SAM-M10Q - Integration manual
conversion parameters are transmitted periodically (every 12.5 minutes) by GPS satellites, but can
also be supplied to the receiver via the UBX-MGA-GPS-UTC aiding message. By contrast when the
receiver has chosen to use the GLONASS time base as its GNSS system time, conversion to GPS
time is more difficult as it requires knowledge of the difference between the two time bases, but as
GLONASS time is closely linked to UTC, conversion to UTC is easier.
When insufficient information is available for the receiver to perform any of these time base
conversions precisely, predefined default offsets are used. Consequently plausible times are nearly
always generated, but they may be wrong by a few seconds (especially shortly after receiver start).
Depending on the configuration of the receiver, such "invalid" times may well be output, but with
flags indicating their state (e.g. the "valid" flags in UBX-NAV-PVT).
u-blox receivers employ multiple GNSS system times and/or receiver local times (in order
to support multiple GNSS systems concurrently), so users should not use UBX messages
reporting GNSS system time or receiver local time. It is recommended to use messages that
report UTC time and other messages are retained only for backwards compatibility reasons.
2.7.4 iTow timestamps
All the main UBX-NAV messages (and some other messages) contain an iTOW field which indicates
the GPS time at which the navigation epoch occurred. Messages with the same iTOW value can be
assumed to have come from the same navigation solution.
Note that iTOW values may not be valid (i.e. they may have been generated with insufficient
conversion data) and therefore it is not recommended to use the iTOW field for any other purpose.
The original designers of GPS chose to express time/date as an integer week number
(starting with the first full week in January 1980) and a time of week (often abbreviated
to TOW) expressed in seconds. Manipulating time/date in this form is far easier for digital
systems than the more conventional year/month/day, hour/minute/second representation.
Consequently, most GNSS receivers use this representation internally, only converting to a
more conventional form at external interfaces. The iTOW field is the most obvious externally
visible consequence of this internal representation.
If reliable absolute time information is required, it is recommended to use the UBX-NAV-PVT
navigation solution message which also contains additional fields that indicate the validity (and
accuracy in UBX-NAV-PVT) of the calculated times (see also the GNSS times section below for
further messages containing time information).
2.7.5 Time validity
Information about the validity of the time solution is given in the following form:
•
Time validity
: Information about time validity is provided in the
valid
flags (e.g.
validDate
and
validTime
flags in the UBX-NAV-PVT message). If these flags are set, the time is known
and considered valid for use. These flags are shown in table GNSS times in section GNSS times
above as well as in the UBX-NAV-PVT message.
•
Time validity confirmation
: Information about confirmed validity is provided in the
confirmedDate
and
confirmedTime
flags in the UBX-NAV-PVT message. If these flags are
set, the time validity can be confirmed by using an additional independent source, meaning
that the probability of the time to be correct is very high. Note that information about time
validity confirmation is only available if the
confirmedAvai
bit in the UBX-NAV-PVT message
is set.
UBX-22020019 - R01
2 Receiver functionality
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