The purpose of these tags is to use the time propagation of a sound message to know the position of the robot.

Steps to measure the distance of the robot 1 tag:

The robot sends a synchronization pulse to the tag 1 and sends its message ultrasound

The tag receives the pulse starts a counter. The message is spread through the air at the speed of sound.

The tag receiving listening time, and detects the time between pulse ZigBee and receiving ultrasound.
The Doppler shift can be measured.

The tag returns the information on propagation time and Doppler robot, which uses them to calculate its position.

The speed of sound depends on temperature.

The distance to more than three tags to improve the measure and compensate for temperature.

Doppler can also be used to improve the result, using a filter Kalmann.