Agriculture Reference
In-Depth Information
resolution is provided by
carrier phase signals
instead of coded signals. Details to
this are dealt by Mansfeld (
2010
) and by van Diggelen (
2009
).
An amazing good georeferencing can be realized by
real-time kinematic dif-
ferential GPS
, abbreviated
RTK-GPS
. This system uses all the possibilities for
improvement that are mentioned above and is - in the original way - equipped with
an own base receiver for corrections. This base receiver is located rather close to the
moving receiver. For farm machines, it often is positioned on the headlands. With
optimal conditions, the positioning error can be as low as 1-3 cm. The inaccuracies
increase with the distance between the two receivers. Per 1 km distance, the increase
in error is about 1 mm (Heraud and Lange
2009
). So even with a distance of 4 km,
the inaccuracies can be below 4 cm.
RTK-GPS technology allows a farmer to return to the exact location again later
during the growing season or even in subsequent years. Hence its precision in geo-
referencing can be relied on not only from pass to pass during a current farm opera-
tion, but from season to season or year to year as well. This feature is important
when
repeatability
in the guidance of farm machinery via positioning systems with
a low error is needed. Prime examples for this are the guidance for no-till sowing
into inter-row strips of the crop from the previous year (Sect.
8.4.1
)
or strip-till sow-
ing when the cultivating of the strips occurs in autumn and the sowing precisely into
the center of the narrow strips in spring. There are additional examples when deal-
ing with row crops. Some farmers pour concrete pads at the headlands to ensure that
the base station is returned to the exact spot for precise guiding.
The maximum distance between the base receiver station and the moving receiver
with real-time kinematic differential GPS - as described above - is between 10 and
20 km. This restriction in distance with an own reference station can be avoided by
using an array or
network of RTK-GPS
base receiver stations within a wide area.
The distance between adjacent
network base receiver stations
can be up to 70 km
(Heraud and Lange
2009
). These network base stations provide for correction data
that are collectively processed. The result is that despite longer distances to the
moving receiver within this network, a similar low error or inaccuracy as shown in
Table
3.4
, bottom is possible (Edwards et al.
2008
). The transmission of the correc-
tion data from the network to the user typically is via mobile phone.
Not all precision farming operations require the accuracy or low error range of
RTK-GPS. In many cases, the error associated with differential GPS operating on
dual frequencies and carrier phase resolution can be tolerated. This system presently
is used widely, since the expenditures are much lower than for RTK-GPS.
References
Abd Aziz S, Steward BL, Tang L, Karkee M (2009) Utilizing repeated GPS surveys from fi eld
operations for development of agricultural fi eld DEMs. Trans ASABE 52(4):1057-1067
Chuvieco E, Huete A (2010) Fundamentals of satellite remote sensing. CRC Press, Boca Raton
CRISP, Centre of Remote Sensing, Imaging & Processing (2010) Principles of remote sensing.
Research tutorial 01.
http://www.crisp.nus.edu.sg/~research/tutorial/intro.htm
