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We present the results of a recent, extensive measurement campaign validating the traceability of the solar irradiance record and earth radiation budget data. The campaign also established future traceability, thus ensuring confidence in the continuing climate-data record. The Total Solar Irradiance Radiometer Facility (TRF) at the Laboratory for Atmospheric and Space Physics (LASP) Boulder, uses a liquid helium cooled cryogenic radiometer as the reference standard for the validation of spaceflight total solar irradiance instrumentation. In 2008 the radiometer was directly compared to the National Institute of Standards and Technology (NIST) Primary Optical Watt Radiometer (POWR) at a wavelength of 532.12 nm. At total solar irradiance power levels, a correction factor of 1.000306 ± 0.000098 (k=1) was reported for the TRF radiometer scale when using external voltage measurement electronics, and not correcting for cavity heating non-equivalence or cavity absorptance. The TRF radiometer has recently been revalidated at LASP using a POWR calibrated silicon photodiode trap transfer standard named TT4. We report a correction factor of 0.999970 ± 0.000294 (k=1) to align the TRF radiometer scale with the current NIST POWR scale. A new room temperature reference standard radiometer NACR, was established. It measured 49 ppm lower than POWR using the same silicon transfer standard as above, and in a separate direct measurement, 15 ppm higher than the TRF radiometer at a shutter modulation of 200 secs. The difference is in agreement with stated uncertainties. A correction of 1.000049 ± 0.000313 (k=1) will align the new radiometer scale with the NIST radiant power scale of POWR. All measurements were made in power mode, whereby the radiometer apertures were underfilled. The presentation will discuss the experimental methodology and results of the comparisons.

Supplemental Materials (Figures S1-S4)

Current uncertainty requirements for the measurement of global variables relevant to climate change modeling in the reflected solar range are 0.5 % to 1 % for solar and lunar spectral irradiance and 0.5 % to 2 % for Earth reflected radiance. Future uncertainty requirements, as defined by CLARREO mission requirements, include laboratory calibration uncertainties are 0.2 % over the 500 nm to 900 nm spectral range and 1 % or less over the rest of the spectral region from 320 nm to 2300 nm. The uncertainties in lamp-illuminated integrating spheres, a principal calibration artifact, ranging from 2% to 3 % in the visible to near-infrared and 3 % to 5 % in the short-wave infrared, do not meet either current or future uncertainty requirements. NIST uncertainties in broad-band lamp-based calibration artifacts are traceable to primary standard blackbodies. The irradiance and radiance responsivity scales disseminated by the NIST laser-based SIRCUS calibration facility are held by primary standard irradiance meters traceable to the Primary Optical Watt Radiometer and dimensional metrology from the NIST Aperture Area Facility. The SIRCUS facility has demonstrated that moving from source-based scales traceable to primary standard blackbodies to detector-based scales offer opportunities to reduce the uncertainties in disseminated standards. This presentation discusses existing primary radiometric standards and the uncertainties in detector-based radiance and irradiance scales, validation of their uncertainty budgets through measurements of primary standard metal melting point blackbodies, development of low temperature active cavity radiometers operating in irradiance mode, and the development of transfer standard spectrographs. Finally, this paper discusses possible further developments at NIST aimed at supporting future climate benchmark instrument uncertainty requirements.

During NASA’s Earth Observing System-era, a series of source radiance validation campaigns were planned and executed by the EOS Project Office with the goal of validating the radiances assigned to laboratory calibration sources, principally lamp-illuminated integrating spheres, and establishing uncertainty budgets for the disseminated radiance scale. Based on an analysis of 7 years’ worth of data, Butler et al.1 assigned an uncertainty in disseminated radiance scales of 2% to 3% in the Vis/NIR (silicon) region, increasing to 5% in the short-wave infrared region. The uncertainty in the radiance scale met EOS requirements. For example, the radiance uncertainty requirement for MODIS was 5% in the Vis/NIR spectral region and the uncertainty in disseminated radiance scales met or exceeded sensor calibration requirements. Looking to the future, uncertainty requirements for radiance are reduced below 1% [2]. Uncertainties in the radiance scale in disseminated sources from NIST cannot be reduced and alternate approaches need to be considered to meet future satellite sensor uncertainty requirements. In this presentation, the radiometric characterization and calibration of a spectrograph along with its uncertainty budget are discussed. Its long-term stability is presented and its potential use as a transfer standard radiometer is discussed. The combined standard uncertainty in the spectrograph responsivity is estimated to be 0.25% (k=1), representing a potential order of magnitude reduction in the uncertainty in the radiance of laboratory calibration sources. [1] Butler, J. J., et al., Validation of radiometric standards for the laboratory calibration of reflected-solar Earth observing satellite instruments, Proc. SPIE 6677, 667707 (2007). [2] Ohring, G., et al. (eds.), Satellite Instrument Calibration for Measuring Global Climate Change, NISTIR 7047 (2004).

Laser-based laboratory calibrations in the facility for Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (SIRCUS) of the National Institute of Standards and Technology (NIST) have achieved detector calibration with uncertainties less than 0.1 % in the silicon spectral region. Because of the high power of lasers, there is also a drastic increase in dynamic range covering more than 5 orders of magnitude. The low measurement uncertainty and high dynamic range allow accurate assessment of out-of-band performance for flight instruments such as the Suomi National Polar-orbiting Partnership (NPP) Visible and Infrared Imager Radiometer Suite (VIIRS) sensor which was calibrated in 2010 for Absolute Spectral Responsivity (ASR) with full aperture illumination using a tunable laser and a large integrating sphere with an approximately 0.5% uncertainty in radiance responsivity. With the calibration data, it is now possible to fully evaluate the effect of out-of-band (OOB) contribution on at-sensor water-leaving radiance and derive strategy to alleviate variations from OOB scattering and reduce data product uncertainties. In this work, we present a sensitivity analysis of the Top-of-Atmosphere (TOA) measurement for varying chlorophyll concentrations and column water vapor based on measured detector Relative Spectral Responsivity (RSR). A conventional band-averaged radiance approach is taken and histograms are presented for different bands of radiometers to illustrate the data uncertainty from these variations as well as the global seasonal differences. We show how conventional lamp calibration at ground and on board solar calibration resulted in large deviation from OOB scattering because of mismatch with ocean color spectra. We will also present similar analysis for the detectors of the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS).