WATER THAT ORIGINATES FROM SNOW PROVIDES A VITAL RESOURCE FOR OVER A BILLION PEOPLE GLOBALLY AND IS VALUED AT OVER A TRILLION US DOLLARS AS A RESOURCE IN THE WESTERN UNITED STATES ALONE. ARGUABLY THE MOST IMPORTANT STEP FORWARD FOR UNDERSTANDING AND MANAGING THIS RESOURCE IS TO DEVELOP METHODS FOR ACCURATELY MEASURING THE AMOUNT OF WATER RETAINED WITHIN THE SNOWPACK RESERVOIR USING SATELLITE REMOTE SENSING. ALTHOUGH SNOW COVER HAS BEEN MAPPED FROM SPACE FOR MORE THAN A HALF-CENTURY QUANTIFYING SNOW WATER EQUIVALENT (SWE) REMOTELY REMAINS A SIGNIFICANT CHALLENGE PARTICULARLY IN REGIONS OF FOREST COVER OR COMPLEX TOPOGRAPHY. THUS THE PRIMARY GOAL OF THE NASA SNOWEX CAMPAIGN IS TO TEST THE VIABILITY OF DIFFERENT SPACE-READY INSTRUMENTS FOR QUANTIFYING THE DISTRIBUTION OF SWE ACROSS A RANGE OF TOPOGRAPHIC AND FOREST CANOPY TYPES. ESSENTIAL TO ACHIEVING THE SNOWEX MISSION GOALS ARE SPATIALLY PERVASIVE GROUND VALIDATION OBSERVATIONS TO CONSTRAIN ALGORITHM DEVELOPMENT. NUMEROUS TECHNIQUES ARE AVAILABLE TO MAKE GROUND OBSERVATIONS OF SWE. AN INCREASINGLY POPULAR CHOICE IS THE USE OF GROUND PENETRATING RADAR (GPR) A MATURE TECHNIQUE THAT HAS BEEN UTILIZED IN SNOW APPLICATIONS FOR DECADES. THE PRIMARY BENEFIT OF GPR OVER OTHER APPROACHES IS THE ABILITY TO RAPIDLY MEASURE SNOW DEPTH DENSITY AND THE RESULTANT SWE OVER MUCH GREATER SPATIAL SCALES. IN ADDITION GROUND-BASED RADAR SURVEYS DIRECTLY MEASURE THE TWO-WAY TRAVELTIME (TWT) AND AMPLITUDE RESPONSE OF THE SNOWPACK PROVIDING IMPORTANT INFORMATION FOR AIRBORNE AND SPACEBORNE RADAR RETRIEVAL ALGORITHMS (E.G. L-BAND INSAR X-/KU-BAND SCATTEROMETERS). THE PRIMARY OBJECTIVES OF THE PROPOSED WORK ARE TO: 1. ANALYZE GPR SURVEYS COLLECTED BY OUR TEAM DURING YEAR 1 OF SNOWEX TO QUANTIFY SNOW DEPTH DENSITY AND SWE DISTRIBUTION THUS YIELDING ROBUST GROUND VALIDATION DATASETS FOR SATELLITE ALGORITHM DEVELOPMENT. 2. COLLABORATE WITH SNOWEX LEADERSHIP AND REMOTE SENSING TEAMS TO DESIGN AND PERFORM GPR SAMPLING STRATEGIES FOR FUTURE SNOWEX CAMPAIGNS. 3. QUANTIFY THE ROLE OF VEGETATION AND TOPOGRAPHY IN CONTROLLING SNOW DENSITY AND SWE DISTRIBUTION. 4. CONTRIBUTE TO THE DEVELOPMENT OF A FUTURE ATBD FOR A SNOW SATELLITE MISSION THROUGH A BETTER UNDERSTANDING OF SPATIAL VARIATIONS IN SNOW DENSITY AND SWE AND THEIR EFFECT ON SWE RETRIEVAL ALGORITHMS. THESE OBJECTIVES WILL PRODUCE HIGH QUALITY DATA PRODUCTS THAT PROVIDE VALUABLE INFORMATION CONCERNING SNOW DENSITY AND SWE ACROSS A RANGE OF CANOPY AND TOPOGRAPHIC CONDITIONS. THE ABILITY OF OUR RADAR SYSTEMS TO RAPIDLY COLLECT GROUND BASED OBSERVATIONS BENEATH DENSE FOREST CANOPIES OVER A GREATER SPATIAL EXTENTS THAN OTHER METHODS IS UNIQUE. THIS ABILITY WILL RESULT IN SPATIALLY PERVASIVE DATASETS OF SNOW DENSITY AND SWE THAT ARE NECESSARY FOR MOVING REMOTE SENSING TECHNIQUES FORWARD. THE THREE TYPES OF AIRBORNE OR SPACEBORNE SWE RETRIEVAL CURRENTLY BEING TESTED DURING THE NASA SNOWEX MISSION ARE 1) DEPTH-BASED SUCH AS LIDAR STRUCTURE FROM MOTION OR KA-BAND INSAR 2) VOLUME-BASED USING ACTIVE OR PASSIVE SIGNALS IN THE MICROWAVE (5-89 GHZ) REGION AND 3) INSAR PHASE-BASED WHICH USES L-BAND INSAR TO ESTIMATE SWE. GROUND-BASED RADAR IS THE MOST EFFICIENT WAY TO PROVIDE INDEPENDENT INFORMATION SENSITIVE TO DENSITY AND MICROSTRUCTURE THAT CAN EFFECTIVELY CONSTRAIN THE LEADING SWE RETRIEVAL APPROACHES. OUR PROPOSED WORK WILL PROVIDE UNIQUE CALIBRATION AND VALIDATION OPPORTUNITIES FOR KEY SWE REMOTE SENSING APPROACHES (LIDAR SFM ACTIVE AND PASSIVE MICROWAVE) AND SNOWEX MISSION INSTRUMENTS (ASO UAVSAR GLISTIN-A AESMIR WISM SNOWSAR). THIS PROPOSED PROJECT WILL TRAIN ONE NEW PH.D. STUDENT PROVIDE SUPPORT FOR TWO EARLY-CAREER SCIENTISTS DEVELOP A NEW COLLABORATION BETWEEN RESEARCH GROUPS WITH SNOW RADAR EXPERTISE AT COLORADO STATE UNIVERSITY UNIVERSITY OF COLORADO AND BOISE STATE UNIVERSITY AND WILL PRODUCE UNIQUE VALUABLE GROUND-BASED RADAR DATASETS FOR THE ENTIRE NASA SNOWEX MISSION.