Agriculture Reference
In-Depth Information
The American, Russian and European satellites orbit the earth about twice per
day in oblique angles to the equator. The vertical distances to the surface of the earth
are between 19,000 and 23,000 km. Hence the satellites move higher than those that
are on polar orbits, yet lower than those on geosynchroneous orbits (Sect.
3.4
). All
systems use about 24 satellites, however, the GPS satellites are on six different
orbits, the GLONASS- and GALILEO systems have only three orbits.
The signals are transmitted via
microwaves
, which operate within an atmo-
spheric window (Fig.
3.3
) and penetrate clouds. So obstructions in the atmosphere
do not exist.
The georeferencing is achieved by the
time interval
, within which radio signals
go from the satellites to the receiver. The latter is
e.g
. on a vehicle or on a farm
machine that moves in the fi eld. The satellites carry highly accurate atomic clocks.
The receivers on the ground synchronize themselves to these clocks. Hence in a
simplifi ed way, every receiver too is a highly accurate atomic clock.
Once the time-interval is known that a radio signal takes from the satellite to the
receiver on earth, the calculation of the respective distance between the satellite and
the target is possible. This only requires taking into account the speed of the elec-
tromagnetic radiation (Sect.
3.1
). And fi nally, when the distances between several
satellites and the receiver are known, the geometric position of the target can be
found out by trigonometric means. A prerequisite for this is the knowledge about
the position of the satellites. This knowledge is at hand. Thus in detail, the signals
can provide the target with four dimensions:
the time
the geographical longitude
the geographical latitude
the geographical altitude.
The last three dimensions together defi ne the respective
geometrical position
.
As a fi rst step in precision farming, geometrical positions can be used for getting
the borders and exact areas of all fi elds. Subsequently, the position can be used as
the site-specifi c reference for all farming operations. This reference allows to link
soil- and crop properties in an intelligent way. In this respect, the position is a
benchmark in precision farming. The site-specifi c altitude can be used as a source
for mapping the contour lines of fi elds. Topographic maps that contain this infor-
mation can be obtained as a by-product of other site-specifi c farming operations
(Abd Aziz et al.
2009
).
Important criteria in georeferencing are the availability of the satellite signals
and the precision of positioning. A general prerequisite of
availability
is that
the radio waves from four satellites simultaneously can get to the receiver.
Clouds are no barrier since microwaves are used, but trees and buildings can
refl ect the signals.
Whether this prevents georeferencing, can depend on the number of satellites
that are operating (Fig.
3.9
). This number has been steadily increasing, not least
because the global positioning system of the USA has been and still is supple-
mented by similar systems from other parts of the world. Different global
