MIT21W_789S11_class6

MIT21W_789S11_class6 - MOBILE LOCATION NETWORKING 21w.789 Spring 2011 Mobile Location Location Tech GPS Cell ID WiFi Mixed Methods(Skyhook Location

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MOBILE LOCATION NETWORKING 21w.789 Spring 2011
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Mobile Location Location Tech GPS Cell ID WiFi Mixed Methods (Skyhook) Location in Android Location on iPhone Example Applications ZoneTag Motion Presence Place Its Jogging the Distance Serendipitous Stories
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Location Fidelity Cell ID: A ZipCode (~150-2000m) WiFi: A block (~100m) GPS: A side of the street (~10m) © Google Maps and unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/fairuse .
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Cell ID Use the cell infrastructure to know when you are in a given location (Zip Code level accuracy) In GSM networks, all cells in the world have a globally unique ID made up of four numbers: cell id, lac, mnc, and mmc Databases exist Cells also can be manually mapped to semantic locations (e.g. Campus, Home, Downtown Boston, etc.)
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Cell Topology Much greater density in cities where places are also more dense (from Chen et al 2006, http://www.intel-research.net/Publications/Seattle/100920061625_366.pdf ) Source: Chen, M., et al. "Practical Metropolitan-Scale Positioning for GSM Phones." Lecture Notes in Computer Science 4206/2006 (2006): 225-242. © Springer Science + Business Media. All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/fairuse .
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Cell ID -> Location Databases exist that map a large number of Cell IDs to locations Google Yahoo Now part of most mobile platforms…transparent to developer/user
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Cell ID - Applications People have used Cell ID to: Determine when someone was at a given place (e.g. home) (see Sohn et al „05) Distinguish between walking/driving/stationary (see Sohn et al ‟06) Provide accurate positioning (e.g. Chen et al ‟06) Down to 95% error of 163m with all visible cells Currently used to save power over GPS/when only approximate locations are needed or indoors (e.g. find nearest movie theater, etc.)
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Wifi Positioning Use available SSIDs to determine location Up to 40m accuracy in cities Issues: Need to have sampled fairly closely to where you want to get location Not practical until recently when combined with other methods to bootstrap (Google trucks, apps that report Cell ID, SSID, GPS to large databases)
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GPS 30 satellites in earth orbit 6 satellites always in line of site of any place on earth Receivers must find 4 or more satellites for a fix Receiver listens for time broadcasts from each satellite and calculates distance based on time delay Satellites also broadcast Navigation Messages which can be used with the time delays to calculate position Receivers often provide access to position data as a NMEA stream (National Marine Electronics Association)
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GPS – NMEA A compound string from the NR203 GPS Receiver containing multiple messages. The actual messages
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This note was uploaded on 01/16/2012 for the course WRITING 21w.789 taught by Professor Edwardbarrett during the Spring '11 term at MIT.

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MIT21W_789S11_class6 - MOBILE LOCATION NETWORKING 21w.789 Spring 2011 Mobile Location Location Tech GPS Cell ID WiFi Mixed Methods(Skyhook Location

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