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## 4.2 Glossary

### 4.2.1 Geometry Glossary

Table 4.2 - Geometry Glossary.

 Horizontal direction measured clockwise from the meridian plane. The vertical distance of a point or object above or below a reference surface or datum (usually mean sea level). Also referred to as altitude or height. Science concerned with the measurement and mathematical description of the size and shape of the earth and its gravitational fields. Geodesy also includes the large-scale, extended surveys for determining positions and elevations of points, in which the size and shape of the Earth must be taken into account. Figure of the Earth visualised as a mean sea level surface extended continuously through the continents. It is a theoretically continuous surface that is perpendicular at every point to the direction of gravity (the plumb line). Angular distance, in degrees, minutes, and seconds of a point north or south of the Equator. Angular distance, in degrees, minutes, and seconds, of a point east or west of the Greenwich meridian. Point on the ground directly in line with the remote sensing system and the centre of the Earth. pointing direction Angle between a look direction lying in the (YS=0) plane in the satellite fixed coordinates system, and the -ZS axis of that system. Positive around YS. satellite fixed coordinate system ZS points in the direction of the Earth outward local normal. XS is perpendicular to the satellite orbit plane. YS completes the right-handed system and is the direction of the opposite of the satellite velocity. This coordinate system is defined and referred to as the "Satellite Relative Actual Reference" system. Sun zenith angle The vertical direction of the Sun relative to the zenith expressed in degrees. The angle measured from a position on the earth ellipsoid pointing towards the satellite. It is defined by its zenith and azimuth angles as shown in the following diagram: WGS84 geoid The Earth geoid model referred to as a "reference ellipsoid". Angle between a look direction in the topocentric coordinate system and the zenith axis of that system (zenith angle + elevation angle = 90°).

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

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