Handling systematic node biases in TDoA positioning systems

Node-specific hardware biases represent significant error source in wireless Time of Arrival (ToA) and Time Difference of Arrival (TDoA) positioning. While bias estimation and treatment are well-established in Global Navigation Satellite Systems (GNSS), their application in wireless Positioning, Navigation, and Timing (PNT) remains a significant challenge. This work proposes a methodology, based on GNSS data processing, to mitigate these systematic biases. Specifically, the concept of Differential Transmitter Bias (DTB) is introduced to account for node-specific hardware variations. These DTB are in fact analogous to the Differential Code Bias (DCB) used in GNSS. This paper includes an analysis based on real data from a network of wireless nodes employed in an indoor navigation context and shows that the DTB exhibits remarkable stability, allowing it to be treated as a constant during an entire positioning session, with small variations as low as 2m, which is in the same uncertainty level as the noise of the measurements used by a TDoA-based positioning engine. In an operating positioning service context, these DTB would be transmitted to the user, similarly to GNSS, where satellite clock corrections are provided through navigation messages. This would allow the user to callibrate the TDoA measurements in order to obtain positioning with errors on the meter level.