This paper describes two important challenges in the development of an ultra-wideband (UWB) positioning system. The first challenge is increasing the accuracy and robustness of the UWB technology. The range measurements are subject to disturbances, which introduce an unwanted bias. This range bias is measured, characterized and modeled to increase the positioning accuracy to an average of 3 cm. The second challenge is a user-friendly deployment of the system. This is achieved by a semi-automated autocalibration procedure. In the proposed method the mobile device (tag) is moved around in the covered area. Based on all the captured data, the coordinates and a range bias model parameter of the static devices (anchors) are obtained. No additional measurement devices are required during the procedure. The positioning accuracy with the autocalibrated parameters decreases to an acceptable 9 cm on average, compared to the situation with exact parameters.