Location Tracking

From SOWNWiki

Wireless location tracking is an area of research currently being undertaken by the SOWN Project group. Specifically it is technology intended to determine a person's location in the world by looking at the Wireless Access Points in the immediate area surrounding them. With sufficient information about the network names (SSID) and signal strengths, it is possible to triangulate the position of the node.

At the moment purely mathematical models are being researched (signal strength decreases at a rate proportional to 1/r^2 (r being the distance from the node)).

Other models/factors to consider can be seen in the notes for ELEC3027 Notes (requires ecs login)

Matlab

Just some quick code to plot some pretty 3D shapes to demonstrate signal strength with distance.

To run: Save as plot3d.m, open and run.

%% Plotting in 3D

% Clear workspace etc
clc;clear;
clear all;  

% Cartesian Coordinates
x=[-2:.1:2];
y=[-2:.1:2];
[X,Y]=meshgrid(x,y);
r=6-((X.^2)+(Y.^2));
z=r.^2;
subplot(2,1,1);mesh(X,Y,z); 

This produces:

% Polar coordinates
phi = (0.4:0.1:1.9)'*pi/30;
theta = (0:1:36)*pi/10;
X = sin(phi)*cos(theta);
Y = sin(phi)*sin(theta);
Z = (1./(phi))*ones(size(theta));
subplot(2,1,2);surf(X,Y,Z);

This produces:

Clearly the equations need to be changed, but this just demonstrates what was being discussed.

Applying the graphs to the real world

If an access point is modelled by the equation

z=(6-((X.^2)+(Y.^2))).^2

cordinates 0,0 represent the center of the access point. If we are modelling a room we need to shift the equations to be centered at diferent cordinates. To do this we have a new equation:

z=(6-(((X + posX).^2)+((Y + posY).^2))).^2

Where posX and posY are the cordinates of the access point.

Diferent access points may have diferent signel strengths. To allow for this we have the following equation:

z=(6-(((X + posX).^2)+((Y + posY).^2))).^2 + gainAP

Where gainAP represents the gain in dbm of the access point.

Links

Random liks of mild relevence to Location Finding

http://www.placelab.org/publications/pubs/wmash-placelab.pdf - Good document on what location aware applications can do

http://www.placelab.org/ - Application for using WiGLE database to work out the location of a device

http://wigle.net/gpsopen/gps/GPSDB/ - Database of wireless access points worldwide

http://www.cs.bgu.ac.il/~benmoshe/RadioMaps/ - How to estimate maps of wireless coverage

http://activecampus.ucsd.edu/ - Locaton aware computing site

http://en.wikipedia.org/wiki/Inverse_distance_weighting - Method of aproximating graphs

http://www.cs.indiana.edu/~yonliu/ - Person who does reserch into location aware computing