Agriculture Reference
In-Depth Information
factor exerts its influence on the noise situation only if soil is in the field of view of
the sensor. In case the sensor views only closed canopies, the factor soil too is
excluded. So with artificial illumination and closed canopies, all these noise factors
get irrelevant.
9.4.2.2
Viewing Directions with Natural Light
The reflectance sensor may have a vertical or an oblique
viewing direction
towards
the crop canopy. The above results from Sects.
9.4.2
and
9.4.2.1
refer to vertical
directions. Sensors that are operating on sprayer booms usually use this view (Fig.
may be appropriate. However, for an on-the-go control of the widely used centrifu-
gal fertilizer spreaders by proximal sensing, additional considerations deserve
attention.
It is essential to have the field of view out of any shades of the field machinery
and out of any tramlines, since this affects the reflectance. With vertical viewing,
this can be achieved by means of a transverse boom. Sprayers and pneumatic
spreaders have this boom anyway, but the dominating centrifugal spreaders do not.
And because of this, sensing with oblique views from the top of the tractor's roof
sidewards into the canopy is widely practiced. This has an additional effect: the sen-
sor is seeing less soil in case the canopy is not closed. Hence the noise that is caused
by soil is reduced (Table
9.4
).
However, an oblique viewing direction into the canopy means also that an addi-
tional solar radiation factor gets important. Whereas in case of vertical viewing only
the
zenith angle
of the solar radiation needs to be considered, with an oblique view-
ing direction it is necessary to take into account the effect of its
azimuth angle
as
well (Fig.
9.21
). Because the oblique direction - as opposed to the vertical direction
- inherently also has a horizontal component. And this horizontal component is
affected in the course of a day by the varying azimuth angle as well as by changing
directions of travel of the machinery in the field. An exception from this holds only
when the sun is precisely in the zenith (zenith angle = 0°). But this happens only in
the tropics and even there just at noon. So in most cases, the effects of solar azimuth
angles cannot be neglected.
Reusch (
2003
) has shown that this adverse effect of the azimuth angle on the
reflectance from natural light can be practically removed by increasing the number
of viewing spots and distributing these evenly around the tractor (Fig.
9.22
). He
investigated the
correction of the azimuth-effect
that can be obtained by multiple
viewing-directions and by averaging the results of these. As a criterion, the ratio of
the near-infrared to red reflectance was used.
In case only one direction supplies the signals, the reflectance ratio goes from 18
to 30 % and back to 18 % while the azimuth angle goes from zero to 360° (Fig.
9.23
).
This wide range for the azimuth angle is encountered while the machinery is operat-
ing in different directions within a field. So this range can affect the signals within
rather short time spans.
