SAM-M10Q - Integration manual
delays so the accuracy of the supplied time is poor. Accuracy of the supplied time can be improved
greatly if the host system has a very good sense of the current time and can deliver an exactly timed
pulse to the EXTINT pin. This pulse informs the receiver when the supplied time assistance data is
to be applied.
UTC time leap seconds and GPS-to-UTC conversion parameters are transmitted periodically by
GPS satellites, but that happens only every 12.5 minutes. The receiver can normally calculate the
correct leap seconds value from other GNSS systems immediately, but in some situations that is
not possible. If the leap seconds information or the difference of time between GPS and UTC system
is important for the host application, the information can be supplied to the receiver via the UBX-
MGA-GPS-UTC aiding message.
2.10.3 Frequency assistance
It is possible to supply hardware frequency assistance by connecting a periodic rectangular signal
with a frequency of up to 500 kHz to the EXTINT pin. The frequency can have an arbitrary duty cycle
but the low/high phase duration must not be shorter than 50 ns. The applied frequency value must
be submitted to the receiver using the UBX-MGA-INI-FREQ message.
Frequency assistance can improve the cold start speed in crystal-based designs. For TCXO-based
designs the frequency assistance has only minimal impact as the receiver is quick to acquire
accurate frequency from satellite transmissions. A stable external reference frequency can be used
to speed up receiver testing in production test setup. The host system may also be able to provide
the reference frequency to improve the cold start speed.
2.10.4 Clock drift assistance
Estimate of the clock drift of the local oscillator can also be fetched from the receiver using the UBX-
NAV-CLOCK message. This estimate can then be sent back to the receiver using the UBX-MGA-INI-
CLKD message.
2.11 Protection level
2.11.1 Introduction
Critical applications need to know how much trust they can place in their GNSS receiver's output at
any given moment. Computed by the GNSS receiver in real time, the protection level (PL) quantifies
the reliability of the position information to allow systems to change their mode of operation and
improve the efficiency and quality of the tasks being performed.
The GNSS receiver's protection level describes the maximum likely position error to a specified
degree of confidence. For example, if a GNSS receiver determines its position with a 95% protection
level of one meter, there is only a 5% chance that the reported position is more than one meter
away from its true position. Like the accuracy estimate of the GNSS receiver, the protection level
constantly fluctuates, influenced by all the common error sources that affect GNSS solutions. Unlike
the accuracy estimate, the confidence level of the protection level is much higher and is validated
against specific operating scenarios to ensure that the output bounds the true error.
UBX-22020019 - R01
2 Receiver functionality
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