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UNIVERSITY of CRETE
Fall04 – HY436: Mobile Computing and Wireless Networks
Location Sensing Overview
Lecture 8Maria Papadopouli
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Reading
• A Survey and Taxonomy of Location Systems for Ubiquitous Computing
Jeffrey Hightower and Gaetano Borriello (2001)
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Roadmap
• Location Sensing Overview♦ Location sensing techniques♦ Location sensing properties♦ Survey of location systems
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Why is location sensing important ?♦ Mapping systems♦ Locating people & objects♦ Wireless routing♦ Supporting smart spaces and location-
based applications
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Location Sensing Techniques
• Triangulation♦ Lateration♦ Angulation
• Scene Analysis• Proximity
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Triangulation - lateration
• Measures distance from reference points
• 2-D requires 3 non-colinear points
• 3-D requires 4 non-colinear points
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Radius 1 Radius 2
Radius 3
Triangulation - Lateration
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Triangulation - LaterationTypes of Measurements♦ Direct touch♦ Time-of-flight
(e.g., sound waves travel 344m/s in 21oC)
♦ Signal attenuation• calculate based on send and receive
strength• attenuation varies based on environment
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Time-of-Arrival: issues
• May require high time resolution (e.g., for light or radio)A light pulse (with 299,792,458m/s) will travel
the 5m in 16.7ns• Clock synchronization issue♦ Possible solution ?
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GPS• 27 satellites • Powered by solar energy (have backup batteries on
board)• Each has 4 rubidium atomic clocks which are locally
averaged to maintain accuracy also updated daily by US Air Force Ground control
• Satellites are precisely synchronized with each other• Receiver is not synchronized with the satellite transmitter• Satellites transmit their local time in the signal• Receivers compute their difference in time-of-arrival• Receivers estimate their position (longitude, latitude,
elevation) using 4 satellites• Differential GPS
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Triangulation - Angulation
• 2D requires: 2 angles and 1 known
distance
• Phased antenna arrays
0°
Angle 1Angle 2
Known Length
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Phased Antenna Array
• Multiple antennas with known separation • Each measures time of arrival of signal• Given the difference in time of arrival and the
geometry of the receiving array, we can compute the angle from which the emission was originated
• If there are enough elements in the array and large separation, the angulation can be performed
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Scene Analysis• Compares scenes to reference scenes♦ Images♦ Electromagnetic readings
• Construct a signature of a position and apply pattern matching techniques with this signature
• Differential scene analysis♦ Tracks differences in scenes
Issues: the observer needs access to the features of the environment against which it will compare its observed scenes
Changes of the environment that affects these features may require their reconstruction
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Proximity
• Physical contact e.g., with pressure, touch sensors or
capacitive detectors• Within range of an access point• Automatic ID systems♦ computer login♦ credit card sale♦ RFID♦ UPC product codes
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Location System Properties
• Physical & symbolic location• Absolute vs. relative• Localized or remote computation• Accuracy & precision requirements• Scale• Device identification/classification/recognition• Cost• Limitations & dependencies (e.g., infrastructure vs. ad
hoc)• Hardware availability• Privacy requirements
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Existing Location Systems• Active Badge• Active Bat• Cricket• RADAR• RICE project• MotionStar Magnetic Tracker• Easy Living• Smart Floor• E911
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Sensor Fusion
• Seeks to improve accuracy and precision by aggregating many location-sensing systems to form hierarchical and overlapping levels of resolution
• Robustness when some location-sensing system becomes unavailable
Issue: assign weight/importance to the different location-sensing systems
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Adaptive Fidelity
General concept: the ability to adjust the service based on the resource availability
• Adjusts its precision in response to dynamic situations such as partial failures or directives to conserve energy
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Active Bat
• User has a mobile “Bat”• Infrastructure of controllers
and ceiling sensors
• Within 9 cm of their true position 95% of the measurements
• Different signal modalities
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Active Bat: Main Concept• Controller sends simultaneously a radio signal and a synchronized
reset signal to the ceiling sensors using a wired serial network• Bat responds to the radio request with a ultrasonic beacon• Ceiling sensors measure time-of-flight (from reset to ultrasonic pulse)• Central system determines location using lateration• Statistical pruning eliminates erroneous sensor measurements caused
by a a ceiling sensor hearing a reflected ultrasound pulse instead of one that traveled along the direct path from the Bat to the sensor
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Active Bat: issues
• Requires large infrastructure• Maintenance cost• Scalability • Easy of deployment
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Easy Living
• Uses 3D cameras• Provides stereo-vision position
capabilities• Designed for home use
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Smart Floor
• Pressure sensor grid installed in all floors• Accurately determines positions of everyone in a
building• Users do not need to wear a tag or carry a device• Cannot specifically identify an individual
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E911
• FCC is requiring wireless phone providers to locate any phone that makes an E911 call
• Different approaches♦ proximity♦ angulation with phased antenna arrays♦ GPS-enabled handsets
• Leads to numerous new consumer services
