Minimize CryoSat FAQs

What is CryoSat?

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?

Where can I access quality and technical information about the CryoSat products?

How can I 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 (ESA) and dedicated to polar observation. The mission aims to determine variations in the extent and thickness of the Earth's continental ice sheets and polar sea ice, and to test the prediction of global ice loss due to global warming.

Launched in April 2010, CryoSat is Europe's first dedicated ice mission. It provides 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 cover 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 contribute to a better understanding of the role ice plays in the Earth system.

Alongside CryoSat's contribution to the sea ice and land ice communities, CryoSat data is used widely by a number of other scientific communities including meteorology, oceanography, marine gravity, coastal zones and hydrology.

For more information, please visit the CryoSat homepage or the Living Planet website. A CryoSat e-book has recently been published and is available to download.

Q: What is SIRAL and what does it do?
A: SIRAL stands for Synthetic Aperture Interferometric Radar Altimeter and is the primary instrument on-board CryoSat. It 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 Radar (SAR) processing, to reduce the surface footprint and enable CryoSat to map smaller ice floes. This mode is used to obtain high-resolution measurements of floating sea ice, enabling the indirect measurement of ea ice 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 (SAR Interrfometry (SARIn)) provides the exact surface location being measured when the surface is sloping and can be used to study steep and mountainous terrains, such as 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 measures 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 takes up to five pictures per second. The images are analysed by a built-in computer and compared to a catalogue of star positions.

Baseline parameters are available in the Baseline C Ice products. The L1B products contain values for the interferometric baseline vector and real beam direction vector. Both L1B and L2 products contain values for the antenna bench angles (Roll, Pitch and Yaw), which are used to calculate the mispointing angle in the L2 products. This is the angle between the antenna pointing (direction of beam) and the nadir direction, and nominally this should be zero.

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

  • The Doppler Orbit and Radio-positioning Integration by Satellite (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 record the time interval before the pulse is reflected back. This provides 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 orbits at an unusually high inclination, of about 92 degrees. The orbit is not sun-synchronous and consequently the orbital plane rotates 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. This cycle provides full coverage of the earth and the cycle number is provided in the CryoSat product headers. The repeat cycle is made up of approximately 30-day sub-cycles, defined and used primarily for statistic and quality reporting.

Q: If this FAQ does not answer my question, what should I do?
A: It is possible your question may have already been raised and answered on the CryoSat Wiki forum, which provides a platform for discussion between the CryoSat user community. All queries regarding CryoSat and SIRAL should be directed to the ESA Earth Observation Help Desk Team (

Q: Where can I access quality and technical information about the CryoSat Products?
A: Users can access quality information about the CryoSat data products on the CryoSat Sensor Performance and Product Algorithms (SPPA) website. The CryoSat Wiki also provides an e-collaboration platform for sharing information, results, resources and documents amongst the CryoSat user community and with ESA.

Q: How can I be informed about the latest operational news on the mission?
A: CryoSat operational news is regularly posted on the CryoSat homepage and on Twitter (follow @esa_cryosat). You can also subscribe to the relevant RSS feeds