Time Difference of Arrival (TDoA)

A positioning technique measuring differences in signal arrival times at multiple receivers to determine tag location. Also called multilateration. Requires precise time synchronization between receivers. Commonly used in UWB systems providing high accuracy. Doesn't require tags to be synchronized.

Time Difference of Arrival (TDoA) is a positioning technique that determines location by measuring the difference in arrival time of a signal at multiple receivers (anchors). The underlying principle involves: a tag transmitting a signal at an unknown time, multiple anchors receiving the signal at slightly different times due to varying distances, anchors precisely timestamping reception with synchronized clocks, and the positioning engine calculating position from the time differences. However, this shifts the synchronization burden to the anchor network - all anchors must share a common time reference with sub-nanosecond precision since time differences translate directly to position estimates (1 nanosecond time error equals approximately 30 cm distance error). In 2D positioning, three anchors provide two independent time differences. In 3D, four anchors are minimum. Industrial implementation considerations include: anchor placement for good GDOP (square or rectangular patterns provide better accuracy than linear arrangements), synchronization infrastructure (wired connections, GPS-disciplined oscillators, or wireless sync protocols), multipath mitigation (algorithms detecting and rejecting reflected signals), and sufficient anchor density (typically one anchor per 100-200 square meters). Accuracy achievable with TDoA in industrial environments ranges from 10-50 cm depending on anchor geometry and density, multipath environment characteristics, synchronization quality, and signal bandwidth. Comparing TDoA to alternative methods: Two-Way Ranging doesn't require anchor synchronization but consumes more tag power and has lower capacity, Time of Arrival requires tag-anchor synchronization challenging for mobile tags, and RSSI-based methods are simpler but far less accurate (3-10 meter accuracy versus 10-30 cm for TDoA).