EVK-F9P-01 - User guide
UBX-22038408 - R03
Troubleshooting
Page 25 of 28
C1- Public
The position is off by hundreds of meters
Position drift may also occur when almanac navigation is enabled. The satellite orbit information
retrieved from an almanac is much less accurate than the information retrieved from the ephemeris.
With an almanac only solution, the position will only have an accuracy of a few kilometers, but it may
start up faster or still navigate in areas with obscured visibility when the ephemeris from one or
several satellites have not yet been received. The almanac information is NOT used for calculating a
position, if valid ephemeris information is present, regardless of the setting of this flag.
In NMEA protocol, position solutions with high deviation (e.g., due to enabling almanac navigation)
can be filtered with the
Position Accuracy Mask. UBX protocol does not directly support this since it
provides a position accuracy estimation, which allows the user to filter the position according to his
requirements. However, the ‘Position within Limits’ flag of the UBX-NAV-STATUS message indicates
whether the configured thresholds (i.e., P Accuracy Mask and PDOP) are exceeded.
TTFF times at start-up are much longer than specified
At startup (after the first position fix), the GNSS receiver performs an RTC calibration to have an
accurate internal time source. A calibrated RTC is required to achieve minimal startup time.
Before shutting down the receiver externally, check the status in MON-HW in field ‘Real Time Clock
Status’. Do not shut down the receiver if the RTC is not calibrated.
The EVK-F9P does not meet the TTFF specification
Make sure the antenna has a good sky view. An obstructed view leads to prolonged startup times. In
a well-designed system, the average of the C/N0 ratio of high elevation satellites should be in the
range of 40 dBHz to about 50 dBHz. With a standard off-the-shelf active antenna, 47 dBHz should
easily be achieved. Low C/N0 values lead to a prolonged startup time.
The EVK-F9P does not preserve the configuration in case of reset
u-blox F9 GNSS technology uses a slightly different concept than most other GNSS receivers do.
Settings are initially stored to volatile memory. In order to save them permanently, sending a second
command is required. This allows testing the new settings and reverting to the old settings by
resetting the receiver if the new settings are not good. This provides safety, as it is no longer possible
to accidentally program a bad configuration (e.g., disabling the main communication port).
The EVK-F9P does not work properly when connected with a GNSS simulator
When using an EVK together with a GNSS simulator, pay attention to proper handling of the EVK. A
GNSS receiver is designed for real-life use, i.e., time is always moving forward. By using a GNSS
simulator, the user can change scenarios, which enables jumping backwards in time. This can have
serious side effects on the performance of GNSS receivers.
The solution is to configure the GPS week rollover to 1200, which corresponds to Jan 2003. Then,
issue the cold start command before every simulator test to avoid receiver confusion due to the time
jumps
.
Power save mode and USB
For communication in power save mode, use the RS232.
