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 half-plane containing the local normal and the Sun, and the half-plane 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.