ZED-F9K - Integration manual
If the receiver only has three satellites for calculating a position, the navigation algorithm uses a
constant altitude to compensate for the missing fourth satellite. When a satellite is lost after a
successful 3D fix (min four satellites available), the altitude is kept constant at the last known value.
This is called a 2D fix.
u-blox receivers do not calculate any navigation solution with less than three satellites.
3.1.6.3 Navigation output filters
The result of a navigation solution is initially classified by the fix type (as detailed in the
fixType
field of UBX-NAV-PVT message). This distinguishes between failures to obtain a fix at all ("No Fix")
and cases where a fix has been achieved, which are further subdivided into specific types of fixes
(e.g. 2D, 3D, dead reckoning).
Where a fix has been achieved, a check is made to determine whether the fix should be classified as
valid or not. A fix is only valid if it passes the navigation output filters as defined in CFG-NAVSPG-
OUTFIL. In particular, both PDOP and accuracy values must be below the respective limits.
Important: Users are recommended to check the
gnssFixOK
flag in the UBX-NAV-PVT or
the NMEA valid flag. Fixes not marked valid should not be used.
UBX-NAV-STATUS message also reports whether a fix is valid in the
gpsFixOK
flag. These
messages have only been retained for backwards compatibility and users are recommended to use
the UBX-NAV-PVT message.
3.1.6.4 Weak signal compensation
In normal operating conditions, low signal strength (i.e. signal attenuation) indicates likely
contamination by multipath. The receiver trusts such signals less in order to preserve the quality of
the position solution in poor signal environments. This feature can result in degraded performance
in situations where the signals are attenuated for another reason, for example due to antenna
placement. In this case, the weak signal compensation feature can be used to restore normal
performance. There are three possible modes:
• Disabled: no weak signal compensation is performed
• Automatic: the receiver automatically estimates and compensates for the weak signal
• Configured: the receiver compensates for the weak signal based on a configured value
These modes can be selected using CFG-NAVSPG-SIGATTCOMP. In the case of the "configured"
mode, the user should input the maximum C/N0 observed in a clear-sky environment, excluding
any outliers or unusually high values. The configured value can have a large impact on the receiver
performance, so it should be chosen carefully.
3.2 Automotive dead reckoning (ADR)
3.2.1 Introduction
u-blox solutions for automotive dead reckoning (ADR) allow high-accuracy positioning in places
with poor or no GNSS coverage. ADR is based on sensor fusion dead reckoning (SFDR) technology,
which combines GNSS measurements with those from external sensors. The ZED-F9K computes
a solution type called GAWT by combining GNSS measurements with the outputs of a 3-axis
accelerometer, a 3-axis gyroscope and wheel tick (sometimes called a speed tick) or speed
measurements. The utilization of these sensors ensures a quick recovery of a high precision
navigation solution after short GNSS signal outage (going under a bridge, signaling panels, and so
on).
UBX-20046189 - R01
3 Receiver functionality
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C1-Public
Early production information
