Wifi Triangulation

Question

What would be the best way to triangulate a wireless network passively. Are there tools available? Algorithms? Libraries?

My goal would be to create a relative map of various objects that sends or receive signals using signal strength (DB's), signal/noise ratio, signal phase, etc. from a few location points. With enough sampling, i'm guessing it would be possible to create a good 2d/3d map.

I'm searching for stuff in any language / platform.

Some keywords: wi-fi site survey, visualization, coverage, location, positioning

Thinking about using kismet to gather the data and then process it. Maybe Free Space Path Loss for RF in the 2.4Ghz range to calculate a relative distance. And optionally try to use RF obstacle attenuation estimation values (based on some user input) to give better estimates. Then use trilateration to generate possible relative coordinates.

Answer 1

Actually I think you should try using an algorithm like the GPS one (wikipedia).. of course you can simplify it according to your need, for example:

• you need to install on every item that should broadcast its position (the navigation signal) an application that actually does it
• you should use a different channel for every single item to be sure not to generate collisions (it depends also on how much you broadcast the signal)

so if you place at least 4 broadcasters you can triangulate on every client to allow it to calculate its position. Naturally the bcasters should be as much similar as possible in response..

by the way these are just ideas..

Answer 2

You can't use the GPS technique because the timing is nothing like accurate enough. The best you can do is Trilateration based on the signal strength from each base station and assume that range is proportional to signal.
You will probably need to force a connection to each base station in turn in order to measure the signal strength.

Answer 3

Using signal strength to judge distance could easily be thrown off by differences in materials blocking line-of-sight to each of the sampling points. It would probably be better to do the sampling with a directional antenna, and from each sampling point, find the bearing that maximizes signal strength to each device you want to locate. With this technique, you can use only two or three sampling locations, depending on the accuracy with which you can estimate the bearings.

Answer 4

Interesting question. Initial thoughts were using output from something like the WiSpy spectrum analyzer. I like the idea of using a directional antenna. Looks like some research (may) be underway.

Answer 5

Instead of trilateration you could use bilinear interpolation. This is said to be better for non-linear distance vs. signal strength data like wifi in an urban environment would be. http://courses.cit.cornell.edu/ee476/FinalProjects/s2007/ayl26_ym82/ayl26_ym82/index.htm has the background math and the what I assume is AVR C for doing it with magnetic field sensors.