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 sea 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 Interferometry (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.