In a vehicle tracking system computers aboard moving vehicles determine periodically their geographical position and transmit it to a control center. This paper presents a commercial vehicle tracking system as an example of a distributed real-time system with mobile components and focuses on the issue of clock synchronization. The mobile components of the system obtain highly accurate timing information from the Global Positioning System which is used to synchronize their local clocks to UTC world time. Thus a tight clock synchronization is achieved without imposing additional load on the communication channels of the system.