Wi-Fi positioning – efficient and low cost indoor location

Wi-Fi positioning systems (WPS) are used when GPS or Cell ID positioning is not working due to signal and multipath blockage indoors or underground. Wi-Fi positioning uses the fact that Wi-Fi networks are rapidly growing in numbers and Wi-Fi infrastructure can therefore be used for both network access and geolocation reducing the need for investments in infrastructure. A big advantage compared to GPS is that Wi-Fi positioning makes it possible to determine floor level.

Accuracy for Wi-Fi positioning

The accuracy for indoor positioning varies from five to fifteen meters – all depending of the physical conditions. The number of Wi-Fi positions that have been entered into the database is also affecting accuracy.

Techniques to make Wi-Fi positioning work

The challenge for Wi-Fi based indoor positioning of a device is about determining the position of a client device with respect to access points. There are several techniques to succeed with this. They can be classified into four types:

  • RSSI technology is measuring the intensity of the received signal. SSID and MAC addresses of the access point is used to locate the Wi-Fi hotspots or wireless access points.
  • Fingerprinting is also RSSI-based, but it depends on recording the signal strength from several access points in range and storing this information in a database along with the known coordinates of the client device in an offline phase.
  • Angle of arrival (AoA) is using Wi-Fi interfaces with multiple antennas and estimates the angle of arrival of the multipath signals and apply triangulation to calculate the location of client devices.
  • Time of flight (ToF) calculate the time of flight of signals and then use this information to estimate the distance and relative position of a client device with respect to access points.

Inaccuracies

Signal fluctuations can cause use inaccuracies. Other data sources like geographical information and time geography can be used to increase accuracy when precision is low.

How Wi-Fi positioning works with GPS

Devices with both Wi-Fi and GPS can be used to send information about a Wi-Fi network back to a location service provider. This makes it possible to determine where the particular Wi-Fi network is. This is done by having the device send the access point’s MAC address along with the location determined by GPS. If GPS is used to determine the location of a device it will also scan nearby networks that can be used to identify the network. When the location of Wi-Fi networks are found the information will be recorded for public access. Next time someone is close to one of those Wi-Fi networks, but lack a GPS-signal, this service will be used to determine a location as the network’s location is recorded.

Vendors collecting Wi-Fi information

Wi-Fi positioning information is gathered by vendors to provide more accurate location services to users. Gathering this kind of network information is public knowledge, Wi-Fi passwords are not needed to do this. Determining user locations in this way is usually part of cell phone carrier’s terms-of-service agreement though most phones allow the user to turn off location services.