Oxford Technical Solutions Ltd Page 45 of 110
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.
11. 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.
12. 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.
13. 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.
7. 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.
14. 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 RT is permanently mounted in a vehicle and the antennas are bolted on.
Any movement of either the RT or the antennas will upset the algorithms.
Multipath effects on dual antenna systems
Dual antenna systems are very susceptible to the errors caused by multipath. This can be from buildings, trees,
roof-bars, etc. Multipath is where the signal from the satellite has a direct path and one or more reflected
paths. Because the reflected paths are not the same length as the direct path, the GNSS receiver cannot track
the satellite signal as accurately.
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