Altimeter looks into the crevasses

Image of the month - October 2006

Ice shelves are the most sensitive parts of the Antarctic continent. The fragmentation of the shelves, which is often highlighted in the media, is sped up by crevasses. Those crevasses are thus indicators of the ice shelves "health".

"Bi-frequency" altimeter (i.e. altimeter emitting radar waves at two different frequencies) have been made firstly to correct altimeter measurements from the path delay induced by electrons in the ionosphere, which is different depending on the radar wave frequency. But those two frequencies also enable to observe other phenomena, in the atmosphere (see e.g. the Image of the Month of June 2005, Cloudy skies), or on solid earth surfaces. Over the ice caps, in particular, the radar wave interaction with the snow cover is very dependent on its frequency. With Envisat, the Ku band (13.6 GHz) and the S band (3.2 GHz) penetrations are very different in the snow: the Ku wave penetration is very low in Summer, because the coarser snow grains diffuse and attenuate the wave. In S band, on the contrary, the wave penetrate the snow cover, and is thus sensitive to the snow characteristics in depth. Over the ice shelves, the radar wave in S band goes into the large crevasses filled by snow and reflects on their walls, thus leading to a very different altitude measurement than the one made in Ku band.

Altimetry should enable to survey crevasses and better understand ice shelves dynamics. Altimeters thus bring a complementary monitoring over the Antarctic ice. With the future Cryosat, planned for March 2009, this monitoring should allow a closer survey, with a better resolution.

Altitude measurements made by Envisat altimeter (left) in S band (red) and Ku band (black). Arrows indicate were the discrepancies between both frequencies are high. Right, an image from Modis sensor over the Amery shelf, where those points are shown in blue. In red, the ground track plotted left. (Credits Legos/CNRS)


See also:

  • Applications: Ice


  • Lacroix P., B. Legrésy, R. Coleman, M. Dechambre, F. Rémy, 2006 Surface and sub-surface properties of snow on the Amery Ice-shelf using the dual-frequency altimeter of Envisat Remote Sensing of Environment, In press
  • Lacroix P., B. Legrésy, R. Coleman, M. Dechambre, F. Rémy, 2006, Envisat radar altimeter as a sounding radar on the Amery Ice shelf, 15 years of progress in radar altimetry Symposium, Venice, Italy, March 2006