The total solar irradiance (TSI) is Earth’s primary source of energy, and accurate knowledge of its value and variability is crucial for understanding Earth’s climate and variability. In order to continue the existing 44 year data record of TSI measurements from space, NASA is developing the Total and Spectral Irradiance Sensors (TSIS) -2 mission. TSIS-2 consists of the Total Irradiance Monitor (TIM) and Spectral Irradiance Monitor (SIM) on a free flyer satellite, with an anticipated launch in the latter half of 2024. The TSIS-2/TIM is the latest iteration of the TIM instrument, prior versions of which flew onboard the SORCE, TCTE and TSIS-1 missions, and a direct rebuild of the TSIS-1 instrument. We present the pre-flight ground calibration of the TSIS-2/TIM instrument and its uncertainties. A key difference between the calibrations of the TSIS-1 and TSIS-2 instruments is the use of a novel low noise ambient temperature radiometer for TSIS-2 that significantly reduces the uncertainty in validating the component level calibrations through an end-to-end measurement. We compare component level (e.g. aperture area, detector reflectance, etc.) measurements and uncertainties for TSIS-2 with those from TSIS-1, and focus on areas where the uncertainty analysis differs from that applied to TIM instruments on prior missions and the implications of these differences.

The Project for On-Board Autonomy-3 (PROBA-3) is the fourth satellite technology development and demonstration precursor mission within ESA's GSTP (General Support Technology Program) series. The primary mission objective is to demonstrate the technologies required for formation flying of multiple spacecrafts. The PROBA-3 mission concept comprises two independent minisatellites in highly-elliptical Earth orbits in precise formation flying, close to one another with the ability to accurately control the attitude and separation of the two satellites. The mission launch is scheduled for end of 2023.PROBA-3 mission consists of a coronograph spacecraft, hosting the coronograph APIICS, and the occulter spacecraft with the Digital Absolute RAdiometer (DARA). The radiometer to record total solar irradiance is mounted on the front satellite pointing to the Sun. DARA is developed and manufactured in Switzerland by the PMOD/WRC. We have done two pre-flight calibration campaigns: one at the World Radiation Center in Davos, Switzerland, and one at the Total Solar Irradiance (TSI) Radiometer Facility of the Laboratory for Atmospheric and Space Physics in Boulder Colorado, USA. We report on the results of the laboratory comparisons and discuss uncertainties of several instrument parameters, which are used to transform the raw measurements, which are voltage and current, into solar irradiance values.

The Project for On-Board Autonomy-3 (PROBA-3) is the fourth satellite technology development and demonstration precursor mission within ESA's GSTP (General Support Technology Program) series. The primary mission objective is to demonstrate the technologies required for formation flying of multiple spacecrafts. The PROBA-3 mission concept comprises two independent minisatellites in highly-elliptical Earth orbits in precise formation flying, close to one another with the ability to accurately control the attitude and separation of the two satellites. The mission launch is scheduled for end of 2023.PROBA-3 mission consists of a coronograph spacecraft, hosting the coronograph APIICS, and the occulter spacecraft with the Digital Absolute RAdiometer (DARA).  The radiometer to record total solar irradiance is mounted on the front satellite pointing to the Sun. DARA is  developed and manufactured in Switzerland by the PMOD/WRC. We have done two pre-flight calibration campaigns: one at the World Radiation Center in Davos, Switzerland, and one at the Total Solar Irradiance (TSI) Radiometer Facility of the Laboratory for Atmospheric and Space Physics in Boulder Colorado, USA. We report on the results of the laboratory comparisons and discuss uncertainties of several instrument parameters, which are used to transform the raw measurements, which are voltage and current, into solar irradiance values.

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 (TSI) 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 TSI power levels, a correction factor of 1.000 306 with an associated standard uncertainty (u) of 0.000 098, 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 0.999 787, u = 0.000 285 to align the TRF radiometer scale with the current NIST POWR scale. A new room temperature reference standard radiometer was established. It measured 133 parts per million (ppm) higher than POWR using the same silicon transfer standard as above, and in a separate direct measurement, 168 ppm lower than the TRF radiometer shuttered at 400 s full shutter cycle. The difference agrees within stated uncertainties. A correction of 0.999 867, u = 0.000 247 will align the new radiometer scale with the NIST radiant power scale of POWR.

Multi-wall vertically aligned carbon nanotubes (VACNTs) are nearly ideal absorbers due to their exceptionally low reflectance over a broad wavelength range. Integrating VACNTs as bolometer absorbers, however, can be difficult due to their high growth temperature and fragile nature. Despite these challenges, we have microfabricated many different types of VACNT bolometers, ranging from cryogenic optical power primary standards to room temperature satellite-based solar irradiance monitors and broadband infrared microbolometers. Advantages our VACNT bolometers provide over the bolometers they replace vary by application, but can be reduced size and time constant, increased absorption, and/or microfabrication instead of hand assembly. Depending on the application and operating conditions, our VACNT bolometers are designed with a variety of thermistors and weak thermal links. The thermistors used include commercial surface mount chips, superconducting transition-edge sensors, and vanadium oxide (VOx). Weak thermal links include silicon nitride (SiNx) membranes, Si bridges, and laser-cut polyimide. We summarize a wide variety of microfabricated bolometers with VACNT absorbers that measure optical power levels spanning over seven orders of magnitude.

The uninterrupted, 41-year-long, spaceborne total solar irradiance (TSI) record has recently undergone several changes in the instruments contributing to these measurements of the net incoming radiant energy providing nearly all the power driving the Earth’s climate system. Two long-term instruments, NASA’s SORCE/TIM and TCTE/TIM, have recently been powered off. This ends the 17-year record from the SORCE/TIM, which established the currently-accepted TSI value of 1361 W m‑2 after its launch in 2003. ESA’s SoHO/VIRGO continues to acquire measurements that extend its 24-year record, but data availability has been on hold as a new processing methodology is implemented. NASA’s recently-launched TSIS‑1/TIM is presently continuing the measurements of these stalwart legacy instruments. This new TSI instrument is demonstrating higher on-orbit accuracy than any prior such instrument has achieved, with daily measurement updates that are available to the community for climate- and solar-research purposes. I will discuss the many recent changes to the spaceborne TSI measurement record, the current measurement-accuracy improvements and stabilities achieved and their implications for Earth energy-balance studies, and the future plans to maintain measurement continuity.

Long-term monitoring of the Earth-reflected solar spectrum is necessary for discerning and attributing changes in climate. High radiometric accuracy enables such monitoring over decadal timescales with non-overlapping instruments, and high precision enables trend detection on shorter timescales. The HyperSpectral Imager for Climate Science (HySICS) is a visible and near-infrared spatial/spectral imaging spectrometer intended to ultimately achieve similar to 0.2% radiometric accuracies of Earth scenes from space, providing an order-of-magnitude improvement over existing spacebased imagers. On-orbit calibrations from measurements of spectral solar irradiances acquired by direct views of the Sun enable radiometric calibrations with superior long-term stability than is currently possible with any manmade spaceflight light source or detector. Solar and lunar observations enable in-flight focal-plane array (FPA) flat-fielding and other instrument calibrations. The HySICS has demonstrated this solar cross-calibration technique for future spaceflight instrumentation via two high-altitude balloon flights. The second of these two flights acquired high-radiometric-accuracy measurements of the ground, clouds, the Earth's limb, and the Moon. Those results and the details of the uncertainty analyses of those flight data are described.

Vertically aligned carbon nanotubes (VACNTs) are excellent broadband (UV–VIS–IR) absorbers of light that can be made even darker with plasma treatments. Modification of VACNTs using O2 and/or CF4 plasmas is shown to have a significant impact on the reflectance and water repellency. Unfortunately, while O2 plasma treatment reduces the reflectance of VACNTs by changing the top surface morphology, it also makes the VACNTs superhydrophilic. Using an additional CF4 plasma treatment, we show that low reflectance and superhydrophobic surfaces are possible — qualities that are desirable when utilizing VACNTs as black body absorbers in cryogenic environments due to the potential for adsorbed water or ice. Using scanning electron microscopy we show that both plasma treatments can change the surface morphology of the VACNTs similarly, which is associated with a corresponding measured reduction in spectral reflectance. Raman spectroscopy of as-grown and plasma treated samples suggest that plasma treatment is introducing defects and functionalizing the nanotube walls and thereby affecting the water repellency. O2 plasma treated VACNTs have a directional-hemispherical reflectance (d/h) at normal incidence (d = 0°) of 94 ± 4 ppm (with a coverage factor of k = 2) at 660 nm and are superhydrophilic. O2 and CF4 plasma treated VACNTs have a 0°/h reflectance of 163 ± 7 ppm (k = 2) at 660 nm and are superhydrophobic with a contact angle of 159°.

The compact lightweight absolute radiometer (CLARA) experiment aims at measuring the total solar irradiance (TSI) in space and is scheduled to fly on the Norwegian NORSAT-1 micro satellite. The CLARA experiment will contribute to the long term monitoring of the TSI variability to support the analysis of potential long term trends in the Sun's variability. CLARA is traceable to the National Institute of Standards and Technology radiometric scale and will provide further evidence for the TSI value on an absolute scale. In this paper we present the design, characterization, and calibration details of the CLARA instrument. The combined measurement uncertainty for the calibrated SI-traceable CLARA flight instrument is 567-912 ppm (k = 1) depending on the measuring channel.

Using on-orbit solar cross calibrations, the HyperSpectral Imager for Climate Science improves radiometric accuracy of measured Earth scenes to <0.5%, helping establish benchmark measurements for space-borne climate studies in the 350–2300 nm spectral range.

The 2007 National Research Council Decadal Survey for Earth Science identified needed measurements to improve understanding of the Earth's climate system, recommending acquiring Earth spectral radiances with an unprecedented 0.2% absolute radiometric accuracy to track long-term climate change and to improve climate models and predictions. Current space-based imagers have radiometric uncertainties of 2% or higher limited by the high degradation uncertainties of onboard solar diffusers or calibration lamps or by vicarious ground scenes viewed through the Earth's atmosphere.The HyperSpectral Imager for Climate Science (HySICS) is a spatial/spectral imaging spectrometer with an emphasis on radiometric accuracy for such long-term climate studies based on Earth-reflected visible and near-infrared radiances. The HySICS's accuracy is provided by direct views of the Sun, which is more stable and better characterized than traditional flight calibration sources. Two high-altitude balloon flights provided by NASA's Wallops Flight Facility and NASA's Columbia Scientific Balloon Facility are intended to demonstrate the instrument's 10x improvement in radiometric accuracy over existing instruments. We present the results of the first of these flights, during which measurements of the Sun, Earth, and lunar crescent were acquired from 37 km altitude.Covering the entire 350-2300 nm spectral region needed for shortwave Earth remote sensing with the Hy SICS's single, flight-heritage detector array promises mass, cost, and size advantages for eventual space- and air-borne missions. A 6 nm spectral resolution with a 0.5 km spatial resolution from low Earth orbit helps in determinations of atmospheric composition, land usage, vegetation, and ocean color.

The HyperSpectral Imager for Climate Science (HySICS) is a spatial/spectral spectrometer for viewing Earth scenes with the ~0.2% (1-σ) radiometric accuracy needed for climate studies. Covering the reflected solar spectral region from 350 to 2300 nm with 6 nm resolution, this instrument will provide 0.5 km spatial resolution while covering a 100 km ground swath from low Earth orbit. A single focal plane array spans the entire spectral region, allowing for reduced mass, volume, and complexity for space flight applications compared to instrument designs with separate spectral regions.

Continuity of the 33-year long total solar irradiance record has been facilitated by corrections for offsets due to calibration differences between instruments, providing a solar data record with precision approaching that needed for Earth climate studies. Recent laboratory tests have (1) improved measurement absolute accuracy to mitigate potential future data gaps, (2) helped explain the causes of instrument offsets and (3) improved consistency between the international references upon which various instrument calibrations are based.

We demonstrate a visible and near-infrared prototype pushbroom hyperspectral imager for Earth climate studies that is capable of using direct solar viewing for on-orbit cross calibration and degradation tracking. Direct calibration to solar spectral irradiances allow the Earth-viewing instrument to achieve required climate-driven absolute radiometric accuracies of <0.2% (1 sigma). A solar calibration requires viewing scenes having radiances 10(5) higher than typical Earth scenes. To facilitate this calibration, the instrument features an attenuation system that uses an optimized combination of different precision aperture sizes, neutral density filters, and variable integration timing for Earth and solar viewing. The optical system consists of a three-mirror anastigmat telescope and an Offner spectrometer. The as-built system has a 12.2 degrees cross track field of view with 3 arcmin spatial resolution and covers a 350-1050 nm spectral range with 10 nm resolution. A polarization compensated configuration using the Offner in an out of plane alignment is demonstrated as a viable approach to minimizing polarization sensitivity. The mechanical design takes advantage of relaxed tolerances in the optical design by using rigid, non-adjustable diamond-turned tabs for optical mount locating surfaces. We show that this approach achieves the required optical performance. A prototype spaceflight unit is also demonstrated to prove the applicability of these solar cross calibration methods to on-orbit environments. This unit is evaluated for optical performance prior to and after GEVS shake, thermal vacuum, and lifecycle tests.

All of the energy tracked by the GEWEX Radiative Flux Assessment and the driving energy for Earth climate is incident at the top of the Earth’s atmosphere as solar radiation. The total solar irradiance (TSI) has been monitored continually for over 30 years from space. Continuity of these measurements has enabled the creation of composite time series from which the radiative forcing inputs to climate models are derived and solar forcing sensitivities are determined.