Location-Sensingand Location Systems
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A positioning system provides the means to determinelocation and leaves it to the user device to calculate itsposition,
whereas a tracking system monitors objects withoutinvolving them in the computation.
A location-sensing system is a hybrid of
a positioning system and a tracking system.
Location-Sensing and Location Systems
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An important aspect of mobile computing is to giveinformation that is relevant to the user’s context.
In most cases, the user’s location is an important factor indetermining the user’s context.
For example, if I am in Ankara and I request for a list ofrestaurants......
Location-Sensing and Location Systems
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For the system to deliver location-sensitive information,the system has to be able to detect my location andtailor the information delivered to me to meet myneeds.
Location-Sensing and Location Systems
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There are two approaches to implement a positioningsystem:
self positioning and remote positioning.
In a self-positioning system, a mobile device uses signalstransmitted by gateways or antennas to calculate its ownposition.
In a remote positioning system, the position of a mobiledevice or a tagged object is determined by measuringsignals detected by a set of receivers. Signalmeasurementsare used to determine the location of theobject of interest.
Location-Sensing Techniques
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There are three classifications of location-sensingtechniques:
triangulation,
proximity,
scene analysis
Location-Sensing Techniques
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Triangulation is divided into lateration and angulation.
Lateration involves using distance measurements.
The 2D position of an object is calculated by measuringits distance from three noncollinear (not on the sameline) points.
“You’re 30 km from Bangi.” You draw a circle on yourmap to mark your possible location.
Triangulation
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“You’re about 40 km from Kajang.” That is slightlybetter. You can narrow down your location to theintersection area of the two circles (Figure 9.1b).
A third person tells you that you are about 10 km fromSerdang.
This is how 2D lateration works.
Triangulation
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2D lateration
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Four non-coplanar points are required to calculate a 3Dposition (latitude, longitude, and altitude) of an object.
A 2D position would be sufficient, for example, in animplementation involving an active badge that tracks thelocation of users or objects in an office environment.
On the other hand, a 3D position would be more helpfulwhen sending a team to rescue climbers trapped in anavalanche.
Triangulation
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There are four approaches to measuring distance:
Distance measuring
1) Direct: Involves using a physical action or movementto take a measurement.
2) Time-of-flight: Involves measuring the distancebetween an object (stationary or moving) to a point P bymeasuring the time it takes to travel between the objectand P at a known velocity.
For example, because sound travels at a speed of 344m/s,.
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Distance measuring
Clock synchronization becomes an issue for the distancemeasuring.
GPS satellites are precisely synchronized with each otherand transmit their local time in the signal so thatreceivers can compute the difference in time-of-flight.
A GPS receiver calculates a 3D position using foursatellites.
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Distance measuring
3) Attenuation (loss of intensity): The decrease in signalintensity as the distance from the signal source increases.
It is possible to measure the distance between an objectand P using a function that correlates attenuation,
the distance for a type of emission,
and the original strength of the emission.
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Distance measuring
4) Angulation: Involves using angle or bearingmeasurements instead of distance. A 2D angulationinvolves two angle measurements and one lengthmeasurement .
3D angulation requires one length measurement, oneazimuth measurement, and two angle measurements.
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Proximity
Proximity is a location-sensing technique that involvesdetermining when an object is near a known location.
There are three approaches:
1. Detection of physical contact using pressure sensors,touch sensors, and capacitive field detectors.
2. Monitoring wireless cellular Access Points (AP) involvesdetecting when a mobile device is within the range ofone or more APs in a wireless cellular network.
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Proximity
3. Observing automatic ID systems involves usingautomatic ID systems such as credit card point-of-sale,telephone records, computer login history, and use ofATM card.
For example, the location of a person driving on ahighway can be inferred from the last time that personused a Touch-n-Go card because the scanner that readsthe card has a static, known location.
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Scene Analysis
Scene analysis uses features of a scene observed from avantage point to deduce the location of the observer toobjects in the scene.
There are two types of scene analysis:
1) In static scene analysis, observed features are cross-referenced to a predefined dataset to map them toobject locations.
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Scene Analysis
2) In differential scene analysis, the differences betweensuccessive scenes are tracked to estimate location.
The differences correspond to movements of the observer.
If the features are identified as being at a specific location,the observer can determine its own position relative tothem.
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The Global Positioning System (GPS) is a space-basednavigation system that provides location and timeinformation in all weather conditions, anywhere on ornear the Earth where there is an unobstructed line ofsight to four or more GPS satellites.
The system provides critical capabilities to military, civil,and commercial users around the world.
The United States government created the system,maintains it, and makes it freely accessible to anyonewith a GPS receiver.
Location-Sensing Techniques
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GPS: An Example of a Positioning System
The GPS uses 24 satellites orbiting the earth.
It was developed by the U.S. military as a militarynavigation system but has been made available to thepublic.
The satellites are at a distance of 20,000 km above earth’ssurface.
At any one time, four satellites cover a certain portion ofthe earth’s surface.
A GPS receiver determines its location based on signals itreceives from the four satellites.
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GPS: An Example of a Positioning System
A GPS receiver picks up radio wave signals that travel atthe speed of light from the satellites.
The receiver determines its distance from the satellitebased on how long a signal transmitted by the satellitetakes to reach it.
A satellite transmits a signal, termed a pseudorandomcode, at predetermined intervals known by the receivers.
When the satellite starts transmitting the code, thereceiver starts to run the same code at exactly the sametime.
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GPS: An Example of a Positioning System
When the satellite’s signal reaches the receiver, its signalpattern will lag (delay) slightly behind the code thereceiver is running.
The amount of time the signal lags behind (the delay) isequal to the signal’s travel time.
The receiver calculates its distance from the satellite bymultiplying the delay by the speed of light (300,000km/s).
This calculation requires a receiver’s clock to besynchronized with the satellite’s clock.
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