Agriculture Reference
In-Depth Information
Fig. 3.9
The position of the receiver on the tractor is the same in both drawings. In the
left
draw-
ing, the signals from only three satellites get to the receiver, which in most cases is not suffi cient
for georeferencing. In the
right
drawing - because of more satellites - georeferencing is possible
(From graphs by Poloni
2009
)
Table 3.3
Availability of georeferencing signals and maximal down time of
receiver with varying numbers of satellites in orbit, which operate with a mask
angle of 5°
Number of satellites
in orbit
Maximal total down time
within 30 days (min)
Availability % of time
24
99.98
8
23
99.97
13
22
99.90
42
21
99.10
386
Compiled from Mansfeld (
2010
), altered
navigation satellite systems provide signals that are compatible. Accordingly, one
receiver can simultaneously use signals that come from separate systems.
The drawings in Fig.
3.9
show schematically the extreme situation when a farm
machine moves along high obstacles of navigation signals. But also within a normal
fi eld without patches of trees it cannot be assumed that all the navigation signals
have unimpeded access to the receiver. There always is a so called “
mask angle
”
between the horizontal line in the fi eld and oblique radiation that is oriented to the
receiver. Oblique radiation that is directed to the receiver at an angle to the horizon-
tal line that is smaller than the mask angle is ineffective. Apart from the situation
close to forests or hedges, mask angles between 5° and 10° must be taken into
account (Mansfeld
2010
).
The data in Table
3.3
hold for a mask angle of 5°, hence for favorable conditions.
The deteriorating effect of a lower number of satellites on the availability in % of the
time appears to be rather small at fi rst sight. However, the availability in % of time
does not indicate the conditions for georeferencing in a way that is easy to under-
stand. The maximal total down time - that can be calculated from the availability in
% of time - provides for a better insight into the situation. It is the maximal sum of
