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Center for Embedded Networked Sensing
Omni-directional WiFi Localization
Rohun Bansal, Jessica Chen, Kevin Chen, Nathan Schloss,
Deborah Estrin, Cameron Ketcham, Rakhee Patel, Martin Lukac
OWL - http://owl.t4so.com/
Introduction: Determining location through use of sensors
Sensors on Mobile Phones
OWL: Localization System
•
•
Modern smart phones (such as the Android G1) have a variety of
sensors
– GPS sensors can generally acquire fairly accurate location data.
– WiFi devices can scan for access points as well as create ad-hoc networks.
•
An Android application for the G1 which
takes advantage of both GPS and WiFi
sensors in a network of phones to estimate
location
– The system is generally functional in many
different situations.
– The application works to minimize battery usage
and time to location fix while trying to improve
accuracy.
Mobile phones are becoming increasingly more widespread
– As the number of phones in use increases, applications involving networks
of phones become progressively more practical to implement.
•
Relies more heavily on the phone itself for location fix
Problem Description: Current localization methods have disadvantages
GPS: Global Positioning System
WPS: WiFi Positioning System
• Requires line-of-sight view to satellites in order to function
• Requires an internet connection
– Although GPS is very accurate outdoors under an unobstructed sky, it
works poorly or not at all in environments lacking such a view (notably
indoors and near large buildings).
• Long time to first fix
– A list of visible access points must be sent in an HTTP request to a central
server; a location estimate will be sent in the server's response.
• Accuracy is largely inconsistent
– Because each GPS satellite only broadcasts ephemeris data every 30
seconds, the time for a receiver to get a fix is the time it takes to acquire
each GPS satellite’s signal plus up to 30 seconds for ephemeris
information.
– Because the system uses a pre-compiled database of access points, the
accuracy of location estimates depends completely on the coverage of
these access points in the area, resulting in rather rough estimates.
Proposed Solution: A networked localization system that uses both GPS and WiFi
OWL: Localization System
• Devices that can acquire an accurate GPS fix become anchors
– An anchor sets up its own ad-hoc networks, with its IMEI number (used as a
unique identifier) and GPS location encoded into the network SSID.
`
X
• Devices that cannot acquire an accurate GPS fix become receivers
– A receiver periodically scans for available wireless networks set up by anchors.
– Distance to each anchor network is estimated using a simplified logarithmic
correlation to signal strength (in dBm).
– Once there are at least 3 visible anchor networks, a receiver can estimate its
position using multilateration.
A
Screenshots of the OWL application:
– initial check of GPS accuracy (left)
– receiver mode (right)
Inside Building
A can determine its location through
multilateration if it knows the positions of X, Y,
and Z, and its respective distances to them.
Comparison with GPS and WPS
Results
Further Work
• Average accuracy of receiver mode is almost that
of GPS
• Fix issues with ad-hoc networks
– 29.16% of receiver calculations were 11m or better.
– 21.09% of GPS locations were 11m or better.
• Location fix time was lower than both GPS and
WPS
• Battery usage was less than WPS and similar to
GPS
Z
Y
– There were numerous problems with the anchor
networks, including limited range, inconsistent
signal strengths, and slow update rates.
– This seems to be a mainly hardware-specific issue.
• More testing
– The system needs testing in more controlled
environments.
UCLA – UCR – Caltech – USC – UC Merced