In general, it does not matter which three among the set of installed beacons are selected for reference. However, coordinate accuracy should improve if the position of the three selected beacons circumscribe the tracking area.
The coordinate system axes are defined by the order of listener placement underneath each of the three reference beacons. The first reference beacon that is being ``calibrated'' by the listener defines the origin of the coordinate system in the lower left corner. The distance measurements can be noisy and ther user must hold the listener still (or tilting the listener slightly) until after the standard deviations of the distance measurements for all of the beacons fall below STDDEV_TOLERANCE_CM (default 10 cm). When this happens, the Beacon Configuration window will then show a dot (and sound a beep), representing the origin defined by this beacon. Note that a dot will not show the distance measurements if any one of the beacons are faulty (i.e. either the distances have high variations or the beacon is out of range or if there is not enough samples to make a distance estimate).
Repeat the same process for the second reference beacon. When done, the Beacon Configuration window will display a second dot (and sound two beeps), representing the position of the second reference in the coordinate system. Thus, the line extended between the first and second beacon becomes the horizontal axis of beacon coordinate system.
Repeat the process again for the third reference beacon. When done, the coordinate system configuration is complete and the Beacon Configuration window will display all the dots representing the positions of all the beacons that have been configured. The third reference beacon does not define the y-axis. Rather, it defines the direction of the positive y-axis. This should become intuitive after the user gains experience with performing the steps described here.
We summarize the steps for configuring the beacon coordinate system below: