Global Positioning System (GPS)

GPS is a universal system of localization. It enables to position the machinery agricultural and the information collected on the pieces and to proceed to the follow-up in real time of the operations.

One can make use of it in agriculture :

• to make the charts of the pieces by locating contours,
• to locate the points of analysis,
• to know in real time the position of an agricultural machine in a piece.

The GPS uses a network of 24 satellites (launched for the American army) around the globe at an altitude of 21 000 km. A receiver on the ground calculates its ground position starting from information which it receives from at least four satellites of the network and calculate a position in coordinates X, Y and Z (longitude, latitude and altitude). This receiver can be placed on a "mobile": tractor, reaping-machine, applicator, cross-country vehicle or a person.

Taking into account the natural inaccuracies of the system, related on the weather phenomena and the systematic "jamming" carried out by the American Ministry of Defense, it is necessary to lay out, in addition to GPS signal, a correction signal to obtain a precision compatible with the precision agriculture. We speak about dGPS or differential GPS . Without this signal of correction the precision is about 30 to 100 m, whereas with this correction the precision of positioning is 1 to 3 m according to the material used. In precision agriculture an accuracy of 1 to 5 m is considered as acceptable.

Origins of the GPS In the beginning, the GPS was intended for the American military applications, to determine the position of the troops, of the air and marine forces, like specifying the shotfiring cable of the missiles.

A system GPS is used to chart the data on the output, the characteristics of the samples of grounds or any other agricultural information (infestation, fields area, etc). Information on the territory is deferred on a chart and in this manner it can be visualized and/or integrated in a geographical information system (SIG). The GPS is a significant tool for the application of the recommendations or the regulations proposed by the advisers to the good places.