Land Surface Processes

• As well as providing information on land cover type and areal extent, one should be able to obtain information from CHRIS on the directional properties of surface reflectance, and hence infer properties about surface structure, especially of vegetation canopies.

• The Bidirectional Reflectance Distribution Function (BRDF) describes the directional reflectance characteristics of the Earth's surface materials. The apparent reflectance of a surface varies according to the angles at which it is illuminated by the Sun and viewed at by the sensor on-board the platform. Because CHRIS has multiple view angle capabilities, it can sample the surface BRDF function.

• The BRDF is important because of two main reasons:

   

  • The albedo of the Earth surface materials, a critical term in the surface radiation budget and hence in climate modelling, can be accurately estimated from the BRDF. Indeed, ignoring the directional character of surface reflectance can lead to serious errors in albedo calculations using single view measuements.

  • The physical and chemical structure of the reflecting surface effect the BRDF in a complex way.Thus the 'shape' of the BRDF contains information on the three-dimensional geometric structure of the surface as well as its chemical composition.

• In principle, it is possible to derive surface biophysical parameters by inverting the BRDF models once the data has been corrected for atmospheric effects.

The above diagram shows a three colour plot of the cross-section of the BRDF of a meadow at red, green and blue wavelengths. The data was collected by a goniometer, and it has been smoothed and extrapolated using spherical harmonics, with an rms error of 0.1 percent. This technique allows the data to be coupled to an atmospheric scattering model.