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From orbitography to geodesy

Uses of orbit determination


A satellite’s altitude can be calculated using a dynamic equation that compares the theoretical position of the satellite and the forces acting upon it in space (atmospheric drag, Earth’s attraction), which may perturb its orbital motion. As knowledge of these forces is imperfect, a tracking system that acquires measurements all along the satellite’s trajectory is needed to determine its precise position.
Crédits : CNES/Ill. D.Ducros

Crédits : CNES/Ill. D.Ducros

Accurate knowledge of satellites’ orbits is essential for altimetry missions that measure sea or ice fields level. The combined results provided by altimetry instruments and the DORIS system make it possible to know the exact level at points under the satellite pass, and to calculate the height and shape of the geoid, the direction and speed of currents, as well as other parameters.



This monitoring of the oceans or ice fields is a major objective for the scientific community. Rises in sea level due to the potential effects of global warming could have disastrous consequences for many population areas in the world. Measuring local sea level variations will also help to gain a better understanding of ocean-atmosphere interactions and their impact on global climate.

Studying the Earth’s shapes and dimensions


Geologists have frequently used the DORIS system to measure continental drift. The Earth’s surface is made up of tectonic plates which move very slowly in relationship to each other. These movements are the cause of earthquakes. The accuracy of DORIS measurements makes it possible to monitor these imperceptible changes over several years.
Tectonic plates and their movements. Crédits : CNES/Ill. D.Ducros

Tectonic plates and their movements. Crédits : CNES/Ill. D.Ducros

Doris watching Earth's every move
Each day, a DGXX generation Doris instrument acquires more than 30,000 measurements, or nearly 100,000 measurements from all of the satellites currently in operation. Thanks to Doris, it was observed for the first time that the African and Eurasian plates are moving towards each other at the rate of nearly 2 cm per year.


Crédits : CNES/Ill. D.Ducros

Crédits : CNES/Ill. D.Ducros

Doris can also accurately calculate the rotation parameters of the Earth, which show certain irregularities. The planet wobbles slightly on its axis in a continuous fashion. These variations of the pole, which may deviate up to ten metres from its theoretical position, can be monitored on a daily basis, thanks to Doris.



Doris can also be used to evaluate seasonal variations of the height of land masses, particularly in the polar regions, where the weight of winter snow cover is such that it pushes down the soil. These minute deformations of the Earth's crust, of an amplitude of a few millimetres per year, have until now been very difficult to quantify.

Earth's centre of gavity evolution. Crédits : CNES/Ill. D.Ducros

Earth's centre of gavity evolution. Crédits : CNES/Ill. D.Ducros

The Earth's centre of gravity constantly changes position within a three-centimetre cube in response to mass redistributions on the surface of the globe, caused by shifting masses within the ocean and by soil moisture, snow cover and groundwater. DORIS measures these seasonal millimetric variations, which impact orbit calculations.

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