The long-term balance between Earth’s absorption of solar energy and emission of radiation to space is a fundamental climate measurement. Total solar irradiance (TSI) has been measured from space, uninterrupted, for the past 40 years via a series of instruments. The Compact Total Irradiance Monitor (CTIM) is a CubeSat instrument that will demonstrate next-generation technology for monitoring total solar irradiance. It includes novel silicon-substrate room temperature vertically aligned carbon nanotube (VACNT) bolometers. The CTIM, an eight-channel 6U CubeSat instrument, is being built for a target launch date in late 2020. The basic design is similar to the SORCE, TCTE and TSIS Total Irradiance Monitors (TIM). Like TSIS TIM, it will measure the total irradiance of the Sun with an uncertainty of 0.0097% and a stability of <0.001%/year. The underlying technology, including the silicon substrate VACNT bolometers, has been demonstrated at the prototype-level. During 2019 we will build and test an engineering model of the detector subsystem. Following the testing of the engineering detector subsystem, we will build a flight detector unit and integrate it with a 6U CubeSat bus during late 2019 and 2020, in preparation for an on-orbit demonstration in 2021.
Greg Kopp, Chris Belting, Zach Castleman, Ginger Drake, Joey Espejo, Karl Heuerman, Bret Lamprecht, James Lanzi, Paul Smith, David Stuchlik, Bill Vermeer
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 10× 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 HySICS’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.
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% (1σ). A solar calibration requires viewing scenes having radiances 105 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° 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.
The total solar irradiance (TSI) climate data record includes overlapping measurements from 10 spaceborne radiometers.
The continuity of this climate data record is essential for detecting potential long-term solar fluctuations, as offsets
between different instruments generally exceed the stated instrument uncertainties. The risk of loss of continuity in this
nearly 30-year record drives the need for future instruments with <0.01% uncertainty on a absolute scale. No facility
currently exists to calibrate a TSI instrument end-to-end for irradiance at solar power levels to these needed accuracy
levels. The new TSI Radiometer Facility (TRF) is intended to provide such calibrations. Based on a cryogenic
radiometer with a uniform input light source of solar irradiance power levels, the TRF allows direct comparisons
between a TSI instrument and a reference cryogenic radiometer viewing the same light beam in a common vacuum
system. We describe here the details of this facility designed to achieve 0.01% absolute accuracy.
Aperture area knowledge is a primary calibration in radiometric instruments. Corrections for edge effects, particularly
diffraction and scatter, must also be taken into account for high accuracy measurements. The Total Irradiance Monitor
(TIM) is a total solar irradiance radiometer on NASA's SORCE mission launched in 2003 and on the NASA/Glory
mission launching in 2008. In order to measure irradiance, the TIM instrument measures the total optical power that
passes through circular diamond-turned precision apertures. The geometric areas of the 8-mm diameter apertures are
measured to approximately 25 parts per million (ppm) at the National Institute of Standards and Technology [1]. Due to
scatter and diffraction, not all light that passes through the geometric area of an aperture will enter the radiometer cavity
of the instrument, and corrections must be made for these edge effects. Diffraction effects are generally well understood
and are calculated from the instrument geometry. Scatter, on the other hand, is dependent on the microscopic edge
quality of each individual aperture, and so must be measured. This paper describes the measurement of aperture edge
diffraction and scatter for the precision apertures on NASA's Glory/TIM instrument.
The Total Irradiance Monitor (TIM) is a total solar irradiance radiometer on NASA's SORCE mission launched in 2003 and on the NASA/Glory mission launching in 2008. The primary sensors in TIM must absorb energy with accurately calibrated efficiency across the entire solar spectrum. To achieve high efficiency and good thermal conduction, the four sensors in each instrument are hollow conical silver cavities with a cylindrical entrance extension and a diffuse black nickel phosphorous (NiP) interior that converts absorbed incident radiation to thermal energy. A stable resistive heater wire embedded in the cone along with thermistors mounted on the cavity exterior are used in a temperature-sensing servo loop to measure the spectrally-integrated incident solar radiation. Characterization of the absorptance properties of the cavities across the solar spectrum is a dominant driver of instrument accuracy, and a dedicated facility has been developed to acquire these calibrations with uncertainties of approximately 50 ppm (0.005%). This paper describes the absorptance calibration facility, presents the preliminary cavity reflectance results for the Glory mission's TIM instrument, and details the uncertainty budget for measuring these cavity reflectances.
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