iBeacon Microlocation Accuracy

Customers often ask us the accuracy when locating beacons. In order to get the answer, you have to understand how locating works and the tradeoffs that are needed to get the different levels of accuracy.

There are two main scenarios. The first is a where the detector, usually a phone or gateway, is at a known location and the beacon moves. The second is where the beacons are fixed and the detector moves. Either way, the detector receives a unique beacon id and the receiving electronic circuitry provides the strength of the received signal, called a received signal strength indicator (RSSI).

The value of the RSSI can be used to infer the distance from the detector to the beacon. The main problem with RSSI is that it varies too much, over time, to be used to accurately calculate distance. The direction also isn’t known when there’s only one beacon and one detector. The varying RSSI, even when nothing is moving, is caused by the Bluetooth radio signals that are reflected, deflected by physical obstcacles and interfered with by other devices using similar radio frequencies. Physical factors such as the room, the beacon not uniformly emitting across a range of 360 degrees, walls, other items or even people can affect the received signal strength. How the user holds a detecting phone can affect the effectiveness of the antenna which in turn affects the signal strength.

The varying RSSI can be smoothed by averaging or signal processing, such as Kalman filtering, to process multiple RSSI values over time. The direction not being known can be solved by using trilateration where three gateways (or beacons depending on the above mentioned scenario) are used to determine the distance from three directions and hence determine the 2D location.

Trilateration

The aforementioned physical factors that affect RSSI can be reduced by measuring the actual RSSI at specific locations and hence calibrating the system.

The change of RSSI with distance is greater when the beacon is near the detector. At the outer reaches of the beacon signal, the RSSI varies very little with distance and it’s difficult to know whether the variance is due to a change of distance or radio noise. Hence for systems that use signal processing, trilateration and calibration tend to achieve accuracies of about 1.5m within a shorter range confined space and 5m at the longer distances.

However, such systems have problems. The multiple RSSI values needed for averaging or signal processing mean that you either have to wait a while to get a location fix or have the beacons transmit more often (with a shorter period) that flattens their batteries much sooner. Trilateration requires at least three devices per zone so can be costly and require significant time to setup and maintain. Using calibration is like tuning a performance car. It works well until something small changes and it needs re-tuning. If someone adds a room partition, desk or even something as simple as lots of people in the room, the calibration values become invaid. Re-calibration takes human effort and, pertinently, it’s not always easy to know when it needs re-tuning.

An alternative to trilateration is zoning. This involves putting a detector (or beacon depending on the above mentioned scenario) in each room or zone. The system works out the nearest detector or beacon and can work on just one RSSI value to get a fix quickly. The nearest zone is often all that’s required of most implementations. With zoning, if you need more accuracy in a particular zone you add more detectors in the area to get up to the 1.5m accuracy of other methods. This will obviously be impractical if you need 1.5m accuracy everywhere over a large area.

BeaconRTLS™ area zones

An alternative to trilateration and zoning is specialist and more expensive Bluetooth hardware that makes use of Angle of Arrival (AoA). One such system is Fathom that has gateways with six antenna that can detect the direction of the beacon signal. The gateways are placed 15m apart and achieve 2m accuracy. Quuppa also provide an AoA solution with a claimed accuracy of 0.5m but we suspect this is ‘best case’ with a very dense gateway infrastructure and beacons advertising very frequently.

If you really need reliable sub 1.5m accuracy over a large area you can look beyond Bluetooth to specialist, very expensive, Ultra Wideband solutions that use Time Difference of Arrival (TDoA). For TDoA the closest detector will receive the signal before other detectors. The difference in the time of receiving the signals gives an indication of the difference in distances allowing the position to be calculated. TDOA systems are more accurate than those based on just RSSI. Their accuracy varies between 5 percent and 10 percent of the distance between the detectors. Detected time differences are of the magnitude of a few nanoseconds. Solutions need expensive electronics and need to be synchronised at the nanosecond level that can make them more complex and expensive. Also, again, at least three (expensive) detectors are needed in each zone for accurate results.

In summary, choosing a solution just because it is more accurate, rather than needed, will cost significantly more not just in hardware but in setup effort and maintenance. Work out what accuracy you need and then seek out an appropriate solution.