Abstract
The NIST Advanced Radiometer (NISTAR) that resides on the Deep Space Climate Observatory (DSCOVR) has been measuring the irradiance from the sun-lit earth in 3 bands for more than 3 years while orbiting the L1 Lagrange point between the Earth and Sun. The bands measure the outgoing total and reflected-solar radiation from the earth at a limited range of solar angles. These measurements assist in answering questions of the earth radiation imbalance and future climate change. The NISTAR absolute radiometric uncertainty goal is less-than 1.5%, which is very challenging and requires a rigorous ground calibration and determination of the on-orbit degradation of the instrument. To this end, the NISTAR radiometers were designed for maximum stability and were calibrated and characterized extensively in the laboratory on the ground at state-of-the art facilities—in 2010 at NIST and most recently in 2013 at L-1 Standards and Technology. Some questions have been raised concerning the accuracy of the short-wave channel due a difference between it and CERES derived models. The difference is significantly larger than the NISTAR measurement uncertainty and has necessitated a re-examination of potential measurement errors. The investigation has indicated two corrections to the NISTAR on-orbit radiometric scale. However, a significant difference between the NISTAR SW channel and CERES derived models remain.
Analysis of the NISTAR On-Orbit Absolute Radiometric Scale
The NIST Advanced Radiometer (NISTAR) that resides on the Deep Space Climate Observatory (DSCOVR) has been measuring the irradiance from the sun-lit earth in 3 bands for more than 3 years while orbiting the L1 Lagrange point between the Earth and Sun. The bands measure the outgoing total and reflected-solar radiation from the earth at a limited range of solar angles. These measurements assist in answering questions of the earth radiation imbalance and future climate change. The NISTAR absolute radiometric uncertainty goal is less-than 1.5%, which is very challenging and requires a rigorous ground calibration and determination of the on-orbit degradation of the instrument. To this end, the NISTAR radiometers were designed for maximum stability and were calibrated and characterized extensively in the laboratory on the ground at state-of-the art facilities—in 2010 at NIST and most recently in 2013 at L-1 Standards and Technology. Some questions have been raised concerning the accuracy of the short-wave channel due a difference between it and CERES derived models. The difference is significantly larger than the NISTAR measurement uncertainty and has necessitated a re-examination of potential measurement errors. The investigation has indicated two corrections to the NISTAR on-orbit radiometric scale. However, a significant difference between the NISTAR SW channel and CERES derived models remain.