Despite the loss of CryoSat, ESA's first Earth opportunity mission, during its launch sequence in Oct 2005 ESA have been fortunate enough to have acquired, processed to Level 1b and analysed a significant amount of campaign data from ESA's demonstration Airborne SAR/Interferometric Radar Altimeter System (ASIRAS) designed to have similar functionality to CryoSat's Synthetic Interferometric Radar Altimeter (SIRAL). This data acquisition took place for the original purpose of validating CryoSat retrievals. Our initial analyses of the level 1b data have revealed some very interesting results both over land and sea ice test sites from respective campaigns conducted in the Arctic during the land-ice spring/autumn campaigns of 2004 and for sea-ice in the Bay of Bothnia during March 2005. Since a further ASIRAS campaign in the Arctic is guaranteed for April/May 2006 we look at how this data can be exploited in view of future ESA Earth observation radar altimeter missions. Verification of the ASIRAS data with coincident laser altimeter and in-situ data collected at test sites is also presented. The paper also provides a review of key space and air borne radar altimeters dating from 1957 including those using the synthetic aperture radar (SAR) technique as applied to radar altimeters leading to the development of CryoSat.