Search

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.

Microfabricated electrical substitution bolometers with vertically aligned carbon nanotube (VACNT) absorbers are being integrated into laboratory standards for optical laser power1 and CubeSats for solar irradiance mesurements2,3. We are building on these successes to extend the single bolometer technology to broadband microbolometer arrays with electrical substitution and VACNT absorbers. A linear array of radiometer elements has been fabricated. Each element is an uncooled bolometer with vanadium oxide (VOx) temperature measurement. Unlike existing VOx microbolometer technology where the thermistor film is incorporated into a narrow band cavity and doubles as the absorber, in these microbolometer arrays we use VACNT absorbers. Additionally, each element can be electrically heated to provide an absolute radiometric calibration through electrical substitution. The array provides two benefits for Earth Science applications that require spectral data across many wavelengths. The first is the electrical substitution calibration on every pixel. Existing instruments are typically calibrated on the ground but then subjected to severe launch and space environments. On-orbit calibration is achieved with a reference such as a blackbody, lamp, or reference material and by ground verification. On-orbit references increase mass and power and may be degraded by the space environment. This technology eliminates the need for on-orbit references by incorporating electrical substitution calibration. By comparing the temperature rise of the bolometer when illuminated to the temperature rise when a known current is injected into a heater of known resistance, a calibrated irradiance is measured. The second benefit is an extended spectral range. A typical microbolometer response is from 8 um to 15 um, but a VACNT absorber can extend the response to a wider 0.3 um to 100 um range. A broader spectral response in a single array can serve to reduce the number of detectors needed to span a broad wavelength range. The response above 20 µm improves measurement capabilities for applications such as Earth Radiation Budget. The combination of broadband spectral response and integrated calibration capability makes this single technology applicable to a broad range of scientific measurements. This talk will describe the design, fabrication, and performance of the microbolometers. To date we have achieved a noise floor of ~100 K/sqrt(Hz) at 0.1 Hz. We show the expected performance of one of these arrays incorporated with a small telescope is appropriate for use on a smallsat. Acknowledgements: This work is funded by the NASA Earth Science Technology Office. 1. M.G. White, et al., “Cryogenic primary standard for optical fibre power measurement”, Metrologia 55(5), 706-715 2018. 2. Erik Richard et al., “The compact spectral irradiance monitor flight demonstration mission”, Proc. SPIE 11131, CubeSats and SmallSats for Remote Sensing III, 1113105 (30 August 2019). 3. David Harber et al., “Compact total irradiance monitor flight demonstration”, Proc. SPIE 11131, CubeSats and SmallSats for Remote Sensing III, 111310D (30 August 2019).

In order to advance understanding of how natural and anthropogenic processes affect Earth’s climate it is critically important to maintain accurate, long-term records of climate forcing. These climate-data records are a time series of measurements of sufficient length, consistency, and continuity to determine true climate variability and change. Quantifying the solar irradiance (both total and spectral) provides the necessary constraint on the total energy input. In particular, the long-term, continuous measurements of solar spectral irradiance (SSI) are needed to characterize poorly understood wavelength-dependent climate processes. The strong reliance on radiative transfer modeling for interpretation and quantification of the deposition of solar radiation in the atmosphere makes it imperative that the spectral distribution of radiant energy entering the atmosphere be known to a high degree of absolute accuracy (tied to international standards). Major measurement challenges in quantifying the influence of SSI variability on climate are achieving sufficient radiometric absolute accuracy and then maintaining (on-orbit) the long-term relative accuracy of the data record. The Total and Spectral Solar Irradiance Sensor (TSIS) Spectral Irradiance Monitor (SIM) is the next generation, space-borne SSI monitor that will fly as part of the dual agency (NASA/NOAA) Joint Polar Satellite System (JPSS) program scheduled for launch in late 2016. The instrument has been designed, characterized and calibrated to achieve unprecedented levels of measurement accuracy (

The Total and Spectral Solar Irradiance Sensor (TSIS) Spectral Irradiance Monitor (SIM) is the next generation, space-borne SSI monitor that will fly as part of the joint agency (NASA/NOAA) Joint Polar Satellite System (JPSS) program scheduled for launch in late 2016. The instrument has been designed and calibrated to achieve unprecedented levels of measurement accuracy (less than 0.25%) and on-orbit stability required to meet the needs of establishing a complete solar spectral irradiance climate data record. The Spectral Radiometry Facility (SRF) at LASP was developed in order to validate the end-to-end performance of the SIM instrument. This facility includes a test chamber with a 5-axis manipulator that houses the SIM instrument. The illumination source is provided by a NIST Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) laser system. This laser system provides narrow tunable light from 210-2700 nm. The facility also includes a cryogenic radiometer with a precision aperture that is used to measure the absolute irradiance of the illumination light. This facility thus allows us to illuminate the instrument with a known irradiance, wavelength, and polarization. Additionally, the 5-axis manipulator permits us to test the pointing sensitivity and off-axis performance of the instrument. A series of detailed calibrations of the SIM instrument in this facility have allowed us to fully characterize the instrument wavelength scale, spectral response functions, pointing sensitivity, and radiometric accuracy to better than 0.25%.