Remote sensing
Data acquisition
Image processing
  Colour composites
  Geometric corrections
  Radiometric corrections
  Contrast enhancement
  Filtering
  Classification
  Visual interpretation
  Post-classification
  Indices
  Principal Component Analysis
  Combination of images
  Geospatial maps
  Combination of images and other data: DEM and DTM
Radar
GIS
 
Radiometric adjustments
 

An ideal Earth observation system should be equipped with a perfect spectro-radiometer that can measure accurately and uniformly the amount of energy that is reflected by objects located on the Earth’s surface.

Unfortunately, the sunlight that illuminates objects is perturbed by its passage through the atmosphere and does not hit all objects at the same angle.

What is more, the light that is reflected by the objects must also cross the atmosphere before being analysed by the satellite’s sensors, and this journey also perturbs the signal. These perturbations are due to the presence of gases and dust that can absorb and/or reflect specific wavelengths, thereby changing the radiation’s spectral properties. What is more, the electronic processing of the rays that reach the sensors is also accompanied by some perturbations. Consequently, it is actually rather difficult to get accurate radiometric values from the data recorded by Earth-observing satellites.

Now, it is sometimes very useful to be able to calibrate these data precisely, for example to compare the data recorded by different satellites or recorded by the same satellite at different times, and several solutions do exist to try to overcome these flaws. Some of them are based on complex mathematical models that describe the main interactions involved. These models are effective.


However, to apply them the values of certain parameters (i.e. the atmospheric composition) when and where the pictures are taken must be known, and this is seldom possible.
Other radiometric correction methods are based on the observation of reference targets whose radiometry is known. The surfaces of bodies of water, glacial ice caps, and expanses of desert sand are often used, but here too you can understand that actually making the corrections often is not that easy. In fact, the overwhelming majority of remote sensing research uses radiometrically uncorrected data.