Overview#

Positioning technologies have many useful applications, one example being GPS, which is widely used all over the world. Unfortunately, GPS does not work very well indoors, so there is a real need for better indoor positioning technologies. The goal is to track the locations (or direction) of individual objects with an external tracking system, or for a device to track its own location in an indoor environment. This kind of locating system can be used to track assets in a warehouse or people in a shopping mall, or people can use it to find their own directions. There are two conventional methods to calculate the position of an asset: trilateration and triangulation.

Trilateration means determining the distance of the asset from multiple fixed-position locators, and determining the position by finding the point that satisfies all distance measurements. The distance can be determined by Received Signal Strength Indicator (RSSI) measurement or by Time of Flight measurement, for example. Unfortunately, RSSI measurement can be very inaccurate and Time of Flight measurement needs highly accurate time measurement.

Triangulation means determining the angles under which the fixed-position locators see the asset (or under which the asset sees the fixed-position locators). The position of the asset is then determined by the intersection point of the lines of sight. This method can give a more accurate position than trilateration with RSSI measurement, and requires less expensive hardware than Time of Flight measurement. However, it needs an antenna array and a method with which the direction (angle) of the incoming signal can be determined.

This document describes the theory behind direction finding, that is, the general problem of estimating arrival and departure angles. It also discusses how Bluetooth 5.1 supports direction finding implementations.