Remote sensing: basics, sensor’s characteristics, image composition ... (2/3)

• We have to notice that the radiation sent, before arrival at the object on the surface, it is subjected to an interaction with the atmosphere (which can reflect a part) and even after the reflection on the object it enters into a second interaction with the atmosphere, and after that, the radiation is captured by sensors. To remedy this problem of interaction with the atmosphere, we exploit the characteristics of electromagnetic radiation, for example we choose wavelengths that allow the penetration of the atmosphere without any reflection on it (atmospheric window)

After scanning and recording the reflectance values, these values are sampled to have gray levels from 0 (black) to 255 (white). The basic unit is the pixel, each pixel has a radiometric value for each band (value for green, red, blue, infrared, ...). The final image is built in the form of a pixel matrix. In which every pixel has a radiometric value.

A satellite sensor has 4 fundamental characteristics: 

• Spatial resolution: this is the size covered by a pixel on the ground.
• Radiometric resolution: the ability of a sensor to detect small variations in the reflected energy.
• Spectral resolution: the wavelengths used by the sensor.
• Temporal resolution: the time required for a satellite to re-observe the same place from the same position in the sky.

After scanning and obtaining the image, it must undergo a georeferencing process, that is to say making the image superimposable on a topographic map. After this step comes a very important step, which is the processing of satellite images.