RT User Manual
Revision: 190902
39
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.
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.
The dual antenna system in the RT works by comparing the carrier-phase
measurements at the two antennas. This tells the system the relative distance between
the two antennas and which way they are pointing (the heading). For the heading to be
accurate the GNSS receivers must measure the relative position to about 3 mm. The
level of accuracy can only be achieved if there is little or no multipath.
In an ideal environment, with no surrounding buildings, trees, road signs or other
reflective surfaces, the only multipath received is from the vehicle’s roof. The antennas
supplied with the RT are designed to minimise multipath from the vehicle’s roof when
the roof is made of metal. For use on non-metallic roofs a different type of antenna is
required.
When stationary the heading from the RT will show some error; the size of the error
depends on the multipath in the environment. Table 19 lists the errors to be expected
when stationary with a 1 m base-line.
