Remote Sensing for Agriculture

GNSS Reflectrometry (GNSS-R) has been already demonstrated in low earth orbit to measure sea surface heights. There is increasing interest in GNSS-R also for land surface applications.Potential land surface applications are soil moisture measurement, vegetation growth, image mapping. Since the GNSS transmitters are active continuously, suitable receivers could measure with much improved temporal sampling compared with existing low Earth orbit Earth observation satellites which only allow repeat viewing every few days at best. GNSS-R has several other advantages, but also brings the challenge of making good quality measurements despite the very low signal strength.

Remotely sensed images can be used to identify vegetation damages such as insect damage, weed damage, nutrient deficiency…etc., and plant populations via electromagnetic spectrum. Data from remote sensing information can be used as base maps in variable applications of fertilizers and pesticides. It allows farmers to treat only affected areas of a field. Problems within a field may be identified remotely before they can be visually identified on a wider scale. Ranchers use remote sensing to identify prime grazing areas, overgrazed areas or areas of weed infestations. Lending institutions use remote sensing data to evaluate the relative values of land by comparing archived images with those of surrounding fields.

The wavelengths used in most agricultural remote sensing applications cover only a small region of the electromagnetic spectrum. The green colour associated with plant vigor has a wavelength that centers near 5 micrometers on the spectrum band. The visible and infrared regions are widely used in agricultural remote sensing.

When electromagnetic energy from the sun strikes plants, depending upon the wavelength of the energy and characteristics of individual plants, the energy will be either reflected, absorbed, or transmitted. Reflected energy which bounces off leaves and is identified by human eyes as the green colour of plants as the chlorophyll in the leaves absorbs much of the energy in the visible wavelengths and the green colour is reflected. Sunlight that is not reflected or absorbed is transmitted through the leaves to the ground. Interactions between reflected, absorbed, and transmitted energy can be detected by remote sensing. The differences in leaf colours, textures, shapes or even how the leaves are attached to plants, determine how much energy will be reflected, absorbed or transmitted. The relationship between reflected, absorbed and transmitted energy is used to determine spectral signatures of individual plants. Spectral signatures are unique to plant species. Remote sensing is used to identify stressed areas in fields by first establishing the spectral signatures of healthy plants. The spectral signatures of stressed plants appear altered from those of healthy plants.

How it works?

(Source: NDSU, 2004)

The sun (A) emits electromagnetic energy (B) to plants (C). A portion of the electromagnetic energy is transmitted through the leaves. The sensor on the satellite detects the reflected energy (D). The data is then transmitted to the ground station (E). The data is analysed (F) and displayed on field maps (G).

Compiled by,

R. Ghadawala