Dual antenna systems
It is often useful to have an understanding of how the RT uses the measurements from the dual
antenna system. This can lead to improvements in the results obtained.
1.
To use the measurements properly the RT needs to know the angle of the GNSS antennas
compared to the angle of the RT. This is very difficult to measure accurately without
specialised equipment, therefore the RT needs to measure this itself as part of the warm-
up process.
2.
The RT 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 RT.
3.
When the vehicle drives forward and reaches the initialisation speed, the RT assumes that
the heading and track are similar and initialises heading to track angle.
If the RT 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 RT is initialised in a turn. This can
lead to problems later.
4.
When the combined accuracy of heading plus the orientation accuracy figure for the
secondary antenna is sufficiently accurate then the RT will solve the RTK Integer problem
using the inertial heading. There is no need for the RT to solve the RTK Integer problem
by searching.
If the antenna angle is offset from the RT by a lot then the RTK Integer solution that is
solved will be incorrect. For a 2 m antenna separation the RT 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.
5.
Once the RTK Integer solution is available, the RT 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.
6.
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 RT can improve the heading using this value. If you want the vehicle heading
to 0.1°, but the angle of the two GNSS antennas is only known to 5°, then the measurements
from the antenna are not going to be able to improve the heading of the vehicle.
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 is available in the
verified in NAVdisplay.
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