The RTLS algorithm needs at least 4 anchors in order to operate properly.
It might seem like the system should be able to use range data from 3 anchors to a tag in order to create a triangle and use a simple geometric solution. However, it turns out that In RTLS mode the tag is not ranging to the anchors. A solution such as this would require multiple TWR (two way range) operations between each tag and each anchor. The result would be a system that is not scalable due to the amount of time spent sending packets between devices.
In RTLS mode the tag is transmitting a single packet that is timestamped at each anchor. This minimizes RF transmissions within the system, and allows for a network capable of thousands of position calculations per second.
In order to generate a good XY position using TOA (time of arrival) from a single it is necessary to have at least our points of data. Traditional TDOA solutions require at least 5 anchors to generate a ‘good’ position in XYZ space. The algorithm used with the DWUSB kit is able to generate good XYZ output using only four points of TOA data.
With this in mind, it is always helpful to have more anchors to improve accuracy and help make the system more robust.