See TN199812A for general information on the GPS system.
GPS time is established by the Control Segment and is referenced to a UTC (as maintained by the U.S. Naval Observatory) zero time point defined as midnight on the night of January 5, 1980. It is maintained to be within one microsecond of UTC (modulo one second). The largest unit used in stating GPS time is one week.
Each satellite has an internally derived 1.5 second epoch which is used to precisely count and communicate time. Time stated in this manner is referred to as a Z-count. This is provided to the user as a 29 bit binary number in two parts. The least significant 19 bits are referred to as the time of week (TOW) and count from 0 to 403,199 starting at (approximately) midnight Saturday night.
The ten most significant bits are called the GPS week number (modulo 1024). The week starting 6 January 1980 was week 0. Week 1023 was reached in August 1999 at which time the week number will revert to 0.
I think that some other bits have been defined in one of the subframes to operate as "more significant bits" of the week number, but I am not sure. In any case, it is possible for GPS receivers to cope with the week number rollover in some manner. However, the version of software in all GPS receivers purchased by the SRC to date does not cope with the rollover. Therefore, we must add special code in to the Guria software to cope with this. This is done by looking at the week number obtained from the GPS receiver and if it is < 960 then 1024 is added.