Understanding the current state of the polar ice sheets is critical for determining their contribution to sea-level rise and predicting their response to climate change.Surface elevation time series especially can be used to study ice-sheet dynamics and the mass or volume balance of the ice sheets,which are relevant to global climate change and sea-level rise.During the last two decades,satellite radar altimetry or airborne laser altimetry could obtain accuracy by an order of magnitude greater than the traditional airborne barometric altimetry,which has a precision of typically several tens of meters at best and only a limited coverage.The widest coverage comes from satellites,especially from the ERS1/2 and ENVISAT,which extends to 81.5°of latitude,covering almost all of Greenland and most of Antarctica.In this paper,an algorithm for time series analysis based on crossover was used to obtain 4-year(September 2002–March 2007)ice-sheet elevation changes from ENVISAT data.The height of the whole Antarctic ice sheet has a decline of about 0.4±0.43 cm from September 2002 to March 2007.The time series data present clearly a seasonal and annual signal feature;that the ice sheet thickens in March.From the time series data,the seasonal and annual signal can be observed clearly.
