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1 Introduction
1.4.3 THE USER SEGMENT
The User Segment consists of equipment (such as a NovAtel MiLLennium-GLONASS GPSCard receiver) which
tracks and receives the satellite signals. This equipment must be capable of simultaneously processing the signals
from a minimum of four satellites to obtain accurate position, velocity and timing measurements. Like GPS,
GLONASS is a dual military/civilian-use system. Selective availability, however, will not be implemented on
GLONASS C/A code. The system's potential civil applications are many and mirror that of GPS.
1.5 TIME
As stated above, both GPS and GLONASS satellites broadcast their time within their satellite messages. NovAtel's
MiLLennium-GLONASS GPSCard is able to receive and record both time references as well as report the offset
information between GPS and GLONASS time (see GCLA/B, Page 91). Although similar, GPS and GLONASS
have several differences in the way they record and report time. Please see the following sections for information
on GPS and GLONASS time, as well as how NovAtel's MiLLennium-GLONASS GPSCard is GPS week rollover
and Y2K compliant.
1.5.1 GPS TIME VS. LOCAL RECEIVER TIME
All logs report GPS time expressed in GPS weeks and seconds into the week. The time reported is not corrected
for local receiver clock error. To derive the closest GPS time, you must subtract the clock offset shown in the
CLKA/B log (field 4) from GPS time reported, refer to the MiLLennium Command Descriptions Manual.
GPS time is based on an atomic time scale. Universal Time Coordinated U.S. Naval Observatory (UTC(USNO))
time (reported in NMEA logs) is also based on an atomic time scale, with an offset of seconds applied to coordinate
Universal Time to GPS time. GPS time is designated as being coincident with UTC(USNO) at the start date of
January 6, 1980 (00 hours). GPS time does not count leap seconds, and therefore an offset exists between
UTC(USNO) and GPS time (at this date: 13 seconds). The GPS week consists of 604800 seconds, where 000000
seconds is at Saturday midnight. Each week at this time (UTC), the week number increments by one, and the
seconds into the week resets to 0 (see Appendix C, Page 54 for an example).
1.5.2 GLONASS TIME VS. LOCAL RECEIVER TIME
GLONASS time is based on an atomic time scale similar to GPS. This time scale is Universal Time Coordinated
as maintained by the former Soviet Union (UTC(SU)).
Unlike GPS, the GLONASS time scale is not continuous and must be adjusted for periodic leap seconds. Leap
seconds are applied to all UTC time references about every other year as specified by the International Earth
Rotation Service (IERS). Leap seconds are necessary because the orbit of the earth is not uniform and not as
accurate as the atomic time references.
GLONASS time is maintained within 1 ms of UTC(SU) by the control segment with the remaining portion of the
offset broadcast in the navigation message. As well, the GLONASS time is offset from UTC(SU) by plus three
hours due to control segment specific issues. The GCLA/B (see Page 91) contains the offset information between
GPS and GLONASS time.
1.6 DATUM
Because a consistent transformation between WGS84 and the Parametry Zemli 1990 (PZ90) or, in English
translation, Parameters of the Earth 1990 (PE-90, see Page 120) geodetic datum has not been defined, we have
allowed for a new command, PZ90TOWGS84, and a new parameter, PZ90, for the DATUM command.
The PZ90TOWGS84 command (see "G.1.2 PZ90TOWGS84", Page 82) is intended to define the PZ90 transform
for transferring Glonass satellite coordinates to WGS84. However, it can also be used, in conjunction with the
DATUM PZ90 command (see "G.2.6 DATUM", Page 85), to allow for position output in a user-defined PZ90
frame. The PZ90TOWGS84 command will override the default values for the DATUM PZ90 command and set
them to the user-defined values. If the PZ90TOWGS84 command is not issued, the DATUM PZ90 command will
use the default PZ90 values (see "G.1.2 PZ90TOWGS84", Page 82) for the output position parameters. The PZ90
transform parameters can be saved in user-configurable memory for immediate use on powerup.
GPS/GLONASS Receiver User Manual Rev 1
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