Le role de l'espace dans l'étude des changements climatiques
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Thales Alenia Space ESA
 

SIRAL 2, a new generation of altimeter

The Siral radar altimeter, derived from the Poseidon family of oceanic altimeters, was designed to cover the wide variety of elevations in ice and land. It incorporates considerable design and technology improvements:

  • The Siral instrument offers an interferometric mode, which opens access to the topography on either side of the CryoSat satellite’s ground track.
     
  • SAR (synthetic aperture radar) mode, allowing Siral to determine an extremely precise and detailed topography along the ground track.
     
  • A new radar tracking and acquisition algorithm means that Siral offers exceptional performance over a wide variety of elevations.

With the CryoSat 2 mission and Siral 2, the European Space Agency (ESA) will have extremely precise data on ice thickness and changes as early as 2009, to better understand their relationship with global warming. CryoSat 2 is one of six Earth Explorer missions, and is part of the Living Planet program, like Goce and SMOS – two programs in which Thales Alenia Space is also involved.


How does it work?
The complete instrument on its flight platform Instrument electronics

The innovative Siral instrument is very compact, weighing just 70 kilograms. It operates in Ku band (13.7 GHz) and offers three measurement modes:

  • Low resolution (LRM), for conventional altimetric measurements, limited to the relatively undifferentiated elevations of inland ice and sea ice.
  • Synthetic aperture radar (SAR), for high-resolution measurement of floating ice.
  • Interferometric radar (SARin), which can distinguish the most contrasted areas, such as the very active transitions between ice sheets and land.


Image Copyrights @ ESA

The main mission instrument is the Siral SAR/Interferometric Radar Altimeter, developed and produced by Thales Alenia Space in Toulouse.
 


The Siral instrument features an interferometric mode, which opens access to the topography on either side of the CryoSat satellite’s ground track.

A Poseidon type radar altimeter measures elevation solely under its own path, i.e., the ground track directly under the satellite.

Just like a car’s two headlights illuminate the road ahead, the twin antennas on the Siral altimeter target the ground track of the CryoSat satellite. If the surface targeted by the radar echo is on this track, the two antennas see it arrive at the same time.

However, if the surface receiving the radar echo is outside this track, for example 5 km to the left, the left-hand antenna will see it arrive prior to the right-hand antenna. Therefore, the time difference between the arrival of the echo at the two antennas enables determining the direction it is coming from.

 

The Siral instrument also offers an SAR (synthetic aperture radar) mode, allowing it to determine an extremely precise and detailed topography along the ground track.

Poseidon type altimeters offer space resolution of several tens of kilometers. This is not enough to distinguish transitions between ice floating on the sea and the surface of the sea. Siral’s SAR mode improves space resolution 40-fold! To understand how synthetic aperture radars work, take the example of a car that approaches an observer, passes him then moves away. The observer hears a changing sound frequency, from sharp to deeper as it moves away. Known as the Doppler effect, this corresponds to the difference in frequency perceived by the observer as a function of the vehicle’s speed and direction. If the speed of the vehicle is known, there is thus a direct relationship between the frequency shift (Doppler frequency) and the direction of the vehicle.

On the satellite, the radar is the observer, and the surface is in movement relative to the satellite. Echos received by the radar therefore include a mix of Doppler frequencies corresponding to all surface directions covered by the radar antenna beam, from the front to the back. Digital processing of these echoes separates the different Doppler sequences at a given resolution. This is called Doppler filtering, or SAR (synthetic aperture radar) mode, because the space resolution is achieved synthetically. After processing, everything proceeds as if the antenna beam had been cut into strips with a space resolution 40 times smaller than the resolution of the radar antenna beam.

 

A new radar tracking and acquisition algorithm means that Siral offers exceptional performance over a wide variety of elevations.



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