To ensure precise application of inputs, accurate machine positioning is crucial. From GPS and dGPS to RTK, let's explore the various guidance systems and signals available.

Geolocation technology has revolutionized modern farming practices, empowering farmers to enhance their productivity and resource efficiency. Equipped with GPS guidance systems, tractors can navigate fields with pinpoint accuracy, eliminating overlaps and optimizing passes. This precision translates into significant savings on fertilizers, seeds, and fuel, fostering sustainable farming practices. Furthermore, GPS guidance reduces wear and tear on machinery, extending the lifespan of equipment and streamlining operational costs. By minimizing overlaps and optimizing field operations, farmers can save valuable time and maximize the utilization of their machinery assets. 

GPS is also extensively employed in precision farming, particularly in variable rate application. To precisely apply the recommended dosage to a specific area of the field, the tractor's location must be determined with high accuracy. However, GPS alone offers an accuracy range of only 5 to 15 meters. To enhance this precision, GPS can be coupled with additional signals such as dGPS (differential global positioning system) or RTK (Real Time Kinematic).

Differential GPS (dGPS): relative accuracy

By correcting GPS signal, dGPS reduces accuracy to around 5-50 cm. Farmers tend to use it for guiding wide machinery, for example for cross passes in cover crops. This avoids overlaps and gaps.

dGPS use a network of fixed reference stations with a maximum range of 300 kilometers. These stations transmit corrections to the GPS signal via geostationary satellites to a receiver installed on the tractor. Corrections are calculated based on the precise station location.  Accuracy of dGPS is influenced by several factors, including tractor's antenna quality and satellite drift. Various levels of accuracy are available for correcting GPS signal. For instance, EGNOS (European Geostationary Navigation Overlay Service), operated by the European Union, provides an accuracy of 15-30 centimeters and is cost-free.

RTK, centimetric accuracy

For activities demanding higher precision, such as mechanical weeding or modulation, RTK GPS is the preferred choice. It consistently delivers accuracy within a remarkable 2 to 3 centimeters. This remarkable accuracy is achieved through a real-time differential correction technique applied to GPS data.

In practical terms, RTK compares data from a base station, also known as an RTK beacon, with that from a mobile receiver in the field (e.g., GPS on a tractor) in real time. The base station, with its precisely known location, measures errors in GPS signal. These errors, caused by factors such as atmospheric disturbances and signal reflections from surrounding objects, are then transmitted to the mobile receiver, enabling it to correct its position. It is important to note that the mobile receiver must not be located more than 10 kilometers from the station to ensure optimal accuracy.

An RTK station represents an investment of over €10,000. Its range needs to be considered, as a single station may not be enough where fields are far away. To reduce costs, it is possible to join forces, for example via a cooperative, or to use an RTK network: Orphéon (Geodata Diffusion), Teria network, etc.

Agricultural geomatics: understanding data

Global Positioning Systems (GNSS), including GPS, revolutionize agricultural data acquisition by providing precise positioning and mapping capabilities. This data is further refined and organized by geographic information systems (GIS), enabling farmers and their technicians to visualize and analyze agricultural information in a spatial context. This convergence of GNSS, GIS, and agricultural practices has fostered development of agricultural geomatics, a scientific discipline dedicated to understanding and optimizing agricultural operations through spatial data analysis.

Thanks to this combination of technologies, farmers can adopt more precise and efficient farming practices, reducing costs and minimizing environmental impact.

* Owned by the United States, GPS is one of the 4 satellite constellations that make up the GNSS (Global Navigation Satellite System) satellite navigation system. The others are European (Galileo), Russian (Glonass) and Chinese (Beidou).  

For further :

• Chambre d’agriculture d’Isère : « systèmes de guidage en agriculture de précision »
• Terre-net : « Gps, Gprs, Dgps, Rtk… Comment s’y retrouver ? »