In regards to your comment about accelerometers... The MEMS based market has had significant improvements thanks to companies like Analog Devices**, I have a couple of their dual-axis devices sitting on my desk right now for another project. That piece of the problem could be easily solved depending on your definition of "precise."
<br><br>The effect of small distance variations on signal strength unfortunately I've never read up on. I'll leave that one up to the RF experts... <br><br>**disclaimer... i have no affiliation or interest in AD, I just think their products are cool. :)
<br><br><br><div><span class="gmail_quote">On 3/15/07, <b class="gmail_sendername">Michal Zalewski</b> <<a href="mailto:lcamtuf@dione.ids.pl">lcamtuf@dione.ids.pl</a>> wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
On Thu, 15 Mar 2007, Dave Aitel wrote:<br><br> [ Repost; Dave, if you get a chance, reject my original post and<br> approve this one instead, I hit Ctrl-X too early; or if it's too late,<br> reject this repost. Thanks. ]
<br><br>> So here's my idea of the day: I want relational triangulation in<br>> SILICA. I want to be able to click "Find this AP" and then have SILICA<br>> say "stay still . . . . signal is 99. Now take 5 steps to the left....
<br>> signal is 91. Now take five steps forward....signal is 102" and then<br>> interpolate in "steps" the distance and direction of the access point.<br><br>Moving several feet to the left or right when not standing next to the
<br>device is almost guaranteed not to measure any appreciable signal<br>differences that would not be overpowered by random reflections, RF<br>interference, attentuation caused by walls, chairs, etc, or residual<br>directional characteristics of an antenna (you need to get one that is
<br>almost perfectly omnidirectional, or else maintain a precise angle while<br>moving around).<br><br>Consider this: when standing 20 meters from the transmitter, facing it in<br>an open, unobstructed, reflection- and interference-free field, moving 2
<br>meters to the left with a perfectly omnidirectional antenna would change<br>the actual distance the signal has to travel by about 0.1%. A precise RF<br>interferometer could work, but signal strength measurement alone are not a
<br>useful indication of your location in this axis.<br><br>Doing it from 5 meters away will of course work better, but then you're<br>close enough to spot the transmitter by simply observing signal strength<br>while walking around. Circling the area of a suspected transmitter site
<br>would yield great results, too, but without a GPS or a set of precise<br>accelerometers, registering or approximating your movements in an indoor<br>environment is unlikely to be easy.<br><br>If you're left with only one axis to take meaningful measurements, you
<br>wouldn't be able to interpolate the actual distance, because you don't<br>know how powerful the signal would be were you standing next to the<br>transmitter - depends on chips, antenna, settings, terror alert level, and
<br>how strong is the initial attentuation is (be it caused by ceiling panels,<br>doors, rack mount or a printer it is sitting behind).<br><br>As such, standing up, making 5 steps to the right, 5 to the front, 5 to<br>the left, 5 to the back is almost guaranteed to give you no benefit over
<br>simply walking around with a traditional meter.<br><br>We happen to hunt "pirate" APs in our office buildings from time to time,<br>and even with specialized, directional receivers and quality software,<br>it's still a mess.
<br><br>That said, there are several tools that allow AP location triangulation in<br>corporate environments, but they usually rely on several fixed measurement<br>points that are 10-50 meters apart, and mounted in a controlled, carefully
<br>measured way, and again, *around* the rogue access point, so that absolute<br>measurements can be made. AirMagnet sells something like this.<br><br>/mz<br>_______________________________________________<br>Dailydave mailing list
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