Le role de l'espace dans l'étude des changements climatiques
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Current situation
Ice in the Earth's poles plays a pivotal role at several levels:
Stability of the global climate
Average sea level
Circulation of major ocean currents

Observation satellites have shown that ocean ice has decreased 10 to 15% since 1950. In certain regions, underwater measurements have shown a 40% decrease in the thickness of ice fields. However, ice in Greenland and the Antarctic seems to have gotten thicker.

Thales Alenia Space ESA
 

Altimetry at a glance
AltimetrieA space altimetry mission allows us to measure, from a spacecraft in orbit, the relative or absolute altitude of the surface it flies over. These are Earth observation (EO) missions, with a particular focus on measuring the topography of oceans and ice. The technique used to measure these altitudes is based on calculating the time for a signal to make the round trip between the sensor on the spacecraft and the surface targeted.

If we know the speed of the signal and the exact orbital position of the spacecraft, it is easy to calculate the elevation of the surface in question. Since the atmosphere and ionosphere affect the propagation speed, we also have to integrate these factors to achieve the desired level of altimetric precision. The major advantage of using radar technology for this purpose is that it is independent of day/night conditions or cloud clover.

Space altimetry helps us observe the “secret life” of oceans and better understand the Earth’s climate. Altimetry is an unrivaled tool to enhance performance in various areas, including weather forecasts, maritime navigation, marine resources management and even large-scale construction projects.

 
Ocean Altimetry - A typical application

Measuring altitude (in calm seas)

The radar altimeter sends a signal (radio-electric impulse) which will reflect off the surface of the sea and return to the original point

  • We know the speed of the signal
  • We time how long it takes to return
  • We calculate the distance covered (equal to twice the altitude)

Our signal is very fast, traveling at 300,000 kilometers/second: it takes just one-thousand millionth of a second to travel 30 centimeter.

But this means that to achieve accuracy down to a centimeter, we need a very, very precise clock!

Wind Force
There are two basic cases, depending on conditions

  1. Whe sea is calm and the surface of the water is like a mirror.
  2. The wind comes up and the mirror is no longer smooth:
    • the signal may be reflected without distortion, but its image is attenuated;
    • the stronger the wind, the greater the attenuation.
Thus, by measuring the attenuation of the reflected signal, we can determine “wind force”.

Wave height

If the wind blows long enough, it will distort the surface of the water by creating waves.
The signal therefore reflects off a distorted surface, and its image is modified.

When the signal returns to the altimeter:
  it is compared to models corresponding to different sea states
  when the most accurate model has been determined, we can calculate:
         - the average height of waves
         - the position of the sea without these waves
   

And therefore... the altitude!



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