Ing. Josef Krška

The application of the impulse-radio ultra-wide band technology is already established in indoor localization. Usually, two-way ranging (TWR) or time-difference of arrival (TDoA) approaches are utilized. In this paper it is shown that it is possible to perform time of arrival (ToA) positioning with accuracy similar to the TDoA. The ToA method estimates the bias between the clocks of the anchors (fixed infrastructure nodes) and the tag (localized user equipment). As a direct consequence, any timestamps taken by the tag may be converted to the common system timescale with accuracy of several nanoseconds. The approach is verified by real measurements. We demonstrate that the positioning solution can be obtained by least-squares optimization on epoch-by-epoch basis. We also show that the extended Kalman filter can be used to estimate tag position along with the bias and drift of its clock.

UWB positioning combines time measurements from several devices to estimate the position of a target device. These measurements, however, are expressed in the device's own timescale that is derived from its free running clock source, generally a crystal oscillator. The timescales are inherently offset and with different and time-variant drift, both of which have to be compensated during the estimation. With the coherent IR-UWB chips it is possible to measure the drift either directly (Carrier Frequency Offset) or indirectly with timestamps. This work provides the stability analysis of such measurements as well as the characterization of the noise that affects them. From the results the usability of drift measurement methods for synchronization and positioning is inferred.

High-capacity impulse-radio ultra-wideband (IR-UWB) indoor localization systems are typically based on the time difference of arrival (TDoA) principle. When the fixed and synchronized localization infrastructure, the anchors, transmit precisely timestamped messages, a virtually unlimited number of user receivers (tags) are able to estimate their position based on differences in the time of arrival of those messages. However, the drift of the tag clock causes systematic errors at a sufficiently high magnitude to effectively deny the positioning, if left uncorrected. Previously, the extended Kalman filter (EKF) has been used to track and compensate for the clock drift. In this article, the utilization of a carrier frequency offset (CFO) measurement for suppressing the clock-drift related error in anchor-to-tag positioning is presented and compared to the filtered solution. The CFO is readily available in the coherent UWB transceivers, such as Decawave DW1000. It is inherently related to the clock drift, since both carrier and timestamping frequencies are derived from the identical reference oscillator. The experimental evaluation shows that the CFO-aided solution performs worse than the EKF-based solution in terms of accuracy. Nonetheless, with CFO-aiding it is possible to obtain a solution based on measurements from a single epoch, which is favorable especially for power-constrained applications.

This paper proposes an approach of TDoA positioning in UWB networks, where user tags localize themselves by means of exploitation of the broadcasted synchronization messages of the anchor network. Such approach promises unlimited number of localized devices, moreover, the position is available directly at the user terminal. The key challenge of this method is to eliminate errors caused by tag clock drifts, which render the TDoA measurements useless when left uncorrected. Our method employs extended Kalman filtering (EKF) for the estimation of position and elimination of the drift-induced errors. It is shown that the system performance is similar to the more common TDoA method, where the tags transmit blinks received by the anchors. However, the anchors are still able to receive the blink messages and estimate position of those tags, since the synchronization messages are exploited. Therefore, it is possible to use both directions of the TDoA positioning concurrently; a limited number of tags is tracked by the infrastructure and all tags may compute their positions. The TDoA solutions have achieved RMS horizontal accuracy of 25.9 cm and 33.6 cm, respectively.

The Time Difference of Arrival is a popular method for UWB-based positioning, since it allows high position update rate even for multiple users. However, it requires the network infrastructure (anchors) to be synchronized, preferably with sub-nanosecond accuracy. Herein, an approach for synchronizing multiple anchors in a wireless, line-of-sight manner is described. This method is able to deal with UWB modules equipped with inexpensive drift-prone oscillators, as such impairments are estimated and compensated. By applying the proposed approach the influence of generally variable environment (e.g. temperature) on timing and positioning performance is reduced. Moreover, the presented algorithm is suitable for straightforward chaining of the line-of sight-segments in order to allow synchronization of distant anchors that cannot be synchronized with the master anchor directly.

In the ultra-wide band (UWB) localization networks the time difference of arrival (TDoA) is often used. The TDoA method is advantageous in comparison with two-way ranging approach, however, requires the UWB infrastructure of the network to be synchronized to a sub-nanosecond level. The wireless synchronization is in favor due to practical reason, nonetheless its application is constrained by the need of line-of-sight between the synchronized nodes. This paper focuses on an experimental evaluation of a Kalman-filter-based chained synchronization algorithm, which allows synchronization of a distant, directly unreachable UWB nodes through multiple nodes and the respective line-of-sight path segments.