4.4. HOW TO OUTPUT A PPS SIGNAL
The Delay argument can be used to compensate for signal delays in the system (including
antenna, antenna cable and PPS cable). For example, if the antenna cable is replaced by
a longer one, the overall signal delay could be increased by say, 20 nsec. If Delay is left
unchanged, the PPS pulse will come 20 ns too late. To re-synchronize the PPS pulse, Delay
should be increased by 20 ns.
By default, PPS pulses are aligned with the satellite time system (TimeSys) as shown in
the Time Scale field. PPS signals can alternatively be aligned with UTC, local receiver time
(RxClock) or GLONASS time.
When Time Scale is set to anything other than RxClock, the accuracy of the time of the
PPS pulse depends on the age of the last PVT computation. During PVT outages, the PPS
generation time, which is extrapolated from the last available PVT information, may start
to drift. To avoid large biases, the receiver stops outputting the PPS pulse when the last
available PVT is older than the specified MaxSyncAge. The MaxSyncAge is ignored when
TimeScale is set to RxClock.
4.4.1 Time synchronisation using the PPS signal
The PPS signal is an electronic pulse synchronised with GPS time clock ticks, it doesn’t itself
specify time. To synchronise a device with GPS time, the AsteRx-U can be configured to
output both a PPS signal and an NMEA ZDA sentence which contains the time. The PPS signal
arrives first followed by the ZDA whose reported time corresponds to the leading edge of the
PPS signal.
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