The first testing of the Galileo program's Galileo System Test Bed(GSTB) looks closely at the space, ground, and user segments of the overall system. In this development and validation phase, testing used the operations of a core spacecraft constellation of four satellites and its ground and user segments for qualification. The three Galileo receiver units being tested that receive and process signals in space from the various satellites are the GTSB-V2 receiver, the Open Service/Commercial Service/Safety of Life (OS/CS/SoL) test user receiver, and the Public Regulated Service (PRS) test user receiver. This test user receiver (TUR) is a software-based receiver in that it will offer a flexible architecture by downloading different configurations. First, Galileo signals will be transmitted in four frequencies in the radio navigation satellite service (RNSS) band as well as the aeronautical radio navigation services (ARNS) band. Using both bands allows for protecting weak signals and safety-critical applications. A TUR breadboard was designed and manufactured for early signal-acquisition purposes. The prototype does not differ much from a basic global positioning system (GPS) receiver. This breadboard has been permanently integrated into the Galileo constellation simulator, and will evolve into the GSTB-V2 receiver by the end of 2005. GSTB-V2 will transmit the navigation signals at all the planned frequencies for Galileo for early experimentation. Further development of the system will occur in the coming years.