BY ALMOST 4 ORDERS OF MAGNITUDE SOLAR IRRADIANCE IS THE PRIMARY SOURCE OF NATURAL FORCING OPERATING ON THE EARTH'S ATMOSPHERE. THERE ARE TWO CRITICAL ASPECTS TO THE MEASUREMENT OF SOLAR IRRADIANCE: 1) MEASURING THE SOLAR IRRADIANCE TRACEABLE TO INTERNATIONAL RADIOMETRIC STANDARDS AND 2) UNDERSTANDING AND CORRECTING LONG-TERM INSTRUMENT DEGRADATION FOR THE DETERMINATION OF FULL-SPECTRAL SOLAR VARIABILITY. FOR THE PLANNED DEPLOYMENT OF THE TOTAL SOLAR IRRADIANCE SENSOR (TSIS) ON THE INTERNATIONAL SPACE STATION SPECTRAL IRRADIANCE WILL BE MEASURED WITH THE NEXT GENERATION SPECTRAL IRRADIANCE MONITOR (SIM) BUILT ON HERITAGE AND LESSONS-LEARNED FROM THE SORCE SIM (SOLAR RADIATION AND CLIMATE EXPERIMENT). TSIS WILL BE LAUNCHED IN NOVEMBER OF 2017 WITH A PREFLIGHT CALIBRATION TRACEABLE TO SI STANDARDS THROUGH THE USAGE OF NIST SIRCUS (NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY SPECTRAL IRRADIANCE AND RADIANCE RESPONSIVITY CALIBRATIONS USING UNIFORM SOURCES). CONTEMPORANEOUS OBSERVATIONS FOR SORCE SIM AND SOLSTICE (SOLAR STELLAR IRRADIANCE COMPARISON EXPERIMENT) WITH TSIS SIM WILL PROVIDE ABOUT 1 FULL YEAR OF OVERLAP THAT WILL 1) PERMIT AN ABSOLUTE SCALE CORRECTION FOR THE SORCE SIM AND SOLSTICE INSTRUMENTS THAT WERE LAUNCHED WITH AN ABSOLUTE IRRADIANCE SCALE LESS ACCURATE THAN TSIS BY ALMOST A FACTOR OF 10. 2) THE 1-YEAR OVERLAP BETWEEN SORCE AND TSIS IS AN ADEQUATE LENGTH OF TIME FOR THE QUANTIFICATION OF WAVELENGTH DEPENDENT LONG-TERM STABILITY OF THESE INSTRUMENTS. THE WELL-DOCUMENTED AND BROADLY CITED METHODOLOGY ADVOCATED BY WEATHERHEAD ET AL. (1997&2017) WILL BE APPLIED TO ANALYZE THE DIFFERENCES IN ABSOLUTE SCALE LONG-TERM DRIFT AND POTENTIALLY DISCONTINUOUS JUMPS IN THE INSTRUMENT IRRADIANCE SCALE. THIS SAME INTERCOMPARISON METHOD CAN BE APPLIED TO THE CUBESAT VERSION OF SIM (COMPACT SIM OR CSIM) CURRENTLY UNDER DEVELOPMENT AND PLANNED FOR LAUNCH IN 2018 - WELL WITHIN THE TIME FRAME OF THIS SIST ALLOCATION. FOR THIS 3-YEAR GRANT WE PROPOSAL THE FOLLOWING PLAN OF ACTION: YEAR 1 ACTIVITY WILL SPECIFY THE DIFFERENCES IN ABSOLUTE SCALE BETWEEN THE SORCE AND TSIS INSTRUMENTS. IN THIS YEAR WE WILL ALSO TAKE ADVANTAGE OF THE HIGH FIDELITY ABSOLUTE CALIBRATION OF TSIS SIM TO RESOLVE THE CONTROVERSY OF THE ABSOLUTE IRRADIANCE AT THE 1.6-MICRON H-MINUS SOLAR OPACITY MINIMUM THAT IS CRUCIAL FOR THE UNDERSTANDING OF SOLAR VARIABILITY IN THE INFRARED CALIBRATION OF OTHER NASA EARTH-OBSERVING SATELLITE ASSETS AND EARTH SYSTEM MODELING. YEAR 2 ACTIVITY WILL CONCENTRATE ON THE DERIVATION OF SORCE AND TSIS LONG-TERM STABILITY ESTIMATES OVER THE FULL 200-2400 NM SPECTRAL RANGE. YEAR 3 ACTIVITY WILL BE TO CONSTRUCT A SOLAR CYCLE 23/24/25 IRRADIANCE COMPOSITE FROM SORCE/TSIS/ CSIM OBSERVATIONS THAT WILL PROVIDE A VALUABLE COMPOSITE WITH ANALYTICALLY DERIVED STABILITY ESTIMATES FROM A SET OF INSTRUMENTS WITH A COMMON CALIBRATION AND DESIGN HERITAGE. FINAL RESULTS WILL BE PRESENTED IN TWO PEER REVIEWED PUBLICATIONS IN YEARS 2 AND 3 AS WELL AS PRESENTATIONS AT PROFESSIONAL MEETINGS IN ALL THREE YEARS.