A hyperspectral imager for high radiometric accuracy Earth climate studies

Abstract

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 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° 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.

Achieving Climate Change Absolute Accuracy in Orbit

“…Figure 2a demonstrates how this is achieved for the thermal infrared interferometer, including independent deep cavity blackbodies with multiple phase change cells for temperature accuracy; an infrared quantum cascade laser to monitor blackbody emissivity as well as spectral response; multiple deep space views to verify polarization sensitivity; and a heated halo on the blackbody to independently verify blackbody emissivity (Anderson et al 2004;Dykema and Anderson 2006;Gero et al 2008Gero et al , 2012Best et al 2008). Figure 2b demonstrates the approach for the refl ected solar spectrometer and its use of the moon as a reference for stability in orbit, the sun with multiple attenuators to verify instrument nonlinearity of gain across the Earth viewing dynamic range, and the ability to directly scan deep space to verify instrument offsets (Espejo et al 2011;Fox et al 2011). Spectral response is verifi ed using solar spectral absorption line features.…”

“…The absolute accuracy of weather spectrometers such as the Atmospheric Infrared Sounder (AIRS), IASI, and CrIS ranges from 0.2 to 0.4 K (k = 2) (Hilton et al 2012;EUMETSAT 2011). For these instruments we rely on much weaker constraints for climate trends: instruments must typically overlap for a year or more (Loeb et al 2009), and we must assume instrument calibration stability (Ohring et al 2005;Ohring 2007).…”

Review of high fidelity imaging spectrometer design for remote sensing

“…An interesting solution for reducing polarization sensitivity in a TMA-Offner design has been shown in Ref. 65.…”

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

“…The instrumentperformance parameters are shown in Table 1, and a schematic of the optical layout, which is an evolution of that described by Espejo et al (2011), is shown in Fig. 1.…”