1. INTRODUCTION
Precision farming is a method of crop management by which
areas of land within a field may be managed with different
levels of input depending upon the yield potential of the crop in
that particular area of land. The benefits of so doing are two
fold:
− the cost of producing the crop in that area can be
reduced;
− the risk of environmental pollution from
agrochemicals applied at levels greater than those
required by the crop can be reduced.
Precision farming is an integrated agricultural management
system incorporating several technologies. The technological
tools often include the global positioning system GPS,
geographical information system GIS, remote sensing, yield
monitor and variable rate technology.
The paper talks about the use of GPS to support agricultural
vehicle guidance. Equipment for this purpose consists on a
yield monitor installed: the system supports human guide by
means of a display mapping with a GIS the exact direction
produced by GPS receiver put on vehicle top: the driver
follows it to cover in an optimal path the full field.
GPS receivers for this applications require, not only an high
accuracy to ensure the reduction of input products, but even an
easy and immediate way of use for farmers; without forgetting
low costs.
Obviously the technology to achieve high precision still exists
but it is too expensive and difficult to use for not skilled people.
Survey modality usually adopted in agricultural applications is
real time kinematic positioning, DGPS RTK, which enable to
have a good accuracy by means of corrections received. In this
experimentation the aim is to obtain a sub-metric accuracy
using low cost receivers, which can provide only point
positioning. These receivers have been developed for maritime
navigation purposes; our aim is their optimization in order to
apply them for land navigation in particular for farming
activities. Some tests using these receivers were carried out, but
results were not satisfying and probably the reason has to be
assigned to the implementation of a Kalman filtering inside the
receiver software. This is the starting point for a new project, at
the moment still in progress, which aim to develop a new
algorithm based on Kalman filter. Its purpose is to improve low
cost receiver outputs in order to optimize trajectories and to
reach needed accuracy in vehicle positioning during agricultural
activities.
2. TRIAL AND ERROR
2.1 Instruments and tests
Experimentation has been carried out using Leica Geosystems
instruments; in particular the low cost receiver discussed in the
paper is the TruRover Leica. Its mainly features are: it is an
antenna-receiver integrated instrument, it has a 5 Hz tracking
time, the report is in the NMEA string format, it cannot neither
store positions nor show them in real time, it requires a
computer to view NMEA data stream. TruRover performances
were compared with geodetic receiver one, which are
considerably better, so they are the perfect comparison
condition to estimate Trurover positioning quality.
Geodetic receiver used is the GX1230 Leica, able to receive
double frequency (both code and phase).
Both static and kinematic tests were performed, simulating the
typical behaviour of an agricultural vehicle (straight and
parallel trajectories with reduced velocity, such as 20÷40 km/h)
and using, at the same time, the two different kinds of GPS
receivers described above. At the top of the vehicle, both
TruRover and geodetic antenna, connected to the receiver, were
placed at a distance of 50 cm. Three static stops with 20
minutes time length were performed, spaced with two steps in
motion. Geodetic receiver were set with a 1 second tracking
time and a cut off angle of 10 degree. Tests length were about
two hours.
Another geodetic receiver were placed for a single point
positioning and used as the Master station for the following data
processing