Gnss Time Bases; Time Pulse Configuration - u-blox ZED-F9P Integration Manual

Figure 37: Timepulse and TIM-TP

4.10.3 GNSS time bases

GNSS receivers must handle a variety of different time bases as each GNSS has its own reference
system time. What is more, although each GNSS provides a model for converting their system time
into UTC, they all support a slightly different variant of UTC. So, for example, GPS supports a variant
of UTC as defined by the US National Observatory, while BeiDou uses UTC from the National Time
Service Center, China (NTSC). While the different UTC variants are normally closely aligned, they
can differ by as much as a few hundreds of nanoseconds.
Although u-blox receivers can combine a variety of different GNSS times internally, the user must
choose a single type of GNSS time and, separately, a single type of UTC for input (on EXTINTs) and
output (via the Time Pulse) and the parameters reported in corresponding messages.
The CFG-TP-* configuration group allows the user to choose between any of the supported GNSS
(GPS, GLONASS, BeiDou, etc) times and UTC. Also, the CFG-NAVSPG-* configuration group allows
the user to select which variant of UTC the receiver should use. This includes an "automatic"
option which causes the receiver to select an appropriate UTC version itself, based on the GNSS
configuration, using, in order of preference, USNO if GPS is enabled, SU if GLONASS is enabled,
NTSC if BeiDou is enabled and, finally, European if Galileo is enabled.
The receiver will assume that the input time pulse uses the same GNSS time base as specified for
the output using CFG-TP-*. So if the user selects GLONASS time for time pulse output, any time
pulse input must also be aligned to GLONASS time (or to the separately chosen variant of UTC).
Where UTC is selected for time pulse output, any GNSS time pulse input will be assumed to be
aligned to GPS time.
u-blox receivers allow users to choose independently GNSS signals used in the receiver (using
CFG-SIGNAL-*) and the input/output time base (using CFG-TP-*). For example it is possible
to instruct the receiver to use GPS and GLONASS satellite signals to generate BeiDou time.
This practice will compromise time-pulse accuracy if the receiver cannot measure the timing
difference between the constellations directly and is not recommended.
The information that allows GNSS times to be converted to the associated UTC times is
only transmitted by the GNSS at relatively infrequent periods. For example GPS transmits
UTC(USNO) information only once every 12.5 minutes. Therefore, if a Time Pulse is configured
to use a variant of UTC time, after a cold start, substantial delays before the receiver has
sufficient information to start outputting the Time Pulse can be expected.

4.10.4 Time pulse configuration

u-blox ZED-F9P receivers provide a time pulse (TP) signal with a configurable pulse period, pulse
length and polarity (rising or falling edge).
UBX-18010802 - R02
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