SI-traceable Spectral Irradiance Radiometric Characterization and Absolute Calibration of the TSIS-1 Spectral Irradiance Monitor (SIM)

Erik Richard,Tom Woods,Matthew Triplett,Odele Coddington,Joel Rutkowski,Ginger Drake,Dave Harber,Peter Pilewskie

doi:10.3390/rs12111818

Abstract

The current implementation for continuous, long-term solar spectral irradiance (SSI) monitoring is the Total and Spectral Solar Irradiance Sensor (TSIS-1) Spectral Irradiance Monitor (SIM) that began operations from the International Space Station (ISS) in March 2018 and nominally provides an SSI spectrum every 12 h. Advances in both instrument design and spectral irradiance calibration techniques have resulted in the TSIS-1 SIM achieving higher absolute accuracy than its predecessor instrument in the wavelength range (200–2400 nm). A comprehensive detector-based Spectral Radiometer Facility (SRF) was developed in collaboration with the US National Institute for Standards and Technology (NIST) to ensure the ties to spectral SI standards in power and irradiance. Traceability is achieved via direct laser calibration of a focal plane electrical substitution radiometer (ESR) against a cryogenic radiometer in power and also irradiance responsivity via calibrated apertures. The SIM accuracy definition followed an absolute sensor approach based on a full radiometric measurement equation where component-level performance characterizations and calibrations were quantified with an associated uncertainty error budget and verified by independent measurements for each parameter. Unit-level characterizations were completed over the full operational envelope of external driving factors (e.g., pointing and temperature ranges) and were allowed for the independent parameterization of sub-assembly performance for expected operating conditions. Validation and final instrument end-to-end absolute calibration in the Laboratory for Atmospheric and Space Physics (LASP)-SRF achieved low combined standard uncertainty (uc < 0.25%, k = 1) in spectral irradiance.

Highlights

Knowledge of the absolute solar spectral irradiance (SSI) and its temporal variation as a function of wavelength are critical to understanding the influence of the Sun’s output on the Earth’s atmosphere and climate [1,2]
For the Spectral Irradiance Monitor (SIM) instrument, we rely on a measurement equation approach that allows for the complete quantification of measurements by detailed characterization and calibration of the unit-level uncertainty contributions
In the Spectral Radiometer Facility (SRF), the L-1 cryogenic radiometer (CR) is coupled to a large vacuum enclosure that allows for direct comparison to the instrument

Summary

Introduction

Knowledge of the absolute solar spectral irradiance (SSI) and its temporal variation as a function of wavelength are critical to understanding the influence of the Sun’s output on the Earth’s atmosphere and climate [1,2]. The solar irradiance Environmental Data Record (EDR) requirements were originally specified in the NPOESS Integrated Operational Requirements Document (IORD) and updated in version II and in the NOAA Climate Sensors Project TSIS Requirements Document (see Table 1) [4,5]. Through these efforts, solar spectral irradiance (along with the total solar irradiance) is recognized as an essential climate variable to emphasize the importance of accurate, long-term measurements for understanding past and present climate and to distinguish it from the higher latency, lower accuracy requirements of operational weather (short-term) data [6,7]. Validation of the climate data record was only possible over ~ 1 month time scales for the ultraviolet through the mid-visible wavelengths implying that new satellite observations of sufficient accuracy and stability are necessary to better understand spectral irradiance variability from the visible through the near infrared and at all wavelengths over longer time scales

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