SEPTENTRIO PolaRx5TR, Справочное руководство

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2 Operation Details
2.3 Time Management
The receiver time is kept in two counters: the time-of-week counter in integer milliseconds
(TOW) and the week number counter (WNc). TOW and WNc follow the GPS convention, i.e.
WNc counts the number of complete weeks elapsed since January 6, 1980, and there are no
leap seconds. The TOW and WNc counters are reported in all SBF blocks.
The synchronization of TOW and WNc involves the following steps:
• Upon powering up the receiver, TOW and WNc are assumed unknown, and set to a
"Do-Not-Use value" in the SBF blocks.
• The transmission time-of-week and week number are decoded from the GNSS satellites
(all constellations considered, not only GPS):
–
As soon as the first time-of-week is decoded, the TOW counter is initialized to
within 20 ms of GPS time and starts counting. This is also the time when the re-
ceiver starts generating GNSS measurements (pseudoranges and carrier phases).
–
As soon as the week number is decoded (which can be either simultaneously with
the time-of-week, or several seconds later), the WNc counter is set and starts
counting.
• After the first position and time fix has been computed (for which measurements from
at least 4 satellites are required), TOW is set to within X milliseconds of GPS time. This is
done by introducing a jump of an integer number of milliseconds in the TOW counter.
X is the maximal allowed offset between the receiver time and GPS time, and is set
by the
setClockSyncThreshold
command (by default, X=0.5ms). This initial clock
synchronization leads to a simultaneous jump in all the pseudorange and carrier phase
measurements.
Note that in cases when the satellite visibility is obstructed, the receiver can also set its time
from NMEA GPRMC or GPZDG sentences received on one of its serial ports. The time in the
NMEA sentences must refer to the GPS time scale (and not to UTC), and the serial port receiv-
ing the NMEA sentences must be configured in ASCIIIn input mode with the
setDataInOut
command. This feature is only enabled when an external frequency and time pulse is pro-
vided on the REF IN and PPS IN connectors.
The synchronization level is given by three status bits (
TOWSET
,
WNSET
and
FINETIME
) avail-
able both in the
ReceiverTime
SBF block and the
ReceiverStatus
SBF block. Once the
FINETIME
bit is set, it remains set until the next reset of the receiver.
The receiver clock can be configured in free-running mode, or in steered mode using the
command
setClockSyncThreshold
.
2.3.1 Free-Running Clock
In free-running mode, the receiver time slowly drifts with respect to GNSS time. The re-
ceiver continuously monitors this time offset: this is the clock bias term computed in the
PVT solution, as provided in the
RxClkBias
field of the
PVTCartesian
and
PVTGeodetic
SBF blocks. A clock jump of an integer number of milliseconds is imposed on the receiver
clock each time the clock bias exceeds X milliseconds by an absolute value (X is set by
setClockSyncThreshold
). This typically results in a saw-tooth profile similar to that
shown in Figure 2-1. In this example, X=0.5ms and each time the clock bias becomes greater
than 0.5ms, a jump of 1ms is applied.
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