New scientific opportunities from Ka-band altimetry

Single frequency Ku-band altimeters were the first instruments designed for sea-surface topography measurements: this was the case for Geosat and Seasat. This has also been the case for the more recent ERS1/2 Ku-band instruments and for the POSEIDON altimeter on board TOPEX/POSEIDON. The need for accuracy for sea surface topography measurement from TOPEX leads to the system choice of the first dual frequency (Ku and C-band) instrument. This choice has been repeated for the near future RA-2 (Ku and S-band) and POSEIDON2 (Ku and C-band) instruments, respectively on board ENVISAT and JASON-1. A new concept of Ka-band altimetry (36 GHz) is now proposed by CNES and ALCATEL. By increasing the frequency, new scientific opportunities appear either directly linked to the exploitation of electromagnetic properties of the interaction between radar-wave and surface or linked to the new geometry between antenna aperture and surface. One major science opportunity of Ka altimetry concerns media such as sand or snow for which unsufficient knowledge of radar penetration is a limitation in the geophysical interpretation of measurements. For instance, this is a major issue for the study of ice sheet mass balance, which is presently of limited accuracy due to penetration effects associated with one-frequency Ku-band ERS1/2 altimeters. Another source of high benefit of Ka-band altimetry lies in the possibility to improve the horizontal resolution of the measurements. This may benefit to oceanic observations near coasts or near sea ice, and may also improve the geographical sampling of estuaries, small inland waters or continental observations.