The ability to accurately measure signal time-of-flight between ultra-wideband (UWB) wireless communication transceivers, even in multipath environments, makes this technology ideally suited to develop ranging-based positioning systems, especially for indoor applications where GPS signals are not available. In recent years, low-cost commercial UWB transceivers have become more easily available and increasingly used to develop custom robot positioning systems. In this paper, we focus in particular on positioning techniques requiring the synchronization of base stations such as Time of Arrival (TOA) and Time Difference of Arrival (TDOA). We present a protocol based on Kalman filtering for simultaneous synchronization of multiple UWB base stations and positioning of an arbitrary number of passive UWB receivers. We illustrate experimentally using our protocol and an EKF-based navigation system design the level of accuracy achievable with small low-power UWB modules for mobile robot positioning. We discuss in details measurement errors and system tuning issues applicable to popular commercial UWB transceivers.