RCB-F9T - Integration manual
receiver obtains some credible timing information from a satellite or an aiding message, it will jump
to an estimate of GNSS time.
3.7.2 Navigation epochs
Each navigation solution is triggered by the tick of the 1 kHz clock nearest to the desired navigation
solution time. This tick is referred to as a navigation epoch. If the navigation solution attempt is
successful, one of the results is an accurate measurement of time in the time-base of the chosen
GNSS system, called GNSS system time. The difference between the calculated GNSS system time
and receiver local time is called the clock bias (and the clock drift is the rate at which this bias is
changing).
In practice the receiver's local oscillator will not be as stable as the atomic clocks to which GNSS
systems are referenced and consequently clock bias will tend to accumulate. However, when
selecting the next navigation epoch, the receiver will always try to use the 1 kHz clock tick which it
estimates to be closest to the desired fix period as measured in GNSS system time. Consequently
the number of 1 kHz clock ticks between fixes will occasionally vary. This means that when producing
one fix per second, there will normally be 1000 clock ticks between fixes, but sometimes, to correct
drift away from GNSS system time, there will be 999 or 1001.
The GNSS system time calculated in the navigation solution is always converted to a time in both
the GPS and UTC time-bases for output.
Clearly when the receiver has chosen to use the GPS time-base for its GNSS system time, conversion
to GPS time requires no work at all, but conversion to UTC requires knowledge of the number of
leap seconds since GPS time started (and other minor correction terms). The relevant GPS-to-UTC
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, pre-defined 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.
3.7.3 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.
UBX-19003747 - R04
3 Receiver functionality
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Early production information
