Mie Scatter: A form of atmospheric scatter that occurs when radiation interacts with atmospheric particles, the diameter of which is approximately equal to the wavelength of the radiation.

Monte-Carlo method: Probabilistic method allowing tracing of the path of each photon knowing the inherent (scattering and absorption) characteristics of the medium.

Optical density: The logarithm to base 10 of the inverse of the transmittance.

Photon: A photon is a particle of light (or other electromagnetic radiation).

Radiative transfer equation: The equation which describes the radiation passage through a scattering and absorbing medium.

Raman scatter: Raman scatter arises when the incident light excites molecules in the sample which subsequently scatters the light. While most of this scattered light is at the same wavelength as the incident light, some is scattered at different wavelengths. This inelastically scattered light is called Raman scatter. It results from the molecule changing its molecular motions.

Rayleigh scattering: A form of atmospheric scattering that is caused when radiation interacts with particles, the diameter of which is much smaller than its wavelength. It therefore affects shorter wavelengths.

Scattering: The process in which a wave or beam of particles is diffused or deflected by collisions with particles of the medium in which it traverses.

Scattering phase function: The angular function which describes the directional scattering probability as a photon interacts with a scattering particle.

Snell's law: Gives the quantitative change of direction of a ray of light in passing from one medium to another. The product n sin z is the same on both sides of a plane interface between two media, where n is the local refractive index, and z is the local angle the ray makes with the normal to the interface.

QA4EO : The team's understanding of the Quality Assurance for Earth Observation (QA4EO) requirement is the need to assign to all data/information products a Quality Indicator (QI) that allows stakeholders to unequivocally evaluate the products' suitability for a particular application. It should be based on a quantitative assessment of its traceability to an agreed reference or measurement standard (ideally SI), but can be presented as numeric or a text descriptor, providing the quantitative linkage is defined.

Geo-location definitions : Specific definitions and conventions to geolocation and re-sampling are provided below:

• The Ellipsoid is defined as the WGS84 Earth reference ellipsoid.
• Satellite Actual Reference Frame : The reference frame is close to the Geodetic Pointing Frame on top of which actual attitude is accounted for. It corresponds to the satellite actual (measured) attitude frame. Regardless of attitude, Z axis is aligned with satellite nadir, X axis with velocity and Y completes the right hand system. Nominal attitude would align Z axis with satellite nadir (the local normal to the ellipsoid that passes through satellite's centre of mass) while X remains in the (Z, Velocity) plane. Actual attitude modifies this frame by three consecutive rotations. It is schematically represented on the figure below.

Satellite Actual Reference Frame and Theoretical OLCI Camera Alignment

• Earth Fixed Reference Frame: is the IERS Terrestrial Reference Frame details. Z axis points toward North Pole, X axis toward longitude 0 in the Equator plane.
• Topocentric Reference Frame: The topocentric is defined at any location at the surface of the Earth reference ellipsoid. Its Z axis corresponds to the outward normal to the ellipsoid; X and Y axes lie in the tangent plane and point toward North and East respectively.
• location coordinates (geodetic latitude, longitude) of a point on the ellipsoid, expressed in the Earth fixed coordinates system.
• AC pointing angle: the angle between the projection of the LOS on the (Y=0) plane in the satellite fixed coordinates system, and the -Z axis of that system. Bears the sign of y. Notation:
• AL pointing angle: the angle between the projection of the LOS on the (X=0) plane in the satellite fixed coordinates system, and the Z axis of that system. Bears the sign of x. Notation:

Across Track and Along Track Pointing Angles