Lassen SQ, Reference Manual

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3 Software Interface
4 6 Lassen SQ GPS Receiver
3.5
Timing Applications
The Lassen SQ GPS receiver is an excellent source for accurate
system timing. Two examples of applications requiring accurate time
are environmental data acquisition and synchronization of
communications networks. The timing functions of the receiver are
supported by the TSIP protocol and the PPS signal. See Report Packet
41 in Appendix A for a description of the time function reports for
TSIP.
Note – GPS time differs from UTC (Universal Coordinated Time) by a
variable integer number of seconds: UTC = (GPS time) - (GPS UTC
Offset)
As of April 2002, the GPS UTC offset was 13 seconds. The offset
increases by 1 second approximately every 18 months. System
designers should plan to read the offset value as a part of the timing
interface to obtain UTC. The GPS week number is in reference to a
base week (Week #0), starting January 6, 1980.
The current GPS UTC offset is contained within the almanac
transmitted by the GPS system. The Lassen SQ GPS receiver must
have a complete almanac before the offset data is valid.
3.5.1
Extended GPS Week Number
The Lassen SQ GPS receiver outputs the Extended GPS Week
Number as the absolute number of weeks since the beginning of GPS
time or January 6, 1980. If the true GPS Week Number is desired,
ignore the extra MSBs of the Extended GPS Week Number and use
only the 10 LSBs (bytes 4 and 5 of Packet 0x41).