Two-Way Ranging (TWR)
A positioning technique where tags and anchors exchange messages to measure round-trip signal propagation time, determining distance. Doesn't require precise time synchronization between tags and anchors, simplifying deployment. Used in some UWB systems providing accurate ranging.
Two-Way Ranging (TWR) is a distance measurement technique where a tag and anchor exchange signals in both directions, calculating distance from round-trip propagation time without requiring precise clock synchronization between them. The distance calculation is: Distance = ((TRound_Trip - TAnchor_Processing) × Speed_of_Light) / 2, where TRound_Trip is the total time from tag's initial transmission to receiving the response, and TAnchor_Processing is the known anchor delay between receiving request and transmitting response. System capacity is limited - the multi-message ranging dialogue takes time (typically 1-5 milliseconds per ranging), limiting how many tags can range to how many anchors per second. With 100 anchors and 1 millisecond per ranging transaction, maximum system capacity is approximately 1,000 ranges per second, supporting perhaps 250 tags at 4 Hz update rate if each tag ranges to 4 anchors. Industrial TWR implementations optimize for these constraints: Asymmetric TWR where only one participant (typically the anchor) requires precision timing, simplifying tag requirements. Scheduled ranging where tags range at predetermined times avoiding collisions and maximizing capacity. TWR accuracy in industrial environments achieves: 10-50 cm distance measurement under typical conditions, comparable to TDoA or other UWB time-based methods, with accuracy depending on clock stability, signal strength, multipath conditions, and NLOS effects. Comparing TWR to alternative methods: TDoA requires anchor synchronization but supports higher tag capacity and simpler (transmit-only) tags with longer battery life, making it preferred for large-scale industrial deployments. ToA requires tag-anchor synchronization (similar challenges to TDoA anchor sync) offering no particular advantage over either TWR or TDoA.