The Two Way Ranging method determines the Time of Flight of the UWB RF signal and then calculates the distance between the nodes by multiplying the time by the speed of light. The TWR process is applied between the Tag and demanded Anchor, just only one Anchor may be involved in TWR at given time slot.

distance = ToF x (speed of light) ToF=[(TRR-TSP)-(TSR-TRP) +(TRF-TSR)-(TSF-TRR)]/4 To measure distance three messages needs to be exchanged. The Tag initializes TWR by sending Poll message to known address of Anchor in time referred to TSP (Time of Sending Poll). The UWB Anchors for Indoor Location Tracking - Woxu RTLS records time of Poll reception (TRP) and replies with the Response message at time TSR. The Tag upon reception of Response message records time TRR and composes Final message, where its ID, TSP, TRR, TSF information are included. Based on time reception of Final message TRF and information provided in the Final message the Anchor can determine the Time of Flight of UWB signal. Optionally, the resulted distance can be send within the Report message back to Tag or any given UWB device in neighborhood. The experiment show that optimal distance between UWB RTLS Tags for Indoor Location Tracking - Woxu RTLS and Anchor for TWR process is in range of 20-30 meters. UWB Sniffer integrated with Wirehark - Woxu RTLS

The RTLS systems based on TWR scheme benefits from idea that no time synchronization is needed between the UWB devices. On the other way such feature limits scalability of Real-Time Location System (RTLS) on Ultra-wideband - Woxu RTLS system since the TWR process is applied gradually with all the Anchors within the system. Furthermore the Tags need to be aware about the address range of the Anchors to gradually poll all the available Anchors. This situation is depicted in figure below.

As described in TWR introduction section, the Report message with estimated distance may be send back through the radio. Such situation leverages possibility to gather all the results wirelessly by dedicated Master Anchor that can directly send information to RTLS server for processing. Such feature brings another limitation in term of scaling of Anchors number, since all the Anchors in the system need to directly communicate with the Master Anchor. The communication distance of Anchors may vary from 20-40 meters according to the indoor environment. The deployment of Master Anchors requires take this limitation into the consideration when planning placement of the Anchors in the facility.

The Two Way Ranging method determines the Time of Flight of the UWB RF signal and then calculates the distance between the nodes by multiplying the time by the speed of light. The TWR process is applied between the Tag and demanded Anchor, just only one Anchor may be involved in TWR at given time slot.