Accuracy Assessment and Learned Error Mitigation of UWB ToF Ranging

Ultra-wideband (UWB) time of flight (ToF) ranging is nowadays one of the most attractive technologies to implement indoor localization solutions with reliable dm-level accuracy. UWB systems are generally resistant to multipath interference. However, non-line-of-sight (NLOS) components with small relative delays may introduce errors in the estimated distances well exceeding the typically achievable accuracy. We present an empirical accuracy assessment of UWB ranging using a commercial UWB system. A particular focus is on the magnitude and spatial patterns of multipath errors. A large dataset comprising distances between 0.2 and 100 m was collected in a geodetic metrology lab and outdoors for this purpose. We derived ground truth of the distances with superior accuracy using a laser tracker and a total station. The results show that multipath errors are highly repeatable if the geometrical configuration does not change, but can degrade the accuracy of the distance measurements locally by almost an order of magnitude. While typical biases were on the order of a few cm, peak values exceeding 60 cm were found, and the standard deviation, typically about 3 cm or less, increased up to almost 15 cm in certain locations. We have used a part of the large dataset to train an artificial neural network (ANN) for error prediction. The ANN uses the measured ranges and additional signal features, also provided by the ranging nodes, as inputs. For error mitigation, the ANN output was subtracted from the original range measurements. This improved the ranging accuracy over the whole test data by practically eliminating the bias on average and reducing the overall standard deviation by a factor of 3. The results indicate that the relation between the output of the UWB node and the range errors can be modeled to a practically useful degree even without in-depth knowledge of the specific UWB node's measurement process and characteristics. The proposed approach can help to significantly improve the UWB ranging performance in presence of multipath and thus also the accuracy of a corresponding localization system.