Teledetection sensors

Teledetection in agriculture refers to a whole of knowledge and techniques used for the observation, the analysis and the interpretation of agricultural surfaces starting from measurements and images obtained using satellite, airborne and embarked platforms. The teledetection is characterized by measurement techniques without contact with the object being studied by exploiting various wavelengths from the electromagnetic spectrum.
The use of teledetection sensors is a means of acquiring information into the plot without human intervention and at lower cost.
The information (qualitative or quantitative), provided by the sensors, can concern :

• the plant,
• ground, bad grasses,
• diseases,
• climate.

They are founded on various principles of measurement: chemical, optical, electric, electromagnetic, mechanical.

The sensor of output for the realization of chart of output is most widespread by far today for the precision agriculture. The satellite and airborne images are used on the other hand more and more within the framework of the precision agriculture like source of information on the plot (ground and plants) and to make estimate of output.
Sensors with high space resolution are besides under development and it will be possible to locate bad grasses, of the nitrogen deficiencies and the stresses by analysis of image using a embarked sensor on the tractor.

The teledetection image, as well as the other cartographic data (chart of output, chart of the grounds...), provide information on the plot (ground and plants). The basic information provided by the images does not correspond to an agronomic characteristic, but simply to a measurement of the reflexion of the sunlight by the ground and/or the plant.

The interpretation of the images and the photographs is based on the spectral signature, its distribution, the color, the shape, the texture and the geographical position of the elements. It is however significant to know the context and to have a preliminary knowledge of the territory to optimize the interpretation and the analysis of the images. The method of visual interpretation gives qualitative information while the treatments and the analyses by computer provide quantitative information.

The data of teledetection are mainly used for the identification and classification of the cultures, followed growth of the plantations, the estimate of the outputs, the evaluation of the space variability of fields, the detection of the disturbances (stress, insects and diseases), evaluation of the state of the grounds. The teledetection data are also used in the realization of chart of recommendations for the plantation in progress or the determination of homogeneous zones within the plot. Basic information can be analyzed and transformed using agronomic models into biological indices (stage of a culture, biomass, nutrition index...) being able to be used directly for the control of the plantations.
From charts of the recommendations and information provided by system GPS, of the aircraft equipped with automatic check carry out a modulation of the adjustments uninterrupted without the intervention of the pilot. According to the type of information used for the modulation, two principles of control exist:

1. systems acting starting from information provided by a sensor in real time. (example: bad grasses on naked ground sensor, chlorophyl rate sensor (Norsk-Hydro prototype NSensor), multi-bands airborne sensor (CASI), infra-red camera, satellite sensor (RADARSAT, Landsat, SPOT, Ikonos, etc.)
2. systems acting starting from information provided by "a chart of computerized recommendation"

It is currently the practice most used for the modulated control of the plots. The spreading of manure and the pulverization of herbicids products are carried out by service providers with specialized applicatons. Several research projects currently in hand in Canada and in Europe will allow the development of satellite, airborne or embarked sensors being able to provide information on the plantation conditions with a spectral resolution (wavelength used), a temporal resolution (the frequency of return on the same plot) and a space resolution (size of the pixel) better adapted has our needs.