Raymond K. Garcia

Atmospheric Emitted Radiance Interferometer. Part I: Instrument Design

Abstract A ground-based Fourier transform spectrometer has been developed to measure the atmospheric downwelling infrared radiance spectrum at the earths surface with high absolute accuracy. The Atmospheric Emitted Radiance Interferometer (AERI) instrument was designed and fabricated by the University of Wisconsin Space Science and Engineering Center (UW-SSEC) for the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program. This paper emphasizes the key features of the UW-SSEC instrument design that contribute to meeting the AERI instrument requirements for the ARM Program. These features include a highly accurate radiometric calibration system, an instrument controller that provides continuous and autonomous operation, an extensive data acquisition system for monitoring calibration temperatures and instrument health, and a real-time data processing system. In particular, focus is placed on design issues crucial to meeting the ARM requirements for radiometric calibration, spectral cali...

Downwelling spectral radiance observations at the SHEBA ice station: Water vapor continuum measurements from 17 to 26μm

Earth loses energy to space in the form of longwave (or infrared) radiation. Much of this energy is radiated through the transparent portion of the water vapor rotational band from 17 to 33 μm (300 to 600 cm−1). Very few measurements have been made in this spectral region to characterize how water vapor absorbs and emits longwave radiation. An Atmospheric Emitted Radiance Interferometer (AERI) with extended longwave spectral coverage has been deployed at the Surface Heat Budget of the Arctic Ocean (SHEBA) ice station 300 miles north of the Alaskan coast to measure downwelling radiances at wavelengths of 3 to 26 μm (380 to 3000 cm−1). The spectral and radiometric performance of the instrument, installation at the ice station, and initial observations are shown. Comparisons to line-by-line radiative transfer calculations for selected clear-sky cases are presented, and air-broadened water vapor continuum absorption coefficients are determined in the wing of the pure rotational band from 17 to 26 μm (380 to 600 cm−1). Comparisons of the coefficients with the widely used Clough Kneizys Davies (CKD) water vapor continuum model suggest empirical modifications to this model are necessary. Comparisons to laboratory measurements of Burch et al. [1974] made at room temperature suggests little or no temperature dependence of the continuum from 400 to 550 cm−1. Implications of these modifications on top-of-atmosphere and surface fluxes, as well as atmospheric cooling rates, are discussed.

Airborne and ground-based Fourier transform spectrometers for meteorology: HIS, AERI, and the new AERI-UAV

Broadband IR high spectral resolution observations of atmospheric emission provide key meteorological information related to atmospheric state parameters, cloud and surface spectral properties, and processes influencing radiative budgets and regional climate. Fourier transform spectroscopy (FTS), or Michelson interferometry, has proven to be an exceptionally effective approach for making these IR spectral observations with the high radiometric accuracy necessary for weather and climate applications, and are currently developing a new airborne instrument for use on an unmanned aerospace vehicle (UAV). These include the high- resolution interferometer sounder aircraft instrument developed for the NASA high altitude ER2, the atmospheric emitted radiance interferometer (AERI) and the new AERI-UAV for application in the DOE atmospheric radiation measurement program. This paper focuses on the design of the AERI-UAV which is novel in many respects. The efforts will help speed the day when this valuable instrumentation is used to improve remote sensing and radiative budget observations from space.

The Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) On-board Blackbody Calibration System

The NASA New Millennium Programs Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) instrument provides enormous advances in water vapor, wind, temperature, and trace gas profiling from geostationary orbit. The top-level instrument calibration requirement is to measure brightness temperature to better than 1 K (3 sigma) over a broad range of atmospheric brightness temperatures, with a reproducibility of ±0.2 K. For in-flight radiometric calibration, GIFTS uses views of two on-board blackbody sources (290 K and 255 K) along with cold space, sequenced at regular programmable intervals. The blackbody references are cavities that follow the UW Atmospheric Emitted Radiance Interferometer (AERI) design, scaled to the GIFTS beam size. The cavity spectral emissivity is better than 0.998 with an absolute uncertainty of less than 0.001. Absolute blackbody temperature uncertainties are estimated at 0.07 K. This paper describes the detailed design of the GIFTS on-board calibration system that recently underwent its Critical Design Review. The blackbody cavities use ultra-stable thermistors to measure temperature, and are coated with high emissivity black paint. Monte Carlo modeling has been performed to calculate the cavity emissivity. Both absolute temperature and emissivity measurements are traceable to NIST, and detailed uncertainty budgets have been developed and used to show the overall system meets accuracy requirements. The blackbody controller is housed on a single electronics board and provides precise selectable set point temperature control, thermistor resistance measurement, and the digital interface to the GIFTS instrument. Plans for the NIST traceable ground calibration of the on-board blackbody system have also been developed and are presented in this paper.

Applications of high spectral resolution FTIR observations demonstrated by the radiometrically accurate ground-based AERI and the scanning HIS aircraft instruments

Development in the mid 80s of the High-resolution Interferometer Sounder (HIS) for the high altitude NASA ER2 aircraft demonstrated the capability for advanced atmospheric temperature and water vapor sounding and set the stage for new satellite instruments that are now becoming a reality [AIRS (2002), CrIS (2006), IASI (2006), GIFTS (2005/6)]. Follow-on developments at the University of Wisconsin-Madison that employ interferometry for a wide range of Earth observations include the ground-based Atmospheric Emitted Radiance Interferometer (AERI) and the Scanning HIS aircraft instrument (S-HIS). The AERI was developed for the US DOE Atmospheric Radiation Measurement (ARM) Program, primarily to provide highly accurate radiance spectra for improving radiative transfer models. The continuously operating AERI soon demonstrated valuable new capabilities for sensing the rapidly changing state of the boundary layer and properties of the surface and clouds. The S-HIS is a smaller version of the original HIS that uses cross-track scanning to enhance spatial coverage. S-HIS and its close cousin, the NPOESS Airborne Sounder Testbed (NAST) operated by NASA Langley, are being used for satellite instrument validation and for atmospheric research. The calibration and noise performance of these and future satellite instruments is key to optimizing their remote sensing products. Recently developed techniques for improving effective radiometric performance by removing noise in post-processing is a primary subject of this paper.

Validation of Atmospheric InfraRed Sounder (AIRS) spectral radiances with the Scanning High-resolution Interferometer Sounder (S-HIS) aircraft instrument

The ability to accurately validate high spectral resolution infrared radiance measurements from space using comparisons with aircraft spectrometer observations has been successfully demonstrated. The demonstration is based on an under-flight of the Atmospheric Infrared Sounder (AIRS) on the NASA Aqua spacecraft by the Scanning High resolution Interferometer Sounder (S-HIS) on the NASA ER-2 high altitude aircraft on 21 November 2002 and resulted in brightness temperature differences approaching 0.1K for most of the spectrum. This paper presents the details of this AIRS/S-HIS validation case and also presents comparisons of Aqua AIRS and Moderate Resolution Imaging Spectroradiometer (MODIS) radiance observations. Aircraft comparisons of this type provide a mechanism for periodically testing the absolute calibration of spacecraft instruments with instrumentation for which the calibration can be carefully maintained on the ground. This capability is especially valuable for assuring the long-term consistency and accuracy of climate observations. It is expected that aircraft flights of the S-HIS and its close cousin the National Polar Orbiting Environmental Satellite System (NPOESS) Atmospheric Sounder Testbed (NAST) will be used to check the long-term stability of the NASA EOS spacecrafts (Terra, Aqua and Aura) and the follow-on complement of operational instruments, including the Cross-track Infrared Sounder (CrIS).

Suomi NPP/JPSS Cross-track Infrared Sounder (CrIS): Calibration Validation With The Aircraft Based Scanning High-resolution Interferometer Sounder (S-HIS)

A summary of the Cross-track Infrared Sounder (CrIS) radiometric calibration validation assessment conducted using the aircraft based Scanning High-resolution Interferometer Sounder (S-HIS) during the SNPP 2013 calibration validation campaign is presented.

Component-oriented design studies for efficient processing of hyperspectral infrared imager data

Future meteorological sounding instrumentation for aircraft and satellite platforms will include hyperspectral imaging infrared spectrometers with high time and space resolution, capable of providing terabytes of raw data per day. In tandem with the development of the instruments themselves, corresponding software must be architected to be capable of timely, efficient and accurate processing of the raw data produced. Design candidates for such a software architecture must respond to use cases including deployment in large-scale distributed production environments with stringent reliability specifications; phasing of research algorithms through testing and validation into production use; marshalling of data product views to metadata-aware analysis and archival systems; maintenance of software supporting multiple similar instrument systems over the course of decades; and most importantly, delivery of fully annotated datasets to end-users with real-time latencies. Consistent techniques in the specification and propagation of metadata for both algorithm software and data content are of paramount concern in manipulating large quantities of data over long stretches of time. Further, long-term maintainability and cost-effectiveness of the system can be assured by improving reusability of both systems software and science software, through defining well-specified interfaces for software components and implementing automated mechanisms for integration and testing. We illustrate current design work, avenues of research and lessons learned on a software component architecture and corresponding development practices addressing the aforementioned concerns.

Vibration-induced tilt error model for aircraft interferometer data

The Scanning High-resolution Interferometer Sounder (S-HIS) instrument is a cross track scanning Fourier-transform interferometer with 0.5 wavenumber resolution. It uses three detectors to cover the upwelling earth spectrum over the range from 3.3 to 17 microns. Vibration experienced during flight on aircraft platforms can cause a significant level of spectrally correlated noise in the calibrated spectra. To allow this interferometric noise to be removed by analysis, a wavefront tilt measurement system that monitors vibration induced optical tilts has been incorporated into the S-HIS instrument. This two-axis tilt measurement system records small changes in wavefront alignment during the data collection of both scene and blackbody interferograms. In general, both amplitude-modulation and sample-position errors can result from these tilts. Here we show that the modulation errors that dominate the interferometric noise in the S-HIS shortwave band can be significantly reduced by using the wavefront tilt measurements to model and remove the interferometric errors. The validation of our vibration induced tilt error model with blackbody data demonstrates a correction technique applicable to correcting all types of scene data.

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.