Minimize CryoSat FAQs

What is CryoSat?

What does SIRAL stand for?

What is SIRAL and what does it do?

How is the baseline orientation measured?

How is the position of the CryoSat satellite measured?

What orbit does CryoSat use?

What is the orbit repeat cycle?

If this FAQ does not answer my question, what should I do?

How can one be informed about the latest operational news on the mission?


Q: What is CryoSat?
A: CryoSat is an altimetry satellite built by the European Space Agency and dedicated to polar observation. It embarks on a three-and-a half-year mission to determine variations in the thickness of the Earth's continental ice sheets and marine ice cover, and to test the prediction of thinning Arctic ice due to global warming.

Launched in April 2010, it is Europe's first ice mission. It will provide multi-year elevation data at latitudes never reached before by a satellite altimeter. CryoSat carries sophisticated technologies to measure changes at the margins of the vast ice sheets that overlay Greenland and Antarctica and marine ice floating in the polar oceans. By accurately measuring thickness change in both types of ice, CryoSat will provide information to complete the picture and lead to a better understanding of the role ice plays in the Earth system.

For more information, please visit the Living Planet website or CryoSat portal.

Q: What does SIRAL stand for?
A: SIRAL stands for Synthetic Aperture Interferometric Radar Altimeter.

Q: What is SIRAL and what does it do?
A: SIRAL is the primary instrument onboard CryoSat and has extended capabilities to meet the measurement requirements for ice-sheet elevation and sea-ice freeboard.

Derived from the well-known Poseidon ocean altimeter on the Jason satellite, SIRAL is a very compact assembly, weighing just 90 kilograms. It combines three measurement modes to determine the topography of land and sea ice masses, as well as ice floes and significant elevation transitions, especially between land and ice fields:

  • Over the oceans and ice sheet interiors, CryoSat operates like a traditional radar altimeter in Low Resolution Mode (LRM).
  • Over sea ice, coherently transmitted echoes are combined via Synthetic Aperture (SAR) processing, to reduce the surface footprint so that CryoSat can map smaller ice floes. This mode is used to carry out high-resolution measurements of floating sea ice (ice sheets), enabling the indirect measurement of the sheets' thickness.
  • CryoSat's most advanced mode is used around the ice sheet margins and over mountain glaciers. Here, the altimeter performs synthetic aperture processing and uses a second antenna as an interferometer to determine the across-track angle to the earliest radar returns. This mode (SARIn) provides the exact surface location being measured when the surface is sloping and can be used to study more contrasted terrains, like the very active areas located at the junction of the ice sheet and the Antarctic continent, or Greenland

Q: How is the baseline orientation measured?
A: Knowledge of the precise orientation of the baseline and the two receiving antennas is essential for the success of the mission. CryoSat will measure this baseline orientation using the position of the stars in the sky. Three star trackers are mounted on the support structure for the antennas. Each contains a camera, which will take up to five pictures per second. The images will be analysed by a built-in computer and compared to a catalogue of star positions.

Q: How is the position of the CryoSat satellite measured?
A: In order to measure accurately, the position of the satellite, CryoSat carries some additional specific equipment:

  • The DORIS radio receiver detects and measures the Doppler shift of signals broadcast from a network of over 50 radio beacons spread around the world.
  • A small laser retro-reflector reflects light back in exactly the direction it came from. A global network of laser tracking stations fire short laser pulses at CryoSat and time the interval before the pulse is reflected back – providing independent reference measurements of CryoSat's position.
  • The International Laser Ranging Service (ILRS) provides tracking from its global network of laser ranging stations to support the project.

Q: What orbit does CryoSat use?
A: CryoSat will orbit at an unusually high inclination, of about 92 degrees. The orbit is not sun-synchronous and consequently the orbital plane will rotate with respect to the sun direction, at an altitude of 717 kilometres.

Q: What is the orbit repeat cycle?
A: The repeat cycle for CryoSat is 369 days and 5344 orbits.

Q: If this FAQ does not answer my question, what should I do?
A: All queries regarding CryoSat and SIRAL should be directed to the ESA Earth Observation Help Desk Team ( in the first instance.

Q: How can one be informed about the latest operational news on the mission?
A: CryoSat operational news is regularly posted on CryoSat's front page and on Twitter (follow @esa_cryosat).