CONCEPT

The wealth of spectral, directional and polarized POLDER measurements opens up new opportunities for determining cloud phase and altitude, and for discriminating aerosols and biosphere parameters.

Directionality

Photographers on Earth know to exploit angular viewing with respect to the sun for improves image quality. Similarly, image geometry with respect to the sun largely determines the reflectance measured by a spaceborne radiometer.

The positions of the imager and the sun relatively to the target are conventionally represented by three angles known as the zenith viewing angle (), the solar zenith angle () and the relative azimuth angle (). The angle between the incident and scattered directions is called the scattering angle ().

Combining the wide field of view with the shift in satellite track from one day to the next makes it possible for a target on the ground to be seen from several viewing angles.

POLDER's image geometry
Two images (A,B) acquired 3-minute apart during the first pass over Europ on September 16th 1996 and their comparative geometry.

Zenith solar angle (°)
	Zenith viewing angle (°)		Azimuth viewing angle (°)

 

At a given location, such as Copenhagen (indicated by a cross), the solar angle remains nearly constant whereas the viewing angle and the azimuth change rapidly. A further 8 images were taken between these two acquisitions, completing the angular sampling represented on the polar diagram (, ).

The satellite over passes the same point at the ground every 41 days (cycle) for POLDER 1 and every 4 days for POLDER 2. Consequently, the satellite passes over the same site every day with a new angular range, leading to a very dense sampling of the bi-directional reflectance of the site.

POLDER thus provides comprehensive sampling of the Bi-directional Reflectance Distribution Function of targets, particularly in the principal plane (=0°) and the perpendicular plane (=90°).

 

Highly significant effects are revealed by POLDER and may be processed in a new way when interpreting satellite-derived measurements.