**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.