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

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.

An Improved Solar Spectral Irradiance Composite Record

“…In particular, SORCE SIM shows more in-phase solar cycle variability in the NUV and more out-of-phase solar cycle variability in the NIR Fontenla et al, 2011;Woods et al, 2018). At the heart of those differences is The new TSIS-1 SIM instrument is experiencing much less degradation than SORCE SIM due to instrument improvements (Richard et al, 2020). TSIS-1 SIM also has three channels,…”

“…The measurement precision and limited long-term stability of the original SORCE SIM design with only two channels have affected the uncertainty for the model fits employed to develop the SSI3 composite. The lessons learned from SORCE SIM have led to improvements for TSIS-1 SIM (Richard et al, 2020) and for the Compact SIM (CSIM) cubesat mission (Richard et al, 2019).…”

“…The OMI long‐term precision (as opposed to accuracy) is about 0.1%–0.2% (Marchenko & DeLand, 2014). The new SIM observations from the Total and Spectral Solar Irradiance Sensor (TSIS‐1) mission are considered the most accurate SSI measurements to date with a total uncertainty of about 0.3% (Richard, Harber, Coddington, et al., 2020). Therefore, the first‐light TSIS‐1 SIM spectrum from March 2018 is used as the reference spectrum to first adjust (offset) the SORCE SIM data, and then the SORCE SIM adjusted data are used in the SSI3 composite for the 500–1,600 nm range.…”

Quantifying the Impact of Solar Spectra on the Inter-Calibration of Satellite Instruments

“…Of primary importance for a reference solar irradiance spectrum are the order-of-magnitude reductions in radiometric accuracy uncertainty achieved relative to SORCE SIM. As described in Richard et al [70], these advances were achieved through an extensive component level calibration program performed in a first-of-its-kind spectral radiometric calibration facility. This calibration program allowed for characterizing the instrument as an absolute sensor across the spectrum in irradiance units using a NIST cryogenic radiometer traceable to Système International (SI) units and stable tunable laser sources.…”

“…This calibration program allowed for characterizing the instrument as an absolute sensor across the spectrum in irradiance units using a NIST cryogenic radiometer traceable to Système International (SI) units and stable tunable laser sources. An instrument-level verification of the irradiance accuracy across the spectrum and final end-to-end absolute calibration placed relative pre-launch accuracy uncertainties at 0.24% (>460 nm) to 0.41% (<460 nm) [70]. Observations made by redundant and independent instrument channels that are exposed to the Sun at varying duty cycles are used to monitor and correct instrument degradation to maintain calibration stability on-orbit.…”