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Dual antenna systems
It is often useful to have an understanding of how the xNAV650 uses the measurements from the dual
antenna system. This can lead to improvements in the results obtained. A warmup needs to be
completed for using the INS but if you have completed one the result of it can be saved and following
warmups only need to be around 3 minutes, see the last step for this. For example in step 1 where a
warm-up process is mentioned, only a short warmup is needed if a warmup has been saved with the
same configuration.
1. To use the measurements properly the xNAV needs to know the angle of the GNSS antennas
compared to the angle of the xNAV. This is very difficult to measure accurately without specialised
equipment, therefore the xNAV needs to measure this itself as part of the warm-up process.
2. The xNAV will lock on to satellites, but it cannot estimate heading so it cannot start. Either motion
or static initialisation can be used to initialise the xNAV.
3. When the vehicle drives forward and reaches the initialisation speed, the xNAV assumes that the
heading and track are similar and initialises heading to track angle.
4. If the xNAV is mounted in the vehicle with a large heading offset then the initial value of heading
will be incorrect. This can also happen if the xNAV is initialised in a turn. This can lead to problems
later.
5. When the combined accuracy of heading plus the orientation accuracy figure for the secondary
antenna is sufficiently accurate then the xNAV will solve the RTK Integer problem using the inertial
heading. There is no need for the xNAV to solve the RTK Integer problem by searching.
6. If the antenna angle is offset from the xNAV by too much then the RTK Integer solution that is
solved will be incorrect. For a 2 m antenna separation the xNAV orientation and the secondary
antenna orientation should be known to within 5°. For wider separations, the secondary antenna
orientation angle needs to be more accurate.
7. Once the RTK Integer solution is available, the xNAV can start to use the dual antenna solution to
improve heading. The level of correction that can be applied depends on how accurately the angle
of the secondary antenna is known compared to the inertial sensors.
8. The Kalman filter tries to estimate the angle between the inertial sensors and the secondary
antenna. The default value used in the configuration software (5°) is not accurate enough so that
the xNAV can improve the heading using this value. The accuracy in this angle will start to improve
as conditions become dynamic.
9. Driving a normal warm-up, with stops, starts and turns, helps the Kalman filter improve the accuracy
of the secondary antenna angle. The accuracy of this angle can be viewed in NAVdisplay. This is
less essential if a warm-up has been saved (see step 11).
10. In the unlikely event that the RTK Integer solution is incorrect at the start then the Kalman filter can
update the secondary antenna orientation incorrectly. If this happens then things start to go wrong.
The Kalman filter becomes more convinced that it is correct, so it resolves faster, but it always
solves incorrectly. Solving incorrectly makes the situation worse.
11. To avoid the Kalman filter from getting things wrong it is possible to drive a calibration run, then use
the Improve Configuration wizard within NAVconfig. This tells the Kalman filter it has already
estimated the angle of the secondary antenna in the past and it will be much less likely to get it
wrong or change it. This step should only be done if the xNAV is permanently mounted in a vehicle
and the antennas are bolted on. Any movement of either the xNAV or the antennas will upset the
algorithms.
12. One way to improve the dual-antenna (in all cases) is to measure the dual-antenna separation to 1
cm and to set it in the Accuracies part of NAVconfig to 1 cm. This will save a lot of incorrect static
initialisations. Can be done even to 5 mm.
Summary of Contents for xNAV650
Page 1: ...xNAV650 Hardware Manual 20 Years Navigation Experience in one small INS ...
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