Glossary – Sentinel-3 Altimetry Technical Guide – Sentinel Online
Absolute dynamic topography: sea surface height with respect to the geoid.
Along track data: data chronologically ordered, following the satellite "ground track". **Altitude: ** distance from the satellite to the reference ellipsoid.
Ascending node: the ascending node of an orbit is the intersection of that orbit, when the satellite goes from the southern to the northern hemisphere, with the x-y plane of the Earth fixed reference frame.
Cycle: in sun-synchronous orbits, the ground track repeats precisely after a constant integer number of orbits and a constant duration. The duration in days of that period is called the repeat cycle, whereas the corresponding number of orbits is called the cycle length.
Geoid: the shape of the sea surface assuming a complete absence of perturbing forces (tides, wind, currents, etc.). The geoid reflects the Earth's gravitational field.
Geophysical corrections: the radar pulse used to measure altimetry is subjected to a number of disturbances as it passes through the atmosphere and when it is reflected by the sea surface.
Ground track: the trace made by the sub-satellite point on the surface of the Earth's reference ellipsoid due to the motion of the satellite along its orbit.
Mean dynamic topography: Oceanic relief corresponding to permanent ocean circulation.
Mean sea level: the sea surface height averaged across all the oceans of the globe. An increase in mean sea level is an indication of possible global warming.
Mean sea surface: a permanent component of sea surface height. The mean sea surface comprises a geoid contribution (approximately 100 m) and a permanent circulation contribution (approximately 1 m).
Orbit number: the absolute orbit number considers the orbits elapsed since the first ascending node crossing after launch. The relative orbit number is a count of orbits from 1 to the number of orbits contained in a repeat cycle. Relative orbit number 1 corresponds to the orbit whose ascending node crossing is closest to the Greenwich Meridian (eastwards).
Range: distance from the satellite to the Earth's surface.
Reference ellipsoid: an arbitrary reference surface that is a raw approximation of the Earth's shape, which is basically a sphere "flattened" at its poles. The length of one of the axes at the Equator is chosen so that the ellipsoid coincides at this latitude with the mean sea level.
Satellite altimetry: a technique for measuring height. Satellite altimetry measures the time taken by a radar pulse to travel from the satellite antenna to the surface and back to the satellite receiver. Combined with precise satellite location data, altimetry measurements yield sea-surface heights.
Sigma 0 (or sigma-naught): backscatter coefficient of the radar wave on the surface.
Sea level anomaly: variations in the sea surface height with respect to mean sea level. SLAs include seasonal variability.
Sea state bias: bias due to the sea-surface state, consisting of two components (electromagnetic bias and tracker bias).
Sea surface height: height of the sea surface with respect to a reference. In altimetry, usually the sea surface height is with respect to the reference ellipsoid.
Significant wave height: average wave height (from trough to crest) of the highest third (33.33%) of the waves in a given sample period.
Sub-satellite point: the normal projection of the position of the satellite in orbit on to the surface of the Earth´s reference ellipsoid. It is also referred to as nadir.
Waveform: the magnitude and shape of the radar altimetry return echoes.