P. Jonathan Gero

A Blackbody Design for SI-Traceable Radiometry for Earth Observation

Abstract Spaceborne measurements pinned to international standards are needed to monitor the earth’s climate, quantify human influence thereon, and test forecasts of future climate change. The International System of Units (SI, from the French for Systeme International d’Unites) provides ideal measurement standards for radiometry as they can be realized anywhere, at any time in the future. The challenge is to credibly prove on-orbit accuracy at a claimed level against these international standards. The most accurate measurements of thermal infrared spectra are achieved with blackbody-based calibration. Thus, SI-traceability is obtained through the kelvin scale, making thermometry the foundation for on-orbit SI-traceable spectral infrared measurements. Thermodynamic phase transitions are well established as reproducible temperature standards and form the basis of the international practical temperature scale (International Temperature Scale of 1990, ITS-90). Appropriate phase transitions are known in the t...

Long-Term Trends in Downwelling Spectral Infrared Radiance over the U.S. Southern Great Plains

AbstractA trend analysis was applied to a 14-yr time series of downwelling spectral infrared radiance observations from the Atmospheric Emitted Radiance Interferometer (AERI) located at the Atmospheric Radiation Measurement Program (ARM) site in the U.S. Southern Great Plains. The highly accurate calibration of the AERI instrument, performed every 10 min, ensures that any statistically significant trend in the observed data over this time can be attributed to changes in the atmospheric properties and composition, and not to changes in the sensitivity or responsivity of the instrument. The measured infrared spectra, numbering more than 800 000, were classified as clear-sky, thin cloud, and thick cloud scenes using a neural network method. The AERI data record demonstrates that the downwelling infrared radiance is decreasing over this 14-yr period in the winter, summer, and autumn seasons but it is increasing in the spring; these trends are statistically significant and are primarily due to long-term change...

A Quantum Cascade Laser–Based Reflectometer for On-Orbit Blackbody Cavity Monitoring

Abstract Satellite measurements pinned to international standards are needed to monitor the earth’s climate, quantify human influence thereon, and test forecasts of future climate change. Credible observations require that measurement uncertainties be evaluated on orbit during a mission’s operational lifetime. The most accurate spaceborne measurements of thermal infrared radiance are achieved with blackbody calibration. The physical properties of blackbody cavity surface coatings are known to change upon extended exposure to the low earth orbit environment. Any such drift must be quantified to continue correctly calibrating observed radiance on orbit. A method is presented to diagnose the effective emissivity of a blackbody cavity in situ using a quantum cascade laser (QCL)-based reflectometer. QCLs provide high-power single-mode output in the thermal infrared and have small mechanical footprints that facilitate integration into existing optical systems. The laser reflectivity in a test blackbody cavity w...

On-Orbit Absolute Radiance Standard for the Next Generation of IR Remote Sensing Instruments

The next generation of infrared remote sensing satellite instrumentation, including climate benchmark missions will require better absolute measurement accuracy than now available, and will most certainly rely on the emerging capability to fly SI traceable standards that provide irrefutable absolute measurement accuracy. As an example, instrumentation designed to measure spectrally resolved infrared radiances with an absolute brightness temperature error of better than 0.1 K will require high-emissivity (<0.999) calibration blackbodies with emissivity uncertainty of better than 0.06%, and absolute temperature uncertainties of better than 0.045K (k=3). 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 (UW) and refined under the NASA Instrument Incubator Program (IIP). This work recently culminated with an integrated subsystem that was used in the laboratory to demonstrate end-to-end radiometric accuracy verification for the UW Absolute Radiance Interferometer. Along with an overview of the design, we present details of a key underlying technology of the OARS that provides 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. In addition we present performance data from the laboratory testing of the OARS.

The University of Wisconsin Space Science and Engineering Center Absolute Radiance Interferometer (ARI): instrument overview and radiometric performance

Spectrally resolved infrared (IR) and far infrared (FIR) radiances measured from orbit with extremely high absolute accuracy (< 0.1 K, k = 3, brightness temperature at scene temperature) constitute a critical observation for future climate benchmark missions. The challenge in the IR/FIR Fourier Transform Spectrometer (FTS) sensor development for a climate benchmark measurement mission is to achieve the required ultra-high accuracy with a design that can be flight qualified, has long design life, and is reasonably small, simple, and affordable. In this area, our approach is to make use of components with strong spaceflight heritage (direct analogs with high TRL) combined into a functional package for detailed performance testing. The required simplicity is achievable due to the large differences in the sampling and noise requirements for the benchmark climate measurement from those of the typical remote sensing infrared sounders for weather research or operations. A summary of the instrument design and development, and the radiometric performance of the Absolute Radiance Interferometer (ARI) at the University of Wisconsin Space Science and Engineering Center (UW-SSEC) will be presented.

On-orbit characterization of blackbody emissivity and spectrometer instrument line-shape using quantum cascade laser based reflectometry

We present a method to characterize the emissivity of a spaceborne blackbody and the instrument line-shape (ILS) of a spectrometer using a quantum cascade laser (QCL) based reflectometer. QCLs allow the realization of on-orbit reflectometry that directly observes blackbody surface properties. We present experimental data verifying that the QCL reflected radiance signal can be measured by an Earth-observing spectrometer. The QCL can also be used to realize a monochromatic, spatially uniform source of infrared radiation to measure the spectrometers ILS, which can be inverted to obtain diagnostic information about the integrity of the detector and nonlinearities in the detector signal-chain.

GOSAT TANSO FTS TIR band calibration: a five year review

The GOSAT thermal infrared (TIR) band calibration is reviewed for the five-year time period from April 2009 through March 2014. Pre-launch characterization has been augmented by post-launch analysis of on-orbit data and comparison with coincident measurements from other satellite, airborne, and ground-based sensors. Successive refinements of the TIR band ground-processing software have incorporated corrections for detector non-linearity and polarization. Estimates of radiometric uncertainty have also been made. The comparison of GOSAT TIR band nadir and off-nadir comparisons (SNOs and SONOs) provide a quantitative spectral assessment of the radiometric bias relative to the NASA AIRS and EUMETSAT IASI sensors.

The heated halo for space-based blackbody emissivity measurement

Reliable calibration of high-accuracy spaceborne infrared spectrometers requires knowledge of both blackbody temperature and emissivity on-orbit, as well as their uncertainties. The Heated Halo is a broadband thermal source that provides a robust and compact method to measure emissivity. We present the results from the Heated Halo methodology implemented with a new Absolute Radiance Interferometer (ARI), which is a prototype space-based infrared spectrometer designed for climate benchmarking. We show the evolution of the technical readiness level of this technology and we compare our findings to models and other experimental methods of emissivity determination.

On-orbit Absolute Blackbody Emissivity Determination Using the Heated Halo Method

The Heated Halo method can be used to accurately measure the spectral emissivity of a blackbody, on-orbit, using a broadband thermal source.

A New Marine Atmospheric Emitted Radiance Interferometer for Shipboard Atmospheric and Oceanic Observations

The Marine-Atmospheric Emitted Radiance Interferometer (M-AERI) is a robust, accurate seagoing instrument that measures thermal emission spectra from the sea surface and marine atmosphere. Results from the first shipboard deployment of a new M-AERI instrument are presented.