Trimble Outdoors SK8 - Manual (Page 49)

Lassen-SK8 Embedded GPS Module 3-7
Software Interface
NMEA data is output in standard ASCII sentence formats. Message identifiers are used to
signify what data is contained in each sentence. Data fields are separated by commas
within the NMEA sentence. In the Lassen-SK8, NMEA is an output only protocol. The
NMEA protocol is described in detail in Appendix E, NMEA 0183.
The receiver is shipped from the factory with NMEA output on Port 2. Port 2 settings can
be changed using
TSIPCHAT and command 0xBC. TSIP command 0x7A changes the
NMEA output sentences and output rates. The new settings are saved to BBRAM or they
can be saved to non-volatile memory using TSIP command 0x8E-26.
3.5 Timing Applications
The Lassen-SK8 is an excellent source for accurate system timing. Examples of
applications requiring accurate time are environmental data acquisition or synchronization
of communications networks. The timing functions of the receiver are supported by the
TSIP protocol. See Report Packet 41 in Appendix A for a description of the time function
reports for TSIP.
*
Note – Note that GPS time differs from UTC (Universal Coordinated Time) by a variable
integer number of seconds:
UTC = (GPS time) - (GPS UTC Offset)
As of July 1997, the GPS UTC offset was 12 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 GPUSTC offset is contained within the almanac transmitted by the GPS
system. The Lassen-SK8 must have a complete almanac before the offset data is valid.
3.5.1 Effect of GPS Week Number Roll-over (WNRO)
At 0000 hours Greenwich Mean Time (GMT) on 21/22 August 1999, the GPS Week
Number will roll-over from 1023 to zero. Trimble receivers have numerous built-in
protections to prevent this from being a catastrophic event. Systems may benefit however,
from extra care with the first power-up after WNRO.
*
Note – GPS Week Numbers occupy a range from zero to 1023 such that the Week
Number Roll Over (WNRO) occurs every 1024 weeks, or approximately every 19 years 8
months. August 1999 is the first roll-over for the GPS system since the beginning of GPS
time on 06 January 1980.