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Location-Based Services
GPS- global position system. uses satellites to determine a user's location.
E911 - the Enhanced 911 Act. voted in US in 1999 to standardize and enhance
911 services. As part of this the bill mandates that all mobile-phone
carriers provide Automatic Location Identification of a caller to withing
125 meters 67% of the time.
Techniques to determine location:
- Cell of Origin - primitive method of locating a wirless user. Each
call is sent to a nearby tower and from this tower the call is sent
to intended recipient. This tower covers a specific area (usually
kilometers). Only know user is in this area.
- Angle of Arrival - measures angles from which a cell phone's signals
are received by 2 or more towers and uses the interesection of these
lines of sights to locate (triangulation) the user. Problem when interference
in one tower.
- Time Difference of Arrival - uses again triangulation but, this
here it usies the time it takes to reach the receiving tower and 2
other towers. An arc can be formed from the information of a receiving
tower time and another tower. Hence you can have 2 arcs. The intersection
of these arcs is the location of the user..
- Enhanced Observed Time Difference (E-OTD) - Like TDOA uses time
to arrive. However, here the towers inititiate the transmission to
the handset not the other way around. The handset does the calcuation
of its location. Requires software on handset unlike TODA.
- Location Pattern Matching - the problem with all of the above techniques
is that in urban areas you can have interference that will effect
the accuracy of the readings. The bouncing and splitting of signals
off of other objects on the way to a tower is produces multipaths.
This technique uses multipaths and they store in a database mulitpath
patterns produced at various locations and match them to identify
a user's location. Problem- how often do they update their database?...2
times are year.
- GPS - gobal positioning system. developed in 70s by US defence department.
Gives lattitutde and lognitude and altitute. Accurate up to 10 meters
(commercial, military??). Uses a number (e.g. 24) satellites to locate
a device. Each satellite sends a signal containing a random code to
the user's device. The code itentifies the time the signal left the
satellite. The user's device, which contains a GPS receiver, determines
the signals travel time and this is multiplied by the speed of light
to determine the distance between the device and each satellite. The
location of each satellite is known. Hence each user to satellite
gives us a 3D sphere the user could be located on. Triangulation is
used...where the spheres interesect say where the user is. Problem...expensive...satellites
are expensive. Satellites are a long way off and hence the siganls
become weak...so, indoor GPS receiving may not be good depending on
building,etc.
Some companies that have been involved in offering location-identification:
- Cell-Loc and subsidary TimesThree
- TruePosition
- SigmaOne
- U.S. Wireless
Languages to develop location-based services:
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