Why satellites struggle indoors
Outdoors, phones can rely on satellite positioning, which How does GPS work? explains. GPS.gov lists satellite signal blockage by buildings, indoor or underground use, and signals reflected off buildings or walls, known as multipath, among the common causes of degraded GPS accuracy. It gives GPS-enabled smartphones as typically accurate to within a 4.9 m radius under open sky, and says accuracy worsens near buildings.
Inside a building, a few metres can be the difference between two rooms or two shop aisles. Indoor positioning fills that gap with radio equipment installed at known positions inside the building. The phone or tag measures something about the signals it exchanges with that equipment, and software turns those measurements into a position.
Three things a system can measure
Indoor systems measure one of three things about a radio signal.
- Signal strength. The weaker a signal arrives, the farther away the sender is likely to be. RSSI is the usual measurement. Working out a position from distances to three known points is called trilateration.
- Angle. An array of antennas can tell which direction a signal came from. Bluetooth direction finding does this with Angle of Arrival and Angle of Departure, and angles from known points can be combined by triangulation.
- Time. Radio travels at the speed of light, so a precise timing of a round trip, or of the difference between arrival times, gives distance. Wi-Fi RTT, UWB and Bluetooth Channel Sounding work this way.
The documentation for each method describes its accuracy differently, and these are the publishers’ own figures rather than guarantees:
| Method | What it measures | What the documentation says |
|---|---|---|
| Bluetooth LE signal strength | RSSI from beacons or locators | The Bluetooth SIG’s 2019 paper puts such systems at metre-level accuracy, about 1 to 10 metres |
| Bluetooth direction finding | Angle of a signal at an antenna array | The Bluetooth SIG says positioning systems can achieve centimetre-level accuracy |
| Wi-Fi RTT | Round-trip time to access points | Android says that with three or more access points the result is typically accurate within 1-2 metres |
| UWB | Ranging, or time difference of arrival from anchors | Android describes ranging to an accuracy of 10 cm; Apple describes sub-metre location from anchors |
| Bluetooth Channel Sounding | Phase and round-trip time between two devices | The Bluetooth SIG says it was designed for centimetre-level accuracy |
Who computes the position
Indoor systems come in two shapes. The Bluetooth SIG’s paper describes both.
In a real-time locating system (RTLS), tags on people or assets transmit, and fixed locators listen. Each locator reports the tags it hears to a location engine, which works out where each tag is. The building knows where the tag is; the tag may know nothing. Factories tracking equipment are one example the SIG gives.
In an indoor positioning system (IPS), the roles reverse. Fixed beacons transmit, and the visitor’s phone listens and calculates its own position, for wayfinding through an airport or a shopping centre. Apple’s UWB approach, downlink time difference of arrival (DL-TDOA), works the same way: anchors send messages, and a receiver such as an iPhone uses the difference in arrival times to calculate its location.
The shape matters for privacy. Android notes that with Wi-Fi RTT only the requesting device can determine its distance to the access point; the access points do not have this information. In an RTLS, by design, the infrastructure does.
What a deployment needs
- Fixed equipment at known positions. On Android 9, an app using Wi-Fi RTT for positioning must already have the access point locations. On Android 10 and higher, access points that support location configuration information can report their own latitude, longitude and altitude. Apple’s DL-TDOA anchors follow the IEEE 802.15.4z standard.
- Supported hardware on both ends. Wi-Fi RTT needs a phone and access points that implement Fine Timing Measurement from IEEE 802.11-2016 or ranging from IEEE 802.11az. The Bluetooth SIG says Channel Sounding may require a new Bluetooth LE chip. Nearby Interaction needs a device with a UWB chip.
- Permissions. Wi-Fi RTT requires location services and Wi-Fi scanning to be on, plus a nearby-devices or fine location permission depending on the Android version the app targets, and it only works while the app is visible or running a foreground service. Apple’s DL-TDOA requires location authorization.
- A map. Coordinates mean little until they are placed on a floor plan the app can show.
Limits
Accuracy depends on the site. The Bluetooth SIG’s paper says the floor plan and the number of locators deployed both influence results. Coverage stops where the installed equipment stops, so a system built for one building does nothing in the next.
Positions can also be wrong on purpose. The Bluetooth SIG describes distance estimates from signal strength as insecure and open to distance spoofing, and designed Channel Sounding’s round-trip timing as a countermeasure to relay attacks. Where a position must be trusted by someone other than the device, see What is proof of location?.
Finally, an indoor position is personal data. A trace of rooms and times can say a lot about a person, as What can a location record reveal? explains.