NASA HAS PIONEERED THE DEVELOPMENT EXPANSION AND OPERATION OF CONTINUOUSLY-OPERATING GPS GROUND NETWORKS AROUND THE WORLD. THESE GPS NETWORKS ARE CRITICAL COMPONENTS OF THE INTERNATIONAL TERRESTRIAL REFERENCE FRAME. THEY ARE USED PRIMARILY FOR POSITIONING AND TO PROVIDE PRECISE ORBITS FOR MANY NASA SATELLITE AND AIRBORNE MISSIONS. IT HAS LONG BEEN RECOGNIZED THAT DATA FROM THESE SAME SITES CAN BE USED TO DETERMINE INTEGRATED MEASUREMENTS OF WATER VAPOR AND TOTAL ELECTRON CONTENT THUS MAKING GPS A STRONG CONTRIBUTOR TO OUR UNDERSTANDING OF THE ATMOSPHERE. MORE RECENTLY IT HAS BEEN SHOWN THAT THESE SAME GPS GROUND NETWORKS IF PROPERLY SITED PROVIDE VALUABLE INFORMATION ABOUT THE LAND SURFACE SURROUNDING THE GPS ANTENNA IN THE FORM OF SOIL MOISTURE SNOW DEPTH AND BIOMASS PRODUCTS. THE BENEFIT OF REFLECTION DATA DERIVED FROM GPS GROUND NETWORKS FOR HYDROLOGICAL STUDIES WOULD BE THREE-FOLD. FIRST BY USING NETWORKS THAT ALREADY EXIST AND HAVE SUPPORT FROM VARIOUS GOVERNMENT AGENCIES NASA WILL BE ABLE TO LEVERAGE USEFUL SCIENTIFIC DATA AT LOW COST. SECONDLY THE FOOTPRINT OF A GROUND GPS SITE IS INTERMEDIATE TO IN SITU PROBES AND SATELLITE SENSORS. THE FORMER (10S OF CMS) FAILS TO CAPTURE THE TRUE SPATIAL VARIABILITY OF THE HYDROLOGICAL PARAMETER WHILE THE LATER IS OFTEN TOO LARGE (10S OF KMS). ASSIMILATION MODELS THAT INCLUDE BOTH IN SITU AND SATELLITE DATA CAN BE IMPROVED BY HAVING SUCH AN INTERMEDIATE-SCALE SENSOR. FINALLY HYDROLOGICAL PARAMETERS PLAY A CRITICAL ROLE IN OUR ABILITY TO UNDERSTAND THE ENVIRONMENT AS SOIL MOISTURE DATA ARE USED IN WEATHER FORECASTS AND CLIMATE MODELS. SNOWPACK AND MELT RATE INFORMATION ARE NEEDED FOR MANAGEMENT OF THE WATER SUPPLY AND FLOOD CONTROL SYSTEMS PARTICULARLY IN THE MORE ARID WESTERN U.S. ESTIMATES OF BIOMASS/VEGETATION STATE ARE ALSO REQUIRED FOR ASSIMILATION MODELS AND FOR VALIDATION OF SATELLITE SURVEYS OF LAND SURFACE CONDITIONS. WHILE THE PRIMARY GOAL OF THIS PROPOSAL IS TO DEVELOP NEW HYDROLOGICAL PRODUCTS FROM GPS REFLECTION DATA KNOWLEDGE OF HYDROLOGICAL SIGNALS IS ALSO IMPORTANT FOR IMPROVING POSITIONING ACCURACY OF GPS. THE LATTER IS A KEY DRIVER IN THE ACCURACY OF THE INTERNATIONAL TERRESTRIAL REFERENCE FRAME. GIVEN THE MATURITY OF GPS NETWORKS IT IS APPROPRIATE NOW TO USE MODELS SUCH AS GLDAS (GLOBAL LAND DATA ASSIMILATION SYSTEM) TO DEVELOP HYDROLOGICAL LOADING CORRECTIONS FOR GPS NETWORKS. TOGETHER WITH ATMOSPHERIC PRESSURE LOADING EFFECTS GPS VERTICAL PRECISION CAN BE SIGNIFICANTLY IMPROVED. THIS RESEARCH WILL SUPPORT NASA MISSIONS SUCH AS GRACE AND DESDYNI. ITS RELEVANCE TO THE ADVANCED GEODETIC CONCEPTS NRA INCLUDES THE CALL FOR NEW MEASUREMENT TECHNIQUES FOR GPS REFLECTIONS. THIS RESEARCH PROJECT WILL ASSEMBLE A COLLECTION OF DATA SETS DERIVED FROM GPS NETWORKS FOR DIRECT USE IN EARTH SYSTEM SCIENCE STUDIES. THE PRIMARY NETWORK USED IN THIS STUDY WILL BE THE EARTHSCOPE PLATE BOUNDARY OBSERVATORY (PBO) BUILT BY NSF IN PARTNERSHIP WITH NASA AND THE USGS. THIS STUDY WILL YIELD A (1) DATABASE OF SNOW DEPTH (AND SNOW WATER EQUIVALENT) (2) DATABASE OF BIOMASS (3) ANCILLARY ENVIRONMENTAL DATA SETS OF PRECIPITATION TEMPERATURE AND VEGETATION PRODUCTS (4) HYDROLOGICAL LOADING EFFECTS (5) ATMOSPHERIC PRESSURE LOADING EFFECTS AND (6) TEST THE ABILITY OF THESE PRODUCTS TO IMPROVE AND UNDERSTAND HOW HYDROLOGICAL FORCING INFLUENCES GLDAS. ALL PRODUCTS DEVELOPED FROM THIS RESEARCH WILL BE INTEGRATED WITH THE NASA-ROSES FUNDED WEB SERVICES-ENABLED GPS SEAMLESS ARCHIVE CENTERS (GSAC-WS).