Trilateration of 3 Calculated Distances from WiFI Strength Signals

Question

I am using android to scan WIFI AP's every frame of time, I am getting from each AP the Strength of Signal (RSSI in dbm) and I am calculating the distance with this formula:

public double calculateDistance(double levelInDb, double freqInMHz) {
    double exp = (32.44 - (20 * Math.log10(freqInMHz)) + Math.abs(levelInDb)) / 20.0;
    return Math.pow(10.0, exp);
}

That is working fine, So I have three or more distances, now I need to draw on a map all AP's with its fixed locations, I made some reading on internet and I found the Trilateration (is the process of determining absolute or relative locations of points by measurement of distances) but it looks like I need at least one Point (x,y), at this moment I just have the calculated distances from the Signal Strength that can be taken as the radius of the different circumferences.

I am confused because I don't have any concrete point (x,y) to start to calculate the location of the Mobile Phone.

I just need to know if there is a way to calculate that point or I can assume that initial point or I am missing something.

Thank you, I really appreciate any.

You need to know the coordinates of the APs in relation to one another. Then you can calculate the intersection of the circles in this coordinate space — Paulw11, Nov 19 '14 at 06:46

score 9 · Accepted Answer · edited Aug 27 '16 at 21:58

As Paulw11 mentioned is his comment, you have to know the exact position of all of the APs or at least the one of them and the relative position of the other twos to this one. Then the Trilateration procedure will produce one circle for each AP and the interception of them will be the device.

Keep in mind that the Trilateration with Wi-Fi will produce an area instead of a point which will result in an area of uncertainty with an accurancy of at least 2-3m. And from what I can see you are calculating the distance based on the free space loss model which is a generic type and it is not the truth for each environment, so, this assumption will make even worse your estimation.

A good approach is to make a radio mapping of your area first with Wi-Fi measurements from the device and then be based on this Wi-Fi fingerprints. Or prepare a training period first. There are many tutorials on that.

Trilateration of 3 Calculated Distances from WiFI Strength Signals

1 Answers1