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ERS Sea Level Analysis – Introduction

Every month, once our precise orbit is available, the heights of the Dynamic Ocean Topography (i.e. the average elevation of the sea above the Earth’s geoid) are calculated from ERS-1 and ERS-2 altimetry using long-arc techniques, by fitting the difference between the orbital height and the corrected altimeter range to an spherical harmonic surface to degree and order 23. Fast-delivery measurements from the Kiruna station -which arrive at ESOC in near real time- are used in the computations, by taking one out of 30 raw measurements (i.e. every 30 seconds) and applying all instrumental and environmental corrections to it. These corrections include dry and wet troposphere, ionosphere, sea-state bias, tides, ocean loading and altimeter bias.

The geoid used as a reference for the determined topographies is our ERS-1 Preliminary Marine Geoid.

Averaging the twelve monthly topographies from 1996 we have determined an annual Mean Dynamic Ocean Topography. This map gives an idea of the main component of the general circulation of the seas resulting from the Coriolis forces acting on the unbalanced masses of water.

Every month, the difference (or anomaly) between the corresponding determined ocean topography and the Mean Dynamic Ocean Topography is plotted. Apart from mismodelling effects, differences between monthly anomalies indicate large-scale ocean variability patterns like El Niño.