UWB positioning systems show promise in improving navigational capabilities and safety of operations for semi-autonomous systems that are emerging in applications such as construction and agricultural vehicles. This paper describes an outdoor UWB positioning system used to measure the position of tracks in a semi-autonomous truck convoy. The system improves safety by providing position the vehicles control system with accurate position measurements. A measurement campaign was conducted to collect experimental range errors in open, forested and urban channels at distances ranging from 2 m to 40 m. The range errors are characterised and a model for range errors is proposed. Position errors using the proposed model are compared to the experimental data. Performance analysis regarding accuracy and position update delay is presented.