



2.3.3 Notations and Conventions
Illumination and Observation Geometry Convention
The following illumination and observation geometry conventions are used in MERIS processing:
· A point on Earth observed by MERIS is taken as a reference.
· The Sun zenith angle is the angle between the local outward normal and the vector from the point towards the Sun.
· The view zenith angle is the angle between the local outward normal and the vector towards the MERIS sensor.
· The azimuth difference is the angle between the halfplane containing the local normal and the Sun, and the halfplane containing the local normal and MERIS. In the principal plane:
 there may be specular reflection of a point source into the MERIS sensor when the azimuth difference is 180° (and the zenith angles are equal),
 there may be backscatter from a point source into the MERIS sensor when the azimuth difference is 0° (and the zenith angles are equal). In general, we assume that an azimuth difference of N degrees is equivalent with respect to MERIS radiometry, to 360°  N, so that ranges from 0° to 180°.
The following figure illustrates these conventions:
Figure 2.5  Illumination and observation geometry angles.



 
Keywords: ESA European
Space Agency  Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,
atmospheric chemistry
