Mitigating a Likely Gap in Total Solar Irradiance Measurements Between the SORCE and TSIS Missions

Total Solar Irradiance (TSI), the measure of solar energy reaching the top of the Earth's atmosphere, is a critical component to understanding and monitoring Earth's climate system. The TSI Climate Data Record (CDR) began in 1978, and has been sustained by a variety of NASA satellites. Although four current missions measure solar irradiance, only the Total Irradiance Monitor (TIM) onboard NASA's SOlar Radiation and Climate Experiment (SORCE) mission can meet the accuracy requirements of climate applications. However, SORCE is well past its design life and is experiencing significant battery degradation. The NOAA-NASA Joint Polar-orbiting Program will sustain the TSI record using the Total Solar Irradiance Sensor (TSIS),however TSIS is not expected to launch until about 2016. This suggests there may be an observation gap between SORCE and TSIS, an event that would jeopardize the quality of the future TSI record since sensor-to-sensor overlap and calibration would be lost. To “bridge” the possibleobservationgap, NOAA is proposing an innovative solution: the Total Solar Irradiance (TSI) Calibration Transfer Experiment (TCTE) designed by the Laboratory for Atmospheric and Space Physics (LASP). TCTE would employ a sensor originally designedfor Space Station use to fly on U.S. Air Force's Space Test Program (STP) Mission 3, slated for launch in 2013. Because of STP's mission requirements, the TCTE sensor would uniquely view its target (the sun) on one orbit each day. If successful, TCTE will provide a NIST-traceable calibration link between SORCE and TSIS. This presentation describes the TSI monitoring concern, the options considered for bridging the likely gap, and NOAA's proposed solution for TSI gap mitigation – the TCTE mission.