# Is it possible to measure the distance of a WiFi access point from the receiver?

I have signal strength in dBm and frequency in MHz is is possible to find the distance??

Any idea about how to do it?

• It helps if the transmitter tells you what its effective power level measured at a reference distance is, but the results will still be of limited reliability, see for example "iBeacon" RSSI-based distancing on 2.4 GHz BLE. – Chris Stratton Jul 21 '14 at 17:08
• No reason to close this - it's a technique in actual use, with known issues, and the question asks if it is possible, not for a how-to on implementing it. – Chris Stratton Jul 21 '14 at 17:08
• @ChrisStratton I totally agree with you, moreover a question like this can produce pretty nice and informative answers from great guys around here. – Vladimir Cravero Jul 26 '14 at 8:06

There are several research papers claiming that there is no correlation between signal strength and distance. I've read it - among others - on Radio Link Quality Estimation in Wireless Sensor Networks: a Survey by Baccour.

Now, I don't think this is true - there must be a correlation, otherwise you could in principle receive wifi signals from across the world. But it is demonstrated that this dependency is overwhelmed by other factors, such as the non-uniform radiation pattern of the antenna, multi-path propagation (highly significant due to the short wavelength), scattering and path loss due to obstacles.

• Quite true, I've looked at this for an application at work (a sort of indoor GPS)... and it is very difficult to get accurate results. Your chances are increased with the more access points you have at your disposal for more accurate triangulation... In the end, I just went with a more basic approach. At least in my application, the user would check in multiple times in between an activity, and thus easier to just use extrapolation for their location. – Jarrod Christman Jul 21 '14 at 11:34

Of course there is a correlation between signal strength and distance! As you can see from this discussion over on Physics there is no fixed equation to tell you distance from signal strength, as you don't know output power, antenna length/ field pattern/geometry, propagation paths, etc, and in fact even things like the weather can change how well the signal propagates.

So what tools like kismet do (extensively used in wifi site surveys and security tests) is to plot signal strength along with location and use triangulation. Broadly speaking, excluding external factors, two points that have the same signal strength are likely to be equidistant from the wireless access point. Using many samples helps pin down locations pretty accurately.

• I disagree with the last part. It's been found that, because of multi-path propagation, moving the receiver a few centimeters (even sideways) can change the received signal by as much as 10dB. Not to mention the effect that walls have. – clabacchio Jul 21 '14 at 11:50
• clabacchio - no, it really works well! This is where kismet wins - it takes > 10 or 15 points and can come up with a very good map. Some materials are worse than others (warehouses storing rolls of paper or whisky seem especially bad) – Rory Alsop Jul 21 '14 at 13:37
• I have updated the wording to make it more obvious. – Rory Alsop Jul 21 '14 at 13:38

All other things being equal (and that is a lot to ask!) there is indeed a relation between transmitter signal strength, distance, frequency, and receiver signal strength.