 The primary instruments aboard CryoSat @-@ 2 are SIRAL @-@ 2 , the SAR / Interferometric Radar Altimeters ; which uses radar to determine and monitor the spacecraft 's altitude in order to measure the elevation of the ice . Unlike the original CryoSat , two SIRAL instruments are installed aboard CryoSat @-@ 2 , with one serving as a backup in case the other fails . 
