THE AIM OF THE PROPOSED STUDY IS TWOFOLD: TO CONSTRAIN UNCERTAINTIES IN SURFACE ELEVATION ESTIMATES DERIVED FROM AIRBORNE AND SATELLITEBASED ALTIMETERS OVER SPECIFIC REGIONS OF THE MARGINS OF THE GREENLAND ICE SHEET (GIS); AND TO RESOLVE SEASONAL AND SHORT-TERM CHANGES IN ICE VOLUME AS WELL AS PROPAGATION OF THINNING TOWARDS THE GIS INTERIOR. FOR THIS PURPOSE WE PROPOSE THREE RESEARCH GOALS: (1) TO QUANTIFY DIFFERENCES IN ALTIMETRY-DERIVED SURFACE ELEVATION ESTIMATES BETWEEN DIFFERENT SYSTEMS CAUSED BY GEOPHYSICAL FACTORS SUCH AS SIGNAL PENETRATION DEPTH AND SLOPE-INDUCED ERRORS OVER AREAS OF THE GIS WITH VARIABLE METEOROLOGICAL CONDITIONS; (2) TO MEASURESHORT-TERM CHANGES IN ICE VOLUME OVER THE SAME REGIONS IN ORDER TO BOTH STUDY THE PROPAGATION OF ICE THINNING TOWARDS THE ICE SHEET INTERIOR AND ASSESS ALTIMETRY SYSTEM RESPONSE TO ANNUAL SIGNALS (I.E. SEASONAL THINNING AND WINTER ACCUMULATION); AND (3) RECONSTRUCT AND LINK LONG-TERM TIME SERIES FROM REPEATED OBSERVATIONS MADE BY ICESAT CRYOSAT-2 AND COMPLEMENTARY AIRBORNE ALTIMETRY DATA ACQUIRED OVER THE MARGINS OF THE GIS. WE INTENT IS TO COMPARE AIRBORNE LASER AND RADAR ALTIMETRY WITH ALTIMETRY DATA ACQUIRED BYICESAT FOR THE PERIOD OF 2004-2009 AND WITH DATA ACQUIRED BY CRYOSAT-2 FOR THE PERIOD OF 2011-2015. SURFACE-RETRIEVAL ALGORITHMS WILL BE VALIDATED WITH EXISTING IN-SITU FIELD MEASUREMENTS FROM PREVIOUS AND ONGOING NASA AND ESA CAMPAIGNS.RECENT ADVANCES IN ALTIMETRY TECHNIQUES COUPLED WITH GROWING CONCERN OVER THE POTENTIAL ICE SHEET CONTRIBUTION TO SEA LEVEL RISE HAVE MOTIVATED BOTH A SERIES OF MISSIONS AND IMPROVED ALTIMETERS DESIGNED SPECIFICALLY TO OBSERVE THE STATE OF THE CRYOSPHERE (E.G. ICESAT GLAS CRESIS AIRBORNE INSTRUMENTS OPERATION ICEBRIDGE ESA'S ASIRAS ENVISAT'S RA-2 CRYOSAT-2 SIRAL). NASA'S GEOSCIENCELASER ALTIMETER SYSTEM (GLAS) ONBOARD ICESAT PROVIDED WITH VERY ACCURATE ELEVATION CHANGE ESTIMATES BY USING LIDAR TO TRACK THE ICE SHEET SURFACE; THE CRYOSAT-2 MISSION CARRYING A KU-BAND INTERFEROMETRIC RADAR ALTIMETER WAS DESIGNED TO OPERATE AT A 92 - INCLINATION ORBIT TO OBTAIN COVERAGE OF VERY HIGH LATITUDE REGIONS WITH A HIGH DENSITY OF CROSSOVERS. CRYOSAT-2 SYNTHETIC APERTURE RADARINTERFEROMETRIC (SARIN) MODE ALLOWS ACCURATE SURFACE TRACKING AND SLOPE DETERMINATION OVER REGIONS WITH HIGH ELEVATION GRADIENTS. EACH SYSTEM HAS ITS OWN ADVANTAGES AND LIMITATIONS AND IT IS IMPORTANT TO ASSESS THE INFLUENCE OF THESE LIMITATIONS HAVE ON ESTIMATES OF ICE MASS FLUCTUATIONS. THEREFORE THE MAIN GOAL OF THE PROPOSED WORK IS TO COMPARE ELEVATION ESTIMATES FROM MULTIPLE ALTIMETRYPLATFORMS TO QUANTIFY BIASES BETWEEN SYSTEMS AND TO ASSESS THE GEOPHYSICAL CAUSES OF THE OBSERVED DIFFERENCES IN KEY AREAS SUCH AS THOSE CLOSE TO FAST FLOWING GLACIERS AND WITH OBSERVED SIGNIFICANT THINNING AS WELL AS REGIONS THAT ARE SUBJECT TO SEASONAL MELT AND REFREEZING CYCLES. THIS STUDY WILL ALSO INCLUDE A FEW REGIONS WITH HIGH SNOW ACCUMULATION RATES TO DETERMINE HOW LARGE VARIABILITY IN ACCUMULATIONAFFECTS SIGNAL PENETRATION DEPTH AND SURFACE RETRACKING ALGORITHMS.THIS WORK DIRECTLY TARGETS THE GOALS OF NASA'S CRYOSPHERIC SCIENCES PROGRAM OF I) IMPROVE OUR UNDERSTANDING OF THE MECHANISMS CONTROLLING MASS BALANCE AND DYNAMICS OF THE ICE SHEETS OF GREENLAND AND II) THE USE OF REMOTE SENSING DATA TO DEVELOP VALIDATE AND IMPROVE PREDICTIVE MODELS OF THE CONTRIBUTION OF LAND-BASED ICE TO SEA LEVEL CHANGE. WE EXPECT TO OBTAIN MULTI-MISSION ALTIMETRY DATA PRODUCTS FOR KEY REGIONS OF THE ICE SHEET MARGINS THAT HAVE BEEN VALIDATED WITH BOTH EXISTING FIELD MEASUREMENTS AND CROSSPLATFORM COMPARISONS. THE ANALYSIS LARGELY FOCUSES ON THE MARGINS OF THE GIS SINCE THE EFFECTS OF CLIMATE CHANGE TO DATE HAVE BEEN MORE SIGNIFICANT IN THESE AREAS. IT IS EXPECTED THAT THIS WORK WILL REDUCE CURRENT UNCERTAINTIES IN ALTIMETRY DATA PRODUCTS IN THE STUDY REGIONS BENEFITING STUDIES OF THE STATE AND DYNAMICS OF GLACIERS MADE WITHIN NASA AND THE WIDER SCIENTIFIC COMMUNITY.