How to calculate distance from Wifi router using Signal Strength

Question

I would like to calculate the exact location of a mobile device inside a building   so no GPS access   I want to do this using the signal strength in dBm  of at least 3 fixed wifi signals 3 fixed routers of which I know the position   Google already does that and I would like to know how they figure out the exact location based on the this data  Check this article for more details   http   www codeproject com Articles 63747 Exploring-GoogleGears-Wi-Fi-Geo-Locator-Secrets

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Distance  km    10   Free Space Path Loss     92 45     20log10 f   20

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the simple answer to your question would be Triangulation  Which is essentially the concept in all GPS devices  I would give this article a read to learn more about how Google goes about doing this  http   www computerworld com s article 9127462 FAQ How Google Latitude locates you  taxonomyId 15 amp pageNumber 2   From my understanding  they use a service similar to Skyhook  which is a location software that determines your location based on your wifi cellphone signals  In order to achieve their accuracy  these services have large servers of databases that store location information on these cell towers and wifi access points - they actually survey metropolitan areas to keep it up to date  In order for you to achieve something similar  I would assume you d have to use a service like Skyhook - you can use their SDK   http   www skyhookwireless com location-technology       However  if you want to do something internal  like using your own routers  locations  - then you d likely have to create an algorithm that mimics Triangulation  You ll have to find a way to get the signal strength and mac address of the device and use that information along with the locations of your routers to come up with the location  You can probably get the information about devices hooked up to your routers by doing something similar to this   http   www makeuseof com tag check-stealing-wifi

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To calculate the distance you need signal strength and frequency of the signal  Here is the java code   public double calculateDistance double signalLevelInDb  double freqInMHz        double exp    27 55 -  20   Math log10 freqInMHz     Math abs signalLevelInDb     20 0      return Math pow 10 0  exp       The formula used is       distance   10     27 55 -  20   log10 frequency     signalLevel  20    Example  frequency   2412MHz  signalLevel   -57dbm  result   7 000397427391188m  This formula is transformed form of Free Space Path Loss FSPL  formula  Here the distance is measured in meters and the frequency - in megahertz  For other measures you have to use different constant  27 55   Read for the constants here     For more information read here

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K   32 44 FSPL   Ptx - CLtx   AGtx   AGrx - CLrx - Prx - FM d   10      FSPL - K - 20 log10  f      20     Here    K - constant  32 44  when f in MHz and d in km  change to -27 55 when f in MHz and d in m  FSPL - Free Space Path Loss Ptx - transmitter power  dBm   up to 20 dBm  100mW    CLtx  CLrx - cable loss at transmitter and receiver  dB   0  if no cables   AGtx  AGrx - antenna gain at transmitter and receiver  dBi Prx - receiver sensitivity  dBm   down to -100 dBm  0 1pW    FM - fade margin  dB   more than 14 dB  normal  or more than 22 dB  good   f - signal frequency  MHz d - distance  m or km  depends on value of K    Note  there is an error in formulas from TP-Link support site  mising      Substitute Prx with received signal strength to get a distance from WiFi AP   Example  Ptx   16 dBm  AGtx   2 dBi  AGrx   0  Prx   -51 dBm  received signal strength   CLtx   0  CLrx   0  f   2442 MHz  7 th 802 11bgn channel   FM   22  Result  FSPL   47 dB  d   2 1865 m  Note  FM  fade margin  seems to be irrelevant here  but I m leaving it because of the original formula   You should take into acount walls  table http   www liveport com wifi-signal-attenuation may help   Example   previous data    one wooden wall   5 dB  from the table    Result  FSPL   FSPL - 5 dB   44 dB  d   1 548 m   Also please note  that antena gain dosn t add power - it describes the shape of radiation pattern  donut in case of omnidirectional antena  zeppelin in case of directional antenna  etc    None of this takes into account signal reflections  don t have an idea how to do this   Probably noise is also missing  So this math may be good only for rough distance estimation

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FSPL depends on two parameters  First is the frequency of radio signals Second is the wireless transmission distance  The following formula can reflect the relationship between them   FSPL  dB    20log10 d    20log10 f    K  d   distance f   frequency K  constant that depends on the units used for d and f If d is measured in kilometers  f in MHz  the formula is   FSPL  dB    20log10 d   20log10 f    32 44   From the Fade Margin equation  Free Space Path Loss can be computed with the following equation   Free Space Path Loss Tx Power-Tx Cable Loss Tx Antenna Gain Rx Antenna Gain - Rx Cable Loss - Rx Sensitivity - Fade Margin  With the above two Free Space Path Loss equations  we can find out the Distance in km   Distance  km    10 Free Space Path Loss     32 44     20log10 f   20   The Fresnel Zone is the area around the visual line-of-sight that radio waves spread out into after they leave the antenna  You want a clear line of sight to maintain strength  especially for 2 4GHz wireless systems  This is because 2 4GHz waves are absorbed by water  like the water found in trees  The rule of thumb is that 60  of Fresnel Zone must be clear of obstacles  Typically  20  Fresnel Zone blockage introduces little signal loss to the link  Beyond 40  blockage the signal loss will become significant   FSPLr 17 32 v d 4f   d   distance  km  f   frequency  GHz  r   radius  m    Source   http   www tp-link com en support calculator

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Don t care if you are a moderator   I wrote my text towards my audience not as a technical writer  All you guys need to learn to navigate with tools that predate GPS  Something like a sextant  octant  backstaff or an astrolabe   If you have receive the signal from 3 different locations then you only need to measure the signal strength and make a ratio from those locations  Simple triangle calculation where a2 b2 c2  The stronger the signal strength the closer the device is to the receiver

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In general  this is a really bad way of doing things due to multipath interference  This is definitely more of an RF engineering question than a coding one   Tl dr  the wifi RF energy gets scattered in different directions after bouncing off walls  people  the floor etc  There s no way of telling where you are by trianglation alone  unless you re in an empty room with the wifi beacons placed in exactly the right place   Google is able to get away with this because they essentially can map where every wifi SSID is to a GPS location when any android user  who opts in to their service  walks into range  That way  the next time a user walks by there  even without a perfect GPS signal  the google mothership can tell where you are  Typically  they ll use that in conjunction with a crappy GPS signal   What I have seen done is a grid of Zigbee or BTLE devices  If you know where these are laid out  you can used the combined RSS to figure out relatively which ones you re closest to  and go from there

[geolocation] How to calculate distance from Wifi router using Signal Strength?

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