PRECIPITATION RETRIEVALS FROM SATELLITE-BASED SENSORS ARE NOW WELL ESTABLISHED. THE OFFICIAL RETRIEVAL SCHEME FOR THE GPM MISSION THE GODDARD PROFILING (GPROF) SCHEME UTILIZES AN A PRIORI DATABASE OF C.400 MILLION PROFILES BASED PRIMARILY UPON THE DPR/GMI COMBINED PRECIPITATION PRODUCT AUGMENTED BY MRMS SURFACE RADAR DATA AND CLOUDSAT CPR OBSERVATIONS. THE GPROF SCHEME THEN UTILIZES THIS DATABASE ACROSS THE RANGE OF SENSORS IN THE GPM CONSTELLATION THROUGH THE MODELLING OF THE DIFFERENCES IN THE FREQUENCIES AND THE RESOLUTIONS OF THE OBSERVATIONS BETWEEN THE SENSORS. QUANTITATIVE PRECIPITATION ESTIMATION FROM PASSIVE MICROWAVE IMAGERS MAY BE CONSIDERED REASONABLY MATURE HOWEVER ESTIMATES FROM PASSIVE MICROWAVE SOUNDERS LESS SO. PRECIPITATION RETRIEVALS FROM CROSS-TRACK SOUNDING INSTRUMENTS PRESENT A NUMBER OF CHALLENGES PRIMARILY RELATED TO THE VARIABLE EARTH INCIDENT ANGLE (EIA) OF THEIR OBSERVATIONS TOGETHER WITH LIMITED CHANNEL SELECTIONS WITH NO POLARIZATION DISCRIMINATION. GPROF V03 USED A MODELED MMF-BASED DATABASE FOR THE MHS (SEE KIDD ET AL. 2016A) WHILE LATER VERSIONS OF GPROF ADAPTED THE IMAGER-BASED DATABASE FOR MHS ATMS AND SAPHIR. THE SAPHIR PRODUCTS CEASED IN 2017 AT GPROF V03 DUE TO THE POOR PERFORMANCE OF THE RETRIEVALS. THE NEED FOR A USABLE PRECIPITATION RETRIEVAL SCHEME FOR THE SAPHIR INSTRUMENT HAS BEEN ADDRESSED BY THE DEVELOPMENT OF THE PRECIPITATION RETRIEVAL AND PROFILING SCHEME (PRPS). THE PRPS SCHEME EXPLOITS DPR-SENSOR MATCHUPS TO GENERATE A PRIORI DATABASES WITHOUT THE NEED OF DYNAMIC ANCILLARY DATA RESULTING IN A COMPUTATIONALLY EFFICIENT OF RETRIEVAL SCHEME. THIS SCHEME IS NOW IMPLEMENTED OPERATIONALLY BY THE NASA PPS WITH PRODUCTS AVAILABLE ONLINE FROM 2011 TO THE PRESENT (SEE KIDD 2018A; KIDD 2018B). THE PROPOSED RESEARCH WILL FURTHER DEVELOP THE PRPS APPROACH THROUGH: I) EXTENSION OF THE PRPS TO OTHER CROSS-TRACK SENSORS INCLUDING EXTENSION OF THE RETRIEVAL SCHEME TO NEW OR UNDERUSED SENSORS; II) IMPROVED DATABASE REPRESENTATION BOTH THROUGH AN INCREASED NUMBER OF ENTRIES IN THE OBSERVATIONAL DATABASE BUT ALSO THROUGH COMPLEMENTARY MODELED AND COMBINED MODEL/OBSERVATIONAL DATABASES; III) IMPROVED DATABASE ACCESS TO MAKE FULL USE OF MULTI-CHANNEL SENSOR INFORMATION; IV) VALIDATION AND VERIFICATION OF THE RESULTS TO IMPROVE ASSESSMENT OF THE ACCURACY OF THE RETRIEVALS OVER DIFFERENT REGIONS AND METEOROLOGICAL CONDITIONS. THE PROPOSED RESEARCH DIRECTLY ADDRESSES KEY ELEMENTS OF THIS SOLICITATION THROUGH THE DEVELOPMENT EVALUATION AND VALIDATION PRECIPITATION RETRIEVAL ALGORITHMS FOR RESEARCH AND OPERATIONAL SENSORS WITHIN THE GPM CONSTELLATION. IN PARTICULAR THE PROPOSED RESEARCH WILL ADVANCE THE ABILITY OF THE CROSS-TRACK INSTRUMENTATION TO PROVIDE RETRIEVALS INCLUDING LIGHT TO HEAVY RAINFALL AND SNOWFALL OVER ALL SURFACE TYPES USING AN OBSERVATIONAL AND PHYSICALLY-BASED RETRIEVAL SCHEME UTILIZING ALL AVAILABLE FREQUENCIES. CONSISTENT MULTI-SENSOR PRECIPITATION RETRIEVALS ARE ENSURED THROUGH THE INTER-COMPARISON OF BOTH THE OBSERVATIONAL AND MODELED A PRIORI DATABASES OF THE CONICALLY-SCANNING AND CROSS-TRACK RETRIEVAL SCHEMES. THE RESEARCH WILL ALSO PROVIDE QUANTITATIVE VALIDATION OF THE RESULTS USING NATIONAL AND INTERNATIONAL GROUND VALIDATION NETWORKS AND INTER-COMPARE THE RESULTS WITH OTHER GPM PRECIPITATION PRODUCTS TO PROVIDE A MEASURE OF RETRIEVAL ABILITY AND CONSISTENCY. ATTENTION WILL BE PLACED UPON THE PROPER REPRESENTATION OF PRECIPITATION ACROSS ALL REGIONS INCLUDING THAT AT THE MID AND HIGH LATITUDES WITH LIGHT RAINFALL AND SNOWFALL REGIMES.