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Oxford Technical Solutions
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 building, 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 28 lists the errors to be expected
when stationary with a 1 m base-line.
Table 28. Typical heading error for when stationary in different environments
Environment
Typical error (3σ
Complete open-sky
0.6°
(0.2° 1σ)
Near trees, buildings 1°
Next to trees, buildings 2°
Typical figures using a 1 m base-line. For accuracy specification of 0.1° RMS a 2 m separation is required.
Using a 2 m base-line can halve the figures shown here.
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