Abstract : The NAVSTAR Global Positioning System (GPS) allows extremely accurate and global determination of time, as well as position and velocity. An STI Time Transfer Unit (TTU) developed for the U.S. Naval Observatory (USNO) has consistently demonstrated the transfer of time with accuracies much better than 100 nanoseconds. A new STI Time Transfer System (TTS), the TTS 502, is currently in development and will be available on the market by the end of 1981. The TTS 502 will be a relatively compact microprocessor-based system with a variety of options that will meet each individual's requirements, and will have the same performance as the USNO system. This paper summarizes the time transfer performance of that USNO system and presents the details of the new system.