Wifi-based trilateration on Android

404px-Sea_island_surveyTriangulation offers a way to locate yourself in space.  Cartographers in the 1600s originally used the technique to measure things like the height of the cliff, which would be too impractical to measure directly.  Later, triangulation evolved into an early navigation system when Dutch mathematician Willebrord Snell discovered three points can be used to locate a point on a map.

While triangulation uses angles to locate points, trilateration uses lateral distances.  If we know the positions of three points P1P2, and P3, as well as our distance from each of the points, r1r2, and r3; we can look at the overlapping circles formed to estimate where we are relative to the three points. We can even extend the technique to 3D, finding the intersecting region of spheres surrounding the points.

In this project, I’d like to show how we can use the Wifi signal strength, in dB, to approximate distance from a wireless access point (AP) or router.  Once we have this distance, we can create a circle surrounding an AP to show possible locations we might occupy.  In the next part of the project, I plan to show how we can use three APs to estimate our position in a plane using concepts of trilateration. (Note: I haven’t had time to implement this, but you can use this Wiki article to implement it yourself).

Trilateration using 3 access points providing a very precise position (a) and a rougher estimate (b)

Trilateration using 3 access points providing a very precise position (a) and a rougher estimate (b)

Determining distance from decibel level

There’s a useful concept in physics that lets us mathematically relate the signal level in dB to a real-world distance.  Free-space path loss (FSPL) characterizes how the wireless signal degrades over distance (following an inverse square law):

Screen Shot 2013-07-05 at 2.36.07 PM

The constant there, 92.45, varies depending on the units you’re using for other measurements (right now it’s using GHz for frequency and km for distance).  For my application I used the recommended constant -27.55, which treats frequency in MHz and distance in meters (m).  We can re-arrange the equation to solve for d, in Java:

Now, there are few drawbacks to this rough approximation:

  1. FSPL explicitly requires “free space” for calculation, while most Wifi signals are obstructed by walls and other materials.
  2. Ideally, we will want to sample the signal strength many times (10+) to account for varying interference.

Problem (1) will be resolved in the future by using the signal-to-noise ratio to more accurately estimate (that sounds like an oxymoron) obstructions to the wifi signal.  Problem (2) can be implemented in code by sampling many times and computing the average signal level.

Using the above code along with Android’s WifiManager and ScanResult classes, I can print out our final measurements:

And we can get back data that appears to be correct when moving further away from my test router (MAC address: 84:1b:5e:2c:76:f2):

[Image lost during host transition, but basically just showed how the distance increased]

23 thoughts on “Wifi-based trilateration on Android

  1. Guilherme

    Hey Ryan,

    That’s a very interesting approach there. I’m working on a similar project myself. Would it be possible to contact you via e-mail so we can perhaps share and discuss findings and information?

    Thanks,
    Guilherme

    Reply
  2. dung hoang

    hi miller,
    im interested in your project but there are some thing that i have not understood
    + the signal level (levelInDb) that we achieved by android app is FSPL ???
    + so if signal level is FSPL, i suppose that the formula should be like this :
    double exp = (27.55 – (20 * Math.log10(freqInMHz)) + levelInDb) / 20.0
    + if you was successful when applying this method, can i know the position accuracy ?
    hope to see your reply soon :D

    Reply
    1. rvmiller Post author

      It looks like you’re correct, the formula should read ” + levelInDb”. I’ve corrected it above. Thanks!

      Reply
  3. Antonis

    Hi,
    i’m basically new to android. My project at uni is about finding the exact coordinates of a mob device through trilateration. I have a question about the code snippet you gave us. Does it calculates all the distances needed(i mean from all the 3 hotspots) or is it just for one? Hope i get an answer as soon as you have time. Thx.
    PS Please excuse any errors in my english.

    Reply
    1. rvmiller Post author

      Hi Antonis,

      The code samples only calculate the distance from one access point (AP). I haven’t had a chance to implement the actual trilateration using multiple APs, but it should be straightforward to extend the code to do so.

      Thanks.

      Reply
  4. tiger

    hi, i have a question :
    you use s.level in calculateDistance((double)s.level, s.frequency)
    but s.level will returm to dBM (in developer.android.com)
    and you use it for FSPL(dB) ?

    Reply
  5. Hao

    Dear Ryan,

    It’s very interesting and impressive!
    When will you be able to give us some advice for calculating the position?
    If three ACs lie in different Z ordinate, the theory still works.
    But I had a hard time figuring out how to locate the point.
    Appreciate that if you can give me any advice.

    Thanks.
    Hao

    Reply
  6. Khawar Raza

    Hi, I was testing the code snippet you mentioned above. I am getting distances like 2.1883797959435243E-8 m which is highly inaccurate. Do you know where I am going wrong?

    Reply
    1. rvmiller Post author

      I think this is related to Tiger’s point above — Android’s ScanResult level is in dBm (mW), while the Wikipedia article uses just dB (decibels). Perhaps someone with more of a background in RF can comment on the conversion.

      Reply
  7. Rahul

    Hi, Thanks for the information. I tried your code. But getting distance as 1.9527402174346173E-6. Can you please help to solve it. Thanks a lot.

    Reply
    1. rvmiller Post author

      Hi Rahul,

      Android’s ScanResult level is actually giving dBm (mW), while the Wikipedia article uses just dB (decibels). For now, you can change the code to calculate FSPL to use Math.abs(levelInDb). I have changed the code sample above. I’m not sure if this is correct, however, but gives more reasonable distance measures :)

      Reply
  8. Throdne

    Hello, I love the post on this subject. I’m currently trying to figure it out myself. But I’m having a some problems.

    So, I took your code and quickly converted it into python to see what results I would get before fully adding it to my c# program. But for some reason I’m getting a distant reading of 1.787094… m. I know for a fact that is not right. I know that my router is on the other side of the house. at least 60ish feet. I I would have bet that It would be a little bit more accurate then what I’m getting.

    Am also getting my dBm and Freq from two different apps with the same signal dbm and freq results.
    but i’m currently using http://www.nirsoft.net/utils/wifi_information_view.html

    here is my python code.
    import math

    freqInMHz = 2457
    levelInDb = -76

    results = (27.55 - (20 * math.log10(freqInMHz)) + math.fabs(levelInDb)) / 20.0

    print results

    Reply
    1. Throdne

      So, I figured out my issues. I didn’t see that when you returned that function you also Math.pow(10, x) So, I corrected my code and it works like I think it should. But I’m really interested in finding a solution for problem one. (FSPL explicitly requires “free space” for calculation, while most Wifi signals are obstructed by walls and other materials.) I can see were I get my SNR and my RSSI. But I’m not to sure where to start on the formula on how to calculate for the difference.

      lets say I have a db of -72 with a SNR of -87 and a freq of 2457. and I know that my router is about 50 to 60 feet away (15.25-18.25m). I want to see if I can’t get accuracy down to at least a meter or 2. Any ideas on how to calculate for SNR?

      Reply
  9. Cristian

    Hi,
    Could you please share with us how do you calculate signal-to-noise ratio in Android?
    Thanks.

    Reply
  10. Raviteja

    Hi,

    I’m just a beginner in android. when I tried this project in my own Wifi, it gives the approximate distance in high signal strengthl. when the signal is low, same distance is changed unexpectedly… may i get the proper solution for this issue soon.

    Reply
  11. Lg Opt

    Im very disappointed in this post… cuz hes taught us something wrong.. and then disappeared!

    dB… is not the same as dBm
    to get distance we need FSPL whose answer will only be in dB

    and what we get from android is dBm… so they arent the same and neither can be converted…

    looking for some direction Mr.Miller?

    Reply
  12. Graham Worsfold

    Hi Ryan

    did you complete this project and achieve accurate results ?

    i understand the RSSI and SNR technique to resolve distance, Did you research the use of TDOA to improve accuracy ?

    i am working on a project that needs trilateration solution over wifi so any help you can give would be appreciated

    Reply
  13. Raviteja

    Hi,

    I’m obtaining the distances from all the wifi devices thye connected to my device. I don’t know how to implement Triangulation/Trilateration. plz suggest me how to implement it…..

    Reply
  14. vjay

    Hello there,

    How to get the triangulation with Wi-Fi triangulation/trilateration?

    links, info, source code, anything will be helpful
    thanks in advance.
    vijay

    Reply

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