Karl Heuerman

The Total Irradiance Monitor (TIM): Instrument Calibration

The calibrations of the SORCE Total Irradiance Monitor (TIM) are detailed and compared against the designed uncertainty budget. Several primary calibrations were accomplished in the laboratory before launch, including the aperture area, applied radiometer power, and radiometer absorption efficiency. Other parameters are calibrated or tracked on orbit, including the electronic servo system gain, the radiometer sensitivity to background thermal emission, and the degradation of radiometer efficiency. The as-designed uncertainty budget is refined with knowledge from the on-orbit performance.

Total solar irradiance data record accuracy and consistency improvements

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.

The TSI Radiometer Facility: absolute calibrations for total solar irradiance instruments

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.

A hyperspectral imager for high radiometric accuracy Earth climate studies

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.

Radiometric absolute accuracy improvements for imaging spectrometry with HySICS

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.

Short-Wave Instrument Development for CLARREO

Benchmarking Earth’s climate via remote sensing from space, as planned by CLARREO, requires radiometry with high absolute accuracy and SI-traceability. We present an on-orbit radiometric calibration approach for hyperspectral imaging from 300 to 2400 nm.

First results from the hyperspectral imager for climate science (HySICS)

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 NASAs 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.

Hyperspectral Radiometric Accuracy Improvements

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.

Calibration of the absorptance cavities for the spaceflight solar radiometer TIM

The Total Irradiance Monitor (TIM) is a total solar irradiance radiometer on NASAs 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 missions TIM instrument, and details the uncertainty budget for measuring these cavity reflectances.

Aperture edge scatter calibration of the cavity radiometers for the spaceflight Total Irradiance Monitor

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 NASAs 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 NASAs Glory/TIM instrument.