Radiometric flight results from the HyperSpectral Imager for Climate Science (HySICS)

Abstract

Abstract. 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 ~ 0.2 % radiometric accuracies of Earth scenes from space, providing an order-of-magnitude improvement over existing space-based 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 currently possible with any manmade spaceflight light-source or detector. Solar- and lunar-observations enable in-flight focal-plane-array 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.

Radiometric Stability of the SABER Instrument

“…SABER and all other instruments presently observing Earth from orbit do not carry onboard, absolute radiance standards that can be used to diagnose calibration changes over time, traceable to Système Internationale (SI) units. Two new instruments now in development, CLARREO Pathfinder (Wielicki et al, ; Shea et al, ; Kopp et al, ) and TRUTHS (Fox et al, ), are designed for measuring the small changes in top‐of‐atmosphere radiation associated with changes in tropospheric climate and carry onboard reference standards for detecting instrument calibration drifts for the express purpose of diagnosing and correcting instrument drifts so they do not impact the observation of trends.…”

An Entropy Framework for Evaluating Reflectance Observations for Climate Studies

“…There are several directions to pursue further work. These studies can be extended to consider spectrally dependent instrument uncertainty (Kopp et al., 2017) and more specific uncertainty budgets, with uncertainty contributions from various random and systematic error sources added in quadrature. Additionally time‐varying instrument artifacts can be considered by modifying the uncertainty level over the time considered, depending on the stability the study is intended to simulate.…”

“…This study also assumes that the instrument is stable within this uncertainty throughout the climate‐relevant time periods considered. Although for a real instrument, there may be additional contributions to the measurement uncertainty, changes in that uncertainty over time, or a spectral dependence of that uncertainty (Kopp et al., 2017), this general consideration of the uncertainty serves to demonstrate the capability of our technique.…”

“…To detect long‐term secular climate trends, highly accurate, stable measurements and the ability to monitor and correct for temporal changes in instrument uncertainty are essential (Kopp et al., 2017; Weatherhead et al., 2018; Wielicki et al., 2013). The need for high accuracy is illustrated, in part, by the small interannual variability of shortwave Earth reflectance (Jin et al., 2014).…”

Direct Solar Viewing Calibration Concept for Future CERES-, GERB-, or Libera-Type Earth Orbital Climate Missions

“…However, it was mentioned earlier that the difficulty of using it to better calibrate Earth-viewing radiometers, is that solar radiance is near five orders of magnitude greater than that leaving Earth. With the units of spectral radiance being J s 21 m 22 mm 21 sr 21 , most nextgeneration concepts have concentrated on attenuating the time and space units, by reducing detector integration time in seconds, and great spatial attenuation using pinholes or slits [i.e., reducing seconds, and area in m 2 as in Kopp et al (2017)]. This proposed concept does not attenuate in space as significantly, or time at all, but instead it does it in wavelength units of mm.…”