method (also called round trip time) do this by sending a ranging signal and waiting for an acknowledgement, a process known as “round tripping”:

• node 1 sends a data signal to node 2 at t

• node 2 starts time measurements at t

when it receives the signal from node 1

• node 2 stops time measurements at t

when it sends the acknowledgement signal to node 1

• node 1 receives the acknowledgement signal at t

• the difference between the time measured by node 1, t

– t

and the time

measured by node 2, t

– t

, is twice the time of signal propagation through

the medium

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3.12 – Round trip time method

In RTT method timestamps on the physical layer and not on application layer are used and the processing time (t

– t

measured by node 2) is assumed to be equal on both nodes.

The round trip time is a rather long interval compared with the time required for a signal to propagate through the air: the time a transceiver uses to transmit and receive packets requires hundreds of microseconds rather than the tens of nanoseconds which are required to propagate a signal through air.

Therefore, although the internodal synchronization is no longer required, the problem of the clock generating oscillator’s quality appears in the TWR scheme.

An acceptable error of the round trip measurement due to oscillator drift should not exceed one nanosecond, but this would require a crystal with a tolerance far beyond that of the crystals typically deployed in RTLS (10 parts-per-million or better).

3.1.7 Symmetrical double sided two way ranging

SDS-TWR is a TOF based method that uses TWR to avoid clock synchronization and double-sided ranging to zero the errors of the first order due to the clock drift.

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3.13 – Symmetrical double sided two way ranging method

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3.14 – Determining position with symmetrical double sided two way ranging

3.1.8 Near field electromagnetic ranging

Technologies based on radio waves, which travel 0.3 meters every nanosecond, have to deal with near nanosecond synchronization that is both expensive and difficult.

NFER technology avoids these problems by exploiting near-field differences

between electric and magnetic waves, providing a low-cost, long-range, high-accuracy

(about 30 centimeters) solution to the problem of tracking people and assets.

A near field electromagnetic ranging system compares the phase between the electric component and the magnetic component of a radio signal to yield a precise measurement of range.

Unlike far field systems that only capture two signal properties like amplitude and phase (or equivalently time of arrival), a NFER system captures phase and amplitude data from both the electric and magnetic components of the signal: this additional information allows an individual receiver to track not only range but also angle of arrival.

In many cases a single receiver can provide accurate tracking information, although multiple receivers improve accuracy and reduce error.

The low frequencies of a NFER system are relatively immune from multipath, yielding precise tracking at distances much further than comparable high frequency systems.

Low frequencies have better propagation characteristics (fewer receivers cover a large area), enable simpler, more economical hardware and are more penetrating, propagating through or diffracting around obstructions that would block comparable high frequency signals.

Furthermore, propagation measurements indicate that NFER systems work well underground or in metal rich environments: thus, a near field electromagnetic range system is ideal for tracking in complicated environments.

Finally, the near-field nature of NFER systems gives ultra-low effective isotropic radiated power and thus a low-probability-of-intercept signal.

3.2 GPS

NAVigation Satellite Timing And Ranging Global Positioning System, or NAVSTAR GPS, is a network of satellites, realized by United States Department of Defense, that continuously transmit coded information, which allows to precisely identifying locations on earth with planimetric and altimetric coordinates by measuring distance from satellites.

The system was born from a U.S. DoD top-secret project during the last years of the

cold war originally intended for military applications; the main target was to support

artillery fires (ballistic and intercontinental missiles launch) and to coordinate the

troop deployment.

In the 1980s the government made the system available for civilian use: satellites yet owned a dedicated civilian channel, L1, planned by military hierarchies for this scope.

GPS can be used everywhere except where it is impossible to receive the signal, for example underwater, inside buildings and in caves, parking garages and subterranean locations.

Basically it allows recording or creating locations from places on the earth and helping to navigate to and from those spots.

The most common airborne applications include navigation by general aviation and commercial aircraft, while at sea, GPS is typically used for navigation by recreational boaters and fishing enthusiasts; land-based applications are more diverse:

• the scientific community uses GPS for its precision timing capability and survey accuracy

• recreational uses are almost as varied as the number of recreational sports available

• GPS is rapidly becoming commonplace in the automotive segment as well:

some basic systems provide emergency roadside assistance at the push of a button (by transmitting the position to a dispatch center), while more sophisticated systems show vehicle’s position on a street map and suggest the best route to follow to reach a designated location

3.2.1 The three segments of GPS

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3.15 – The three segments of GPS

The space segment consists of 30 satellites (21 active and the other operating spares) orbiting at an altitude of approximately 20’200 kilometers.

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3.16 – GPS satellite constellation

These Space Vehicles are constantly moving at speeds of roughly 11’265 kilometers per hour, making two complete orbits in less than 24 hours.

The satellite orbits repeat almost the same ground track once a day, as the earth turns beneath them.

There are six orbital planes (with nominally five SVs in each), equally spaced (60 degrees apart) and inclined at about fifty-five degrees with respect to the equatorial plane; this constellation provides the user with between five and eight SVs visible from any point on the earth.

GPS satellites are powered by solar energy and have backup batteries onboard to keep them running in the event of a solar eclipse; small rocket boosters on each satellite keep it flying in the correct path.

The first GPS satellite was launched in 1978 and a full constellation of 24 satellites

was achieved in 1994, completing the system.

Each SV weighs approximately 900 kilograms, is about 5 meters across with the solar panels extended, has a transmitter power of 20 – 50 watts and is built to last about 10 years.

GPS provides two levels of service:

• SPS – Standard Positioning Service, a positioning and timing service available to all GPS users

• PPS – Precise Positioning Service, a highly accurate military positioning, velocity and timing service available only to users authorized by the U.S.

SVs transmit two microwave carriers: the L1 signal (1575.42 MHz) carries the navigation message and the SPS code signals, while the L2 signal (1227.60 MHz) is used to measure the ionospheric delay by PPS equipped receivers.

The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.

Three binary codes are used to phase modulate the carriers as described below.

• The Coarse Acquisition Code modulates the L1 carrier phase; the C/A code is a 1’023 bits long pseudo-random noise (PRN) code transmitted at 1.023 Mbps so it repeats every millisecond.

Since there is a different C/A code for each SV, GPS satellites are often identified by their PRN number.

• The Precise Code modulates both the L1 and L2 carrier phases; the P-code is a 6’187’100’000’000 bits long PRN code transmitted at 10.23 Mbps so it only repeats once a week since it is.

To prevent unauthorized users from using or potentially interfering with the military signal through a process called spoofing, it was decided to encrypt the P-code modulating it with the W-code, a special encryption sequence, to generate the Y-code, which is what the satellites have been transmitting since the anti-spoofing module was set to the on state.

The encrypted signal is referred to as the P(Y)-code, is the basis for the PPS, requires a classified AS module for each receiver channel and is for use only by authorized users with cryptographic keys.

• The Navigation Message modulates both the C/A and P(Y) ranging codes at

50 bps and contains the satellite orbital and clock information, general system

status messages and an ionospheric delay model.

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3.17 – GPS signals

The satellite signals are timed using highly accurate atomic clocks; their main purpose is to allow the calculation of the travel time (TOA) from the satellite to the GPS receiver on the earth.

The control segment consists of ground stations (five of them, located in Hawaii, Ascension Island, Diego Garcia, Kwajalein and Colorado Springs) that make sure the satellites are working properly; the Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado.

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3.18 – GPS Mater Control and Monitor Station Network

These control stations compute precise orbital data (ephemeris) and SV clock corrections for each satellite with the signals from the satellites which are incorporated into orbital models: the Master Control station uploads ephemeris and clock data to the satellites; the satellites then send subsets of the orbital ephemeris data to GPS receivers over radio signals.

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3.19 – GPS control monitor

The user segment consists of the GPS receivers and the user community.

GPS receivers convert satellites signals into position, velocity and time estimates;

four satellites are required to compute the four dimensions relative to position (latitude, longitude

and altitude) and time.

Navigation in three dimensions is the primary function of GPS; navigation receivers are made for aircraft, ships and ground vehicles and for hand carrying by individuals.

Precise positioning is possible using GPS receivers at reference locations providing corrections and relative positioning data for remote receivers; surveying, geodetic control and plate tectonic studies are examples.

Research projects have used GPS signals to measure atmospheric parameters, while astronomical observatories, telecommunications facilities and laboratory standards can be set to precise time signals or controlled to accurate frequencies by special purpose GPS receivers; this is accomplished with time and frequency dissemination based on the satellites’ precise clocks that are controlled by the monitor stations.

1

See paragraph 3.2.6.3 at page 81 for details.

3.2.2 GPS at work

3.2.2.1 Location

The GPS receiver picks up two kinds of coded information from the satellites:

almanac and ephemeris data.

Almanac data contains the approximate locations of the satellites; this data is periodically updated with new information as the satellites move around and is stored in the memory of the GPS receiver so it knows the orbits of the SVs and where each satellite is supposed to be.

Sometimes, when the GPS unit is not turned on for a length of time, the almanac can get outdated or “cold” and the receiver could take longer to acquire satellites.

A receiver is considered “warm” when the data has been collected from the satellites within the last four to six hours.

Any satellite can travel slightly out of orbit, so the ground monitor stations keep track of the satellite orbits, altitude, location and speed; the ground stations send the orbital data to the master control station, which in turn sends corrected data up to the satellites.

This corrected and exact position data is called the ephemeris data, is valid for about four to six hours and is transmitted in the coded information to the GPS receiver.

By receiving the almanac and ephemeris data, the GPS receiver knows the position of the satellites at all times.

3.2.2.2 Time

Even though the GPS receiver knows the precise location of the satellites in space, it still needs to know how far away the satellites are to determine its position on Earth.

To compute the distance, the receiver has to know the time necessary to a radio signal to travel from the satellite to the receiver and any delay due to the signal travel through the earth’s atmosphere.

When a satellite is generating the pseudo-random code, the GPS receiver is

generating the same code and tries to match it up to the satellite’s code.

The receiver compares the two codes to determine how much it needs to delay (or shift) its code to match the satellite one; this delay time is multiplied by the speed of light to get the distance.

Since a GPS receiver clock is not as precise as the satellite clocks, each distance measurement is affected by an error that needs to be corrected; for this reason, the range measurement is referred to as a “pseudo-range”.

3.2.2.3 Position

The position of the receiver is where the pseudo-ranges from a set of satellites intersect: having satellite location and distance, the receiver can determine a position.

If the receiver is 20’000 kilometers from one satellite, its location would be somewhere on an imaginary sphere that has the satellite in the center with a radius of 20’000 km.

If the same receiver is 22’000 km from another satellite the second sphere would intersect the first sphere to create a common circle:

Figure 3.20 – Intersection of two spheres

If a third satellite is added, at a distance of 24’000 km, two common points where the three spheres intersect are found; the two possible positions differ greatly in latitude, longitude and altitude, thus to determine which of the two is the receiver actual position, it is necessary to enter the approximate altitude into the GPS receiver.

If the receiver distance from a fourth satellite is 20’000 km, there is a fourth sphere

intersecting the first three spheres at one common point.

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3.21 – Intersection of three spheres

To determine position using pseudo-range data, a minimum of four satellites must be tracked and the four fixes must be recomputed until the clock error disappears.

3.2.3 GPS receiver technology

Most modern GPS receivers have a parallel multi-channel design since older single- channel designs were limited in their ability to continuously receive signals in the toughest environments, such as under heavy tree cover.

Parallel receivers typically have from between 5 and 20 receiver circuits, each devoted to one particular satellite signal, so strong locks can be maintained on all the satellites at all times.

Parallel-channel receivers are quick to lock onto satellites when first turned on and they are unequaled in their ability to receive the satellite signals even in difficult conditions such as dense foliage or urban settings with tall buildings.

3.2.4 Error sources

Civilian GPS receivers are affected by inaccuracy caused by the accumulation of positioning errors due to some of the following issues.

Because of these errors civilian GPS horizontal position fixes are typically only

accurate to about 15 meters.

• Ionosphere and troposphere delays – The satellite signal slows as it passes through the atmosphere; the NAVSTAR system uses a built-in model that calculates an average, but not an exact, amount of delay.

• Signal multi-path – It occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver; this increases the travel time of the signal, thereby causing errors.

• Receiver clock errors – Since it is not practical to have an atomic clock in a GPS receiver, the built-in clock can have very slight timing errors.

• Orbital errors – Also known as “ephemeris errors”, these are inaccuracies of the satellite’s reported location.

• Number of visible satellites – The more satellites the receiver can “see”, the better the accuracy is; buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception causing position errors or possibly no position reading at all: the clearer the view, the better the reception is, thus GPS units will not work indoors, underwater, or underground.

• Satellite geometry/shading – This refers to the relative position of the satellites at any given time: ideal satellite geometry exists when the satellites are located at wide angles relative to each other; poor geometry results when the satellites are located in a line or in a tight grouping.

• Intentional degradation of the satellite signal – The U.S. military’s intentional degradation of the signal is known as Selective Availability and is intended to prevent military adversaries from using the highly accurate GPS signals; SA was turned off May 2, 2000 and is currently not active, thus typical GPS accuracies in the range of 6 - 12 meters can be expected.

3.2.5 Techniques to improve accuracy

Augmentation methods of improving accuracy rely on external information being integrated into the calculation process.

Some systems transmit additional information about sources of error (such as clock

drift, ephemeris or ionospheric delay), others provide direct measurements of how

much the signal was off in the past, while a third group provide additional

navigational or vehicle information to be integrated in the calculation process.

A Satellite Based Augmentation System (SBAS) is a system that supports wide-area or regional augmentation through the use of additional satellite-broadcast messages.

Such systems are commonly composed of multiple ground stations, located at accurately-surveyed points; the ground stations take measurements of one or more of the GPS satellites, the satellite signals or other environmental factors which may impact the signal received by the users and using these measurements, information messages are created and sent to one or more satellites for broadcast to the end users.

The Wide Area Augmentation System (WAAS) is an example of SBAS.

The terms Ground Based Augmentation System (GBAS) and Ground-based Regional Augmentation System (GRAS) describe a system that supports augmentation through the use of terrestrial radio messages.

As with the satellite based augmentation systems detailed above, ground based augmentation systems are commonly composed of one or more accurately surveyed ground stations, which take measurements concerning the GPS and one or more radio transmitters, which transmit the information directly to the end user.

Generally, GBAS networks are considered localized, supporting receivers within 20 kilometers and transmitting in the VHF or UHF bands, whereas GRAS is applied to systems that support a larger, regional area, and transmit in the LF and MF bands.

The United States Local Area Augmentation System (LAAS) and Differential GPS (DGPS) are examples of GBAS and GRAS, respectively.

The augmentation may also take the form of additional information being blended into the position calculation; many times the additional navigation sensors operate via a different principle than the GPS receiver, thus are not necessary subject to the same sources of error or interference.

A system such as this is referred to as an Aircraft Based Augmentation System (ABAS); the additional sensors may include eLORAN receivers , Automated Celestial navigation systems , Inertial Navigation Systems and Simple Dead reckoning systems composed of a gyro compass and a distance measurement sensor.

Precise monitoring methods improve the accuracy of a calculation through precise monitoring and measuring of the existing GPS signals in additional or alternate ways.

The first is called Dual Frequency monitoring , and refers to systems that can compare

two or more signals, such as the L1 frequency to the L2 frequency; since these are

two different frequencies, they are affected in different, yet predictable ways by the atmosphere and objects around the receiver, so, after monitoring these signals, it is possible to calculate and nullify errors; this technique is currently limited to specialized surveying equipment and to PPS.

A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS).

It corrects the error that arises because the pulse transition of the PRN is not instantaneous and thus the correlation (satellite-receiver sequence matching) operation is imperfect.

The CPGPS approach utilizes the L1 carrier wave, which has a period 1000 times smaller than that of the C/A bit period, to act as an additional clock signal and resolve the uncertainty.

The phase difference error in the normal GPS amounts to between 2 and 3 meters of ambiguity; CPGPS working to within 1% of perfect transition reduces this error to 3 centimeters of ambiguity.

By eliminating this source of error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters of absolute accuracy.

Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system; in this approach, determination of range signal can be resolved to an accuracy of less than 10 centimeters.

This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver.

This can be accomplished by using a combination of DGPS correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests, possibly with processing in real-time.

3.2.5.1 WAAS

The Wide Area Augmentation System is an extremely accurate navigation system developed for civil aviation by the Federal Aviation Administration (FAA), a division of the United States Department of Transportation (DOT).

The system augments the GPS to provide the additional accuracy, integrity, and

availability necessary to enable users to rely on it for all phases of flight, from en

route through GLS approach for all qualified airports within the WAAS coverage

area.

Before WAAS, the U.S. National Airspace System (NAS) did not have the ability to provide horizontal and vertical navigation for precision approaches for all locations, as ground-based systems are quite expensive.

“Wide Area” refers to a network of 25 ground reference stations that cover the entire U.S. and some of Canada and Mexico and are located at precisely surveyed spots to compare GPS distance measurements with known values.

Each reference station is linked to a master station, which puts together a correction message and broadcasts it via satellite.

WAAS capable receivers typically have accuracies of 3 - 7 meters.

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3.22 – The Wide Area Augmentation System

3.2.5.2 Differential GPS

Differential GPS works by placing a GPS receiver (called a reference station) at a known location; since the reference station knows its exact location, it can determine the errors in the satellite signals.

This is accomplished by measuring the ranges to each satellite using the signals received and comparing these measured ranges to the actual ranges calculated from its known position.

The difference between the measured and calculated range for each satellite in view

becomes a “differential correction”; the differential corrections for each tracked

satellite are formatted into a correction message and transmitted to DGPS receivers that will apply corrections to calculations, removing many of the common errors:

typical DGPS accuracy is 1-5 meters.

The level of accuracy obtained is a function of the GPS receiver and the similarity of its “environment” to that of the reference station, especially its proximity to the station.

The reference station receiver determines the error components and provides corrections to the GPS receiver in real time; corrections can be transmitted over FM radio frequencies, by satellite or by beacon transmitters maintained by the U.S. Coast Guard.

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3.23 – Differential GPS

3.2.6 Mapping

Paper maps, most reliable when used with a compass to determine orientation and direction, have traditionally been the primary navigation tool for centuries.

With the advent of GPS technology, it can be thought that maps have become obsolete; instead they have become even more useful.

While paper maps provide great detail and a large view of the map area, electronic

maps can display only a small area in detail, they can ultimately depict the entire earth

and have the added convenience of zoom in and zoom out map scale selection: GPS

maps do not typically display at the same scale as a paper map but can be zoomed in or out to approximate a paper map scale.

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3.24 – A paper map

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3.25 – A zoomed in and out electronic map

3.2.6.1 Mapping the earth

The basic method of determining location or anything else on the earth is to use a global reference system.

The system most generally accepted is a coordinated grid system comprised of hypothetical lines that encircle the earth in vertical and horizontal directions.

The horizontal lines make parallel circles around the globe called parallels of latitude while the vertical lines divide the earth into segments that meet at each pole and are called meridians of longitude.

The equator, one of the lines of latitude, circles the earth exactly midway between the north and south poles; the location of each of the other parallels is determined by measuring the angular distance from the parallel and the center of the earth expressed in degrees, minutes and seconds.

The lines of longitude begin with the one that runs through Greenwich, England and is designated as the prime meridian; the angular distance between meridians ranges from 0 degrees at the prime meridian to 180 degrees at the meridian located on the opposite side of the globe (International Dateline).

Together parallels and meridians construct a grid measurement system known as graticule.

When describing a particular location by latitude and longitude, the latitude is always stated first in degrees, minutes and seconds followed by the designation N for north or S for south depending on which side of the equator the location rests; the longitude is given next in degrees, minutes and seconds, followed by E for east or W for west of the prime meridian.

The process of measuring latitude and longitude on a map is commonly expressed as

“easting” and “northing” since it involves first moving right (east) from the vertical grid line nearest the desired map location and then up (north) from the nearest horizontal grid line to the map location.

Now because the graticule defines a spherical shape, it is not possible to create a flat paper map of the earth without some distortion, so, in response for the need to chart locations of things and people on the earth and create a route to others, various types of maps have been devised to fit the various needs for navigation, research and documentation of the earth’s physical features.

The distortion is the result of depicting (the technical term is projecting) the curved surface of the earth on a flat surface; the smaller the section of the earth portrayed on a map the less the distortion.

The most ingenious of the map makers have devised methods of projecting the earth

onto a map that provides the best possible solution for accurate measurement of

distance and thus locating a position on the earth from another position and traveling to it (accurate navigation).

Maps and charts are essentially grids created from a starting reference point called datum, a mathematical model of the earth’s shape used to determine a position; the most common US map datums are World Geodetic System 1984 (WGS 84), North American Datum 1983 (NAD 83) and North American Datum 1927 (NAD 27).

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3.26 – Constructing a graticule to map the earth

3.2.6.2 Map projections

The most used type of map is the Mercator cylindrical projection; it is most useful for navigation because a straight line on the map corresponds to a compass heading.

Mercator projection preserved length by projecting the earth’s surface onto a cylinder that shares the same axis as the earth; this causes latitude and longitude lines to intersect at right angles to eliminate the problem of longitudinal lines drawing closer together as they approach the earth’s poles.

Meridians are equally spaced, but parallels are not because the Mercator projection straightens the lines of longitude and increases the space between the lines of latitude equal to the space of longitudinal widening.

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3.27 – Mercator projection

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3.28 – The world in Mercator projection

Maps made this way are most accurate within 15 degrees of the equator and since

distortion is high at the northern and southern portions of the map (land masses at

these limits appear to be much larger than they really are in relation to land masses near the equator), the projection stops at the 84th parallel.

The Universal Transverse Mercator projection is a variation of the Mercator projection that rotates the cylinder so that its axis passes through the equator and then it could be turned to line up with any area of interest.

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3.29 – Universal Transverse Mercator projection

The UTM system, probably the most commonly used map projection, was developed to set a universal world-wide system for mapping.

The UTM grid was created using the Transverse Mercator projection to cover that part of the earth between latitude 84° N and latitude 80° S.

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3.30 – The UTM grid

The Transverse Mercator projection was applied in sixty positions to create sixty zones around the world, each six degrees in width and with its own origin at the intersection of its central meridian and the equator.

All sixty zones are identical in grid pattern; the equator and the central meridian of each zone are assigned a value in meters and serve as base lines for each zone in the grid.

Gridlines are drawn at regular intervals parallel to these two base lines and each grid line is assigned a value to indicate its distance from the origin.

Although it would appear more logical to assign a value of zero to the two base lines and measure outwardly from the intersection, this would require N, S, E or W direction designations or negative numbers west of the central meridian and south of the equator.

Each zone is divided into horizontal bands associated to a letter from south to north beginning with C (omitting I and O to avoid confusion with numbers) and ending with X; bands are separated by 8° of latitude, except the latitudinal band X, which spans 12°.

When using UTM coordinates, the zone number and band letters are included in the description:

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3.31 – An example of UTM coordinates

Positions are measured using eastings and northings and are measured in meters instead of degrees and minutes as with latitude and longitude.

Eastings begin at 500’000 on the center line (central meridian) of each zone; in the northern hemisphere, northings begin at the equator, i.e. 0 and increase as they move toward the pole, while in the southern hemisphere, northings begin at 1’000’000 at the equator and decrease as they move toward the pole to eliminate the use of negative numbers.

To determine a location on the globe it must be also known which hemisphere and

zone it is in, as coordinates will be identical from zone to zone without the zone

number and zone grid letter.

Another variation of the standard Mercator projection is the Space Oblique Mercator projection which lines the cylinder up with the orbital path of a satellite in order to accurately map the earth from satellites with little or no distortion, much like GPS satellites.

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3.32 – Space Oblique Mercator projection

3.2.6.3 Latitude and longitude

Reading a map requires an understanding of how locations on the map are measured.

Almost all maps indicate their location using latitude and longitude that has been used as a grid measurement system for navigating the earth for hundreds of years.

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3.33 – Latitude and longitude

Latitude is the angle, at the centre of the coordinate system, between any point on the earth's surface and the plane of the equator.

Lines joining points of the same latitude are called parallels and trace concentric circles on the surface of the earth.

Each pole is 90 degrees: the North Pole 90° N and the south pole 90° S; the 0°

parallel of latitude is the equator, an imaginary line that divides the globe into the northern and southern hemispheres.

Longitude is the angle, at the centre of the coordinate system, between any point on the earth's surface and the plane of an arbitrary north-south line between the two geographical poles.

Lines joining points of the same longitude are called meridians, not parallel halves of great circles that converge at the north and south poles.

The line passing through Greenwich (near London in the UK) is the international zero-longitude reference line, the prime meridian; the antipodal meridian of Greenwich is both 180° W and 180° E.

Degrees are the unit of measurement for latitude and longitude; they are divided into sixty minutes and the minutes into sixty seconds.

There are three possible representations for geographic coordinates:

• DMS – Degree Minute Second, e.g. 58° 39' 13.51818"

• DD – Decimal Degree, e.g. 58.65375505° that becomes 58.6537° since generally 4 decimal numbers are used

• DM – Degree Minute, e.g. 58° 39.225303'

The point where the prime meridian and the equator meet is defined as N/S 00, E/W 00 depending on whether the location is in the northern or southern hemisphere and east or west of the prime meridian.

This system of measurement is precise enough to allow locating an object or position on the earth within meters.

Because this system of dividing the earth into measurable segments was used

primarily for ocean navigation where no landmarks exist for reference in navigation,

at the equator and on all lines of longitude (meridians) one nautical mile equals one

minute of latitude and a degree of latitude is sixty nautical miles.

Since the length of lines of parallel decrease moving away from the equator, but still maintain a division of the same amount of degrees, the width of a degree of latitude will decrease proportionately, the closer that location is to the north or south pole.

Because this measurement system is based on a spherical model, a formula for a relatively accurate calculation of distance at any latitude would be:

A measurement of sixty nautical miles for one degree at the equator would be calculated at 42.426 nautical miles for one degree at 45 degrees latitude, so the actual distance for latitude will decrease moving away from the equator.

3.2.6.4 True, magnetic and grid north

True north uses the North Pole as a 0° reference, whereas magnetic north uses the magnetic North Pole, which is actually in northern Canada; the difference between true and magnetic north at the current location is known as “magnetic declination”.

There are three indicators found at the bottom of most maps, referred to as the declination diagram; they provide information about the direction of magnetic north in relation to the geographic north orientation for the portion of the earth represented by the map.

If the map is also designed to be measured using a grid, then the grid north value is also represented.

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3.34 – Grid, true and magnetic north

The location of magnetic north is in continual flux and is controlled by movement in the molten iron compounds beneath the earth’s crust, thus the year of the magnetic declination is usually printed next to the indicators (in ten years the location of magnetic north may changes significantly).

3.2.6.5 Map scales

Map scale is the relationship between distance on the map and distance on the ground; a map scale is most often expressed as a fraction or ratio, for example 1/24’000 or 1:24’000 mean that one unit of measurement is equal to 24’000 of the same units on the ground.

The first number of the scale is always 1 and he second number on the scale is different for each scale of the map (the larger the number the smaller the map scale).

Large scale maps generally display more detail but less area, while small scale maps inversely provide a view of more area with less detail.

One of more scale bars indicating the scale length can be found; moreover, since UTM grids typically use meters to measure distance, if contour intervals for topographic maps are not measured in meters conversion factors are also included in the map scale information.

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3.35 – An example of map scale

3.3 RFID locating systems

The most common approach to locate objects with RFID is to tag them with transponders and then to have a variety of readers at known locations scan for nearby tags: when a tag is scanned, it is known that it is in the location of the interrogator.

In this way, supply chain systems can determine the location of objects as they pass near RFID scanner: proximity to a single scanner implies the presence of the item, but its location is only known as somewhere within the operating range of the reader.

Another approach is to have movable readers that identify the presence of RFID tags in their vicinity, wherein the location of the scanner at any time can be identified:

one or more scanners could be mounted on overhead tracks or rails capable of traveling over shopping aisles to scan objects (rails may be single-direction rails or may be interconnected orthogonal systems capable of x-y traversing).

Readers could also traverse aisles by being passed through a plastic tube through the aisle display itself (e.g. near ground level or immediately above the aisle) by a line, belt or other means to pull or push the scanner.

Interrogators could also be attached to a fixed number of employees, carts, automated guided vehicles or other robots and the like, such that the scanners pass through the aisles at irregular intervals but with sufficient frequency to perform an inventory and determine the location of objects.

Rather than using one or more readers at known location to determine the location of an object, another approach is to equip moving transport vehicles with an RFID reader and to tag the environment in which they move with RFID transponders at known locations that serve as reference.

For example readers can be mounted on forklifts and RFID tags embedded in the concrete floor at numerous locations, with the code of each chip associated in a database with its position in the warehouse: as the forklifts move in the warehouse, they can read the tags on objects being moved as well as the reference tags installed in known locations, thereby identifying the location of the objects as they are moved.

Rather than using a transport vehicle equipped with an RFID reader, an operator may use a handheld device equipped with an RFID reader.

The reference tags may be embedded in flooring, placed under carpet or tiles,

mounted in walls (e.g. within drywall, wood, or, plaster), installed in wooden beams,

placed in ceiling tiles, attached to light fixtures and the like.

They can be installed close enough to one another, that an RFID reader in any location of interest can read at least one nearby reference tag; when multiple tags are sensed any ranging method can be used to determine location more precisely.

Smart shelves, that is instrumented shelves that are associated with one or more RFID readers for locating tagged objects, can be a valuable tool to keep the shelves properly stocked.

However, the smart shelves demonstrated in public trials employed numerous expensive RFID readers adapted to read sections of a single shelf and required the use of expensive and bulky coaxial cable for each of the readers, resulting in cluttered cumbersome equipment around the shelves.

Furthermore, these shelf units may not be suitable for end-aisle display units or for use in the storage area, where items may be stored in boxes or on pallets in a disorderly way.

Improved smart shelves have been proposed, in which a single antenna or a single array of interconnected antennas, built into the shelf, can be used with a single reader to read discrete sections of a shelf.

Another technology that is said to eliminate the need for coaxial cable at all and provide good resolution on a shelf at low cost, is the recirculating phase array antenna system of AWID coupled with their fast look-ahead decay sensing system (additional electronics for signal reading and processing).

A film with the antennas and conductive leads is provided in roll-to-roll form for easy retrofitting of existing shelves or rapid placement on their surface (where it may be hidden under paper or other materials).

Even with these improved systems, numerous readers may be needed for each of the many shelves in a store and even then, objects may be placed in regions not adjacent to a smart shelf, where they may effectively disappear from an RFID based inventory tracking system.

While advances in technology promise to bring the cost of RFID scanners down substantially, the cost of numerous smart shelves may still be excessive for some applications, especially in the large quantities necessary to handle the entire product inventory within a retail facility and particularly in environments where shelves per se cannot be installed, or where wired shelves may pose problems for wiring access or safety.

Thus, there is a need for a system that permits tracking of objects in a retail store or

other environment without the need for numerous readers and without the need for

objects to be adjacent to a fixed hardwired reader.

A more expensive solution to locate items is to tag objects with active RFID tags associated with a miniature GPS receiver or positional sensors, such that the transponder can read its location and transmit it to the interrogator.

Honeywell magnetic positional sensors, miniature electronic compasses and other devices using anisotropic magnetoresistive thin film technology can be used: by affixing a magnet or sensor element to an angular or linear moving object with its complementary sensor or magnet stationary, the relative direction of the resulting magnetic field can be quantified electronically.

One approach for scanning and locating objects across long ranges without the need for nearby smart shelves is to use long-range readers with directional antennas, that sweep selected areas of a store and highly sensitive electronics, that allow to resolve faint signals at larger distances, relying on filtering systems to separate the RFID signal from background noise.

Smart antenna technologies may be useful in amplifying weak signals and determining the direction of a radio signal source.

These technologies employ antenna arrays associated with beam-forming and/or beam steering technologies; examples of smart antenna technologies are the directional steerable antenna systems of Antenova, switched-beam antenna arrays and adaptive antenna arrays that include a plurality of antennas cooperatively associated with processors continually adjusting the radio signals to obtain extremely precise directionality and great amplification.

A similar solution is to use RFID tags with high read ranges, that is active transponders.

The advent of printable, low-cost batteries on paper or film could enable active RFID to be used widely instead of passive RFID tags.

Using long-range readers or active tags, location must be determined employing any of the ranging methods explained above; this is also needed to achieve a greater precision with the other approaches presented in this paragraph.

Active RFID tags are the solution on which rely the most important RTLS providers:

• AeroScout

• WhereNet

• Ekahau

• Ubisense

• Multispectral Solutions

3.3.1 AeroScout

This company introduced the industry’s first Wi-Fi based active RFID tag.

AeroScout Visibility System employs Wi-Fi based active RFID tags with TDOA and RSSI ranging methods to offer multiple location types with a single infrastructure:

real-time location, presence detection, choke-point visibility and telemetry.

The AeroScout system is suitable for use in both indoor and outdoor environments, does not require infrastructure changes and works in this way:

1. AeroScout’s Wi-Fi active RFID tags and/or standard Wi-Fi devices send a tiny wireless signal at a regular interval

2. the signal is received by standard wireless access points or AeroScout Location Receivers and is sent to a location engine

3. the engine uses signal strength and/or time of arrival algorithms to determine location coordinates and sends this data to AeroScout MobileView

4. AeroScout MobileView is an end-user application for visualization, asset tracking, alerting and reporting, as well as an integration platform for developers to design and deliver location-based services to third-party applications: it uses location data to display maps, enable searches, create alerts, manage assets and more

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3.36 – AeroScout Visibility System

3.3.2 WhereNet

WhereNet has guided RTLS toward TDOA based ISO 24730-2 standard; this company produces WhereTag IV, the first multimode active tag that combines IEEE 802.11b Wi-Fi technology with the ISO 24730-2 standard, for a complete asset visibility solution.

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3.37 – WhereNet WhereTag IV (802.11b /ISO 24730-2 compliant)

Since this tag is compliant with both the Cisco Compatible Extensions (CCX) for 802.11 tags and the ISO 24730-2 RTLS standard, it can be used with Cisco’s Unified Wireless Network, utilizing the Cisco Wireless Location Appliance.

The WhereTag IV “blinks” an RF transmission at pre-programmed rates ranging from 1 second to multiple hours between blinks and the application-matched infrastructure receives these transmissions and utilizes the appropriate algorithms to determine the location of the tag with an accuracy of about 2 meters.

3.3.3 Ekahau

The Ekahau real time location system is a wireless radio frequency solution that continually monitors and reports real-time locations of tracked resources.

Unlike competing solutions which are based on Time Difference of Arrival (TDOA)

technology, Bluetooth or active RFID and therefore require deployment of costly RF

antennas or receivers, Ekahau’s patented RF modeling technology leverages standard

Wi-Fi access points, so no proprietary infrastructure is needed.