The current downlink data format for the GPS satellites stores the difference between GPS system time and UTC using 8-bits. This means that the maximum difference ( GPS - UTC ) that can be stored is 255 seconds. If leap seconds continue to be inserted into UTC, then the current fleet of GPS satellites and all receivers of GPS signals will eventually become obsolete.
When will the current GPS data format fail? It depends on the deceleration of earth rotation. The approximate year of failure is 2114, 2149, or 2174 for decelerations of 42, 31, or 25.6 s/cy2, respectively. This means that a new series of GPS satellites and a new set of receivers must be designed and deployed sometime within the next century. Furthermore, navigational satellite systems such as GPS and Galileo place constraints on what can and cannot be used for civil time. Suppose that human society deems that atomic time should be used for system operations, but that some form of mean solar time other than UTC with leap seconds is desirable for civil purposes. This new form of mean solar time might have leap-milliseconds and be something like UTC was before 1972. It is not feasible to consider use of a time scale like this unless the satellite systems can broadcast it. Therefore it is imperative as soon as possible to decide what form of time is needed for civil purposes. I have not kept up on the progress of the Galileo signal format of the next generation of GPS satellites. I don't know if they are considering any such issues. -- Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064 [EMAIL PROTECTED] Voice: +1 831 459 3046 http://www.ucolick.org/~sla PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E 49 89 0E FE 26 B4 14 93