IEEE 802.15.4a Ultra-wideband based wireless positioning has recently gained attention for precise localization. However, the multi-user scalability of those, mostly two-way ranging based, approaches is not considered. Due to the exchange of multiple frames per ranging, two-way ranging has significant downsides in terms of scalability. This paper proposes and validates a novel approach for a multi-user time-difference of arrival based localization system using wireless clock synchronization. The system accuracy is assessed using a complex experiment, covering robotic movement and an optical reference system for comparable results. It is shown, that the accuracies achievable by time-difference of arrival positioning with wireless clock synchronization are comparable to similar two-way ranging based approaches. All raw samples, reference data and processed positions are provided alongside this work.