Trimble Outdoors SK8 - Manual (Page 93)

Lassen-SK8 Embedded GPS Module A-29
Trimble Standard Interface Protocol
A.17.22 Report Packet 0x41
This packet provides the current GPS time of week and the week number. The GPS
receiver sends this packet in response to Packet
0x 21 and during an update cycle. Update
cycles occur approximately every 5 seconds. The data format is shown below.
*
Note – UTC time lags behind GPS time by an integer number of seconds
UTC = (GPS time) - (GPS UTC offset).
I
Caution – GPS week numbers run from 0 to 1023 and then cycles back to week #0. Week
# 0 began January 6, 1980. There will be another week #0 beginning August 22, 1999.
The extended GPS week number however, does not cycle back to 0. For example, August
22, 1999 starts week number 1024.
The seconds count begins with “0” each Sunday morning at midnight GPS time. A
negative indicated time-of-week indicates that time is not yet known; in that case, the
packet is sent only on request. The following table shows the relationship between the
information in Packet
0x 41, and the Packet 0x 46 status code.
*
Note – Before using the GPS time from Packet 0x41, verify that the Packet 0x46 status
code is 00 (“Doing position fixes”). This will ensure the most accurate GPS time.
Table A-25. Report Packet 0x41 Data Formats
Byte Item Type Units
0-3 GPS time of week SINGLE seconds
4-5 Extended GPS week number INTEGER weeks
6-9 GPS/UTC offset SINGLE seconds
Table A-26. Packets 0x41 and 0x46 Status Code Relationships
Approximate Time
Accuracy
Time Source Sign (TOW)
Packet 46
Status Code
none no time at all - 0x01
unknown approximate time
from real-time
clock or Packet
2E + 0x01
20-50 msec + clock drift time from satellite + not 0x01
full accuracy time from GPS
solution + 0x00