GPS C/A Code Signal Structure - Fundamentals of Global Positioning System Receivers: A Software Approach - page 81

FIGURE 5.7 TLM and HOW words.
5.10 GPS TIME AND THE SATELLITE Z COUNT ( 3,16 )
GPS time is used as the primary time reference for all GPS operation. GPS time
is referenced to a universal coordinated time (UTC). The acronym UTC is an
English-French mixture for Coordinated Universal Time (CUT) in English or
Temps Universal Coordonne (TUC) in French ( 16 ) . It was internationally agreed
to write Universal Coordinated Time as UTC, rather than CUT or TUC, making
it language-independent. The GPS zero time is defined as midnight on the night
of January 5/morning of January 6, 1980. The largest unit used in stating GPS
time is one week, defined as 604,800 seconds ( 7 × 24 × 3600 ) . The GPS time
may differ from UTC because GPS time is a continuous time scale, while UTC
is corrected periodically with an integer number of leap seconds. The GPS time
scale is maintained to be within one µ s of UTC (modulo of one second). This
means the two times can be different by an integer number of seconds. A history
of the difference of UTC and GPS time will be shown in Section 5.14.
In each satellite, an internally derived 1.5-second epoch, the Z count, provides
a convenient unit for precise counting and communication time. The Z count has
29 bits consisting of two parts: the 19 least-significant bits (LSBs) referred to
as the time of the week (TOW) and the 10 most-significant bits (MSBs) as the
week number. In the actual data transmitted by the satellite, there are only 27 Z
count bits. The 10-bit week number is in the third word of subframe 1. The 17-bit