UNIVERSITY of CRETE Fall04 – HY436: Mobile Computing and Wireless Networks Location Sensing Overview Lecture 8 Maria Papadopouli

University of Crete 2 Reading A Survey and Taxonomy of Location Systems for Ubiquitous ComputingA Survey and Taxonomy of Location Systems for Ubiquitous Computing Jeffrey Hightower and Gaetano Borriello (2001)

University of Crete 3 Roadmap Location Sensing Overview ♦ Location sensing techniques ♦ Location sensing properties ♦ Survey of location systems

University of Crete 4 Why is location sensing important ? ♦ Mapping systems ♦ Locating people & objects ♦ Wireless routing ♦ Supporting smart spaces and location-based applications

University of Crete 5 Location Sensing Techniques Triangulation ♦ Lateration ♦ Angulation Scene Analysis Proximity

University of Crete 6 Triangulation - lateration Measures distance from reference points 2-D requires 3 non-colinear points 3-D requires 4 non-colinear points

University of Crete 7 Radius 1 Radius 2 Radius 3 Triangulation - Lateration

University of Crete 8 Triangulation - Lateration Types of Measurements ♦ Direct touch ♦ Time-of-flight (e.g., sound waves travel 344m/s in 21 o C) ♦ Signal attenuation calculate based on send and receive strength attenuation varies based on environment

University of Crete 9 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 ?

University of Crete 10 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

University of Crete 11 Triangulation - Angulation 2D requires: 2 angles and 1 known distance Phased antenna arrays 0° Angle 1 Angle 2 Known Length

University of Crete 12 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

University of Crete 13 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

University of Crete 14 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

University of Crete 15 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

University of Crete 16 Existing Location Systems Active Badge Active Bat Cricket RADAR RICE project MotionStar Magnetic Tracker Easy Living Smart Floor E911

University of Crete 17 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

University of Crete 18 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

University of Crete 19 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

University of Crete 20 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

University of Crete 21 Active Bat: issues Requires large infrastructure Maintenance cost Scalability Easy of deployment

University of Crete 22 Easy Living Uses 3D cameras Provides stereo-vision position capabilities Designed for home use

University of Crete 23 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

University of Crete 24 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