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initial/assumed value of the altitude is 500 m. If the 2D position fix occurs after the
3D position fix (number of observable satellites drops from at least 4 to 3), the value
of the altitude will keep the last known value of the altitude from the previous 3D
position fix.
4.6.2 Dead Reckoning
As the module loses the tracks for all observable GPS signals because of, for example,
an external blockage, the navigation algorithm implements the Dead Reckoning
strategy. The strategy assumes the same velocity and direction as the last known
values of velocity and direction, i.e., the constant velocity and direction, during the
event. Under the assumption, the positions are predicted (extrapolated) but with
indication “NoFix” until the Dead Reckoning timeout is reached. The value of the
timeout is set by the UBX-CFG-NAV message.
4.7 Almanac Navigation
With Almanac Navigation enabled, based on valid almanac, the position can be
estimated without valid ephemeris data. This is a possible scenario that the position is
fixed while ephemeris data have not been downloaded completely. Therefore, the
TTFF times are much faster for Almanac Navigation than “normal navigation” (using
ephemeredes to estimate position). However, the deviation of position can be up to a
few kilometers. However, this event might be particularly useful when users or
carriers need position desperately, such as emergency and security systems, but
“ephemeris” position is not available.
The activation of Almanac Navigation is implemented by the UBX-CFG-NAV
message. By controlling the position accuracy, use parameters in the UBX-CFG-NAV
message, such as “PDOP Mask” and “Position Accuracy Mask”, to filter out the
“outsiders”.
4.8 DGPS – WAAS, EGNOS, & RTCM
The module utilizes the correction data from WAAS, EGNOS, or RTCM to obtain
better position accuracy. Use the UBX-CFG-SBAS message, the functions for
enabling WAAS or EGNOS tracking can be activated. For activation of RTCM, the
users need an extra antenna-micro controller set, which has ability to receive and
retrieve correction data from the signal transmitted from the near service station,
connected to one of the comm. ports of the module. The corresponding comm. port
needs correct setting, which is set by the “$PUBX,41,…” message. The module
supports RTCM Correction Type Messages 1, 2, 3, and 9. For more information about