Results from recent vacuum testing of an on-orbit absolute radiance standard (OARS) intended for the next generation of infrared remote sensing instruments

Abstract

Future NASA infrared remote sensing missions will require better absolute measurement accuracies than now available, and will most certainly rely on the emerging capability to fly SI traceable standards that provide irrefutable absolute measurement accuracy. To establish a CLARRREO-type climate benchmark, instrumentation will need to measure spectrally resolved infrared radiances with an absolute brightness temperature error of better than 0.1 K, verified onorbit. This will require an independent high-emissivity (>0.999) verification blackbody with an emissivity uncertainty of better than 0.06%, an absolute temperature uncertainty of better than 0.045K (3 sigma), and the capability of operation over a wide range of (Earth scene) temperatures. Key elements of an On-Orbit Absolute Radiance Standard (OARS) meeting these stringent requirements have been demonstrated in the laboratory at the University of Wisconsin and have undergone further refinement under funding from NASA's Earth Science and Technology Office, culminating in an endto-end demonstration under vacuum with a prototype climate benchmark instrument. We present the new technologies that underlie the OARS, and the results of testing that demonstrate the required accuracy is being met in a vacuum environment. The underlying technologies include: on-orbit absolute temperature calibration using the transient melt signatures of small quantities (<1g) of reference materials (gallium, water, and mercury) imbedded in the blackbody cavity; and on-orbit cavity spectral emissivity measurement using a carefully baffled heated halo placed in front of the OARS blackbody viewed by the infrared spectrometer system. Emissivity is calculated from the radiance measured from the blackbody combined with the knowledge of key temperatures and radiometric view factors.

Ground‐Based Far Infrared Emissivity Measurements Using the Absolute Radiance Interferometer

“…The Absolute Radiance Interferometer (ARI) is comprised of a Calibrated Fourier transform spectrometer (FTS) and an On-orbit Verification and Test System (OVTS) which provides end-to-end calibration verification (originally for satellite-based sensors) with direct traceability to absolute standards (Best et al, 2008(Best et al, , 2010(Best et al, , 2012(Best et al, , 2014Gero et al, 2009Gero et al, , 2010Gero et al, , 2012. Taylor et al (2020) (and references therein) provides an in-depth description of these components and the ARI design, assesses the radiometric uncertainty, provides a demonstration of performance, and discusses the calibration methodology.…”

The Infrared Absolute Radiance Interferometer (ARI) for CLARREO

“…The On-Orbit Absolute Radiance Standard (OARS) is a measurement standard that provides traceability to the SI, the International System of units [20][21][22][23]. It is a variable temperature (−50 to 50 • C), high emissivity (>0.998-0.999), cavity blackbody that serves as a standard radiance source based on the fundamental quantum mechanical theory of the Planck function.…”

“…Then the blackbody controller is switched into constant power mode using a power level that would bring the cavity to about 100 mK above the expected phase change temperature, and provide a melt duration of 6,500 seconds. If no gallium were present, the cavity would follow an The concept for using miniature phase transition cells to calibrate imbedded blackbody cavity thermistors in the ARI Prototype instrument is illustrated in Figure 18, which shows a typical transient temperature response from one of the blackbody cavity thermistors during a gallium (<30 mg) melt event [20][21][22][23]. At the start of the melting process, the blackbody cavity is brought to thermal stability in the constant temperature mode about 50 mK under the expected phase change temperature.…”

“…The Absolute Radiance Interferometer (ARI) consists of a Calibrated Fourier Transform Spectrometer (CFTS), calibrated on-orbit in the traditional way using an ambient onboard blackbody reference and a space view, and an On-orbit Verification and Test System (OVTS) that provides end-to-end calibration verification with direct on-orbit traceability to absolute standards. The OVTS consists of fundamentally new subsystems including the On-orbit Absolute Radiance Standard (OARS) [20][21][22][23], On-orbit Cavity Emissivity Modules (OCEMs) [24][25][26][27][28], and an On-orbit Spectral Response Module (OSRM). The ARI CFTS measures absolute spectrally resolved infrared radiance with a spectral coverage of 200-2000 cm −1 at 0.5 cm −1 spectral sampling (unapodized) and a radiometric measurement accuracy of <0.1 K (k = 3) brightness temperature at scene temperature.…”

Retrieving Decadal Climate Change from Satellite Radiance Observations—A 100-year CO2 Doubling OSSE Demonstration

“…CLARREO is essentially an International Radiometric Standard in space. The new technologies needed to make the highly accurate observations of infrared spectra for CLARREO have been developed and tested (Best, et al, [11]; Taylor et al, [12,13]; Gero et al, [14]). As described by Wielicki et al [9], the CLARREO Infrared spectrometer is designed to have a spectral range from 200-2000 cm −1 , with an unapodized spectral resolution of 0.5 cm −1 .…”