Nick N. Ciganovich

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).

On-orbit absolute blackbody emissivity determination using the heated halo method

The Climate Absolute Radiance and Refractivity Observatory is a satellite mission that will measure the Earths outgoing spectral radiance with accuracy better than 0.1 K in radiance temperature for climate benchmarking and forecast testing. Part of the high-accuracy calibration system is the heated halo, which provides a robust and compact method to measure the spectral emissivity of a blackbody. Measurement of the combined radiance of a blackbody, the reflection from a thermal source, and knowledge of key temperatures and the viewing geometry allow the blackbody spectral emissivity to be calculated. This allows the determination of blackbody radiance, and thus calibration of the CLARREO instrument, with high accuracy.

Scanning High-resolution Interferometer Sounder (S-HIS) aircraft instrument and validation of the Atmospheric InfraRed Sounder (AIRS)

S-HIS developments improve aircraft capabilities for observing the earth-emitted spectrum in great detail and high accuracy. With its spatial mapping, S-HIS is a powerful tool to validate spectra from AIRS on the NASA Aqua satellite.

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.

Techniques used in improving the radiance validation of Atmospheric Infrared Sounder (AIRS) observations with the Scanning High-Resolution Interferometer Sounder (S-HIS)

The ability to accurately validate high spectral resolution infrared radiance measurements from space using comparisons with a high altitude aircraft spectrometer has been successfully demonstrated (Tobin, et al. 2006). A comparison technique which accounts for the different viewing geometries and spectral characteristics of the two sensors was introduced, and accurate comparisons were made for AIRS channels throughout the infrared spectrum. Resulting brightness temperature differences were found to be 0.2 K or less for most channels. Continuing work on additional cases has shown some channels to have brightness temperature differences larger than 0.2 K. Atmospheric contribution from above the aircraft is a suspected factor in producing the larger differences. The contribution of upper atmosphere HNO3 and O3 are studied as contributors to the brightness temperature differences. Improved forward model calculations are used to understand and compensate for the above aircraft atmospheric contribution. Results of this effort to understand the observed temperature differences are presented. The methodology demonstrated for the NASA AIRS instrument is expected to be used in the validation of the CrIS sensor radiances from the operational NPP/NPOESS platforms and the IASI sensor radiances from the METOP platforms.

Performance of an infrared sounder on several airborne platforms: the Scanning High Resolution Interferometer Sounder (S-HIS)

A comparison of S-HIS instrument performance on various airborne platforms, and during ground characterization is presented. Specific emphasis is placed on instrument improvements, 1998 to present day, and the engineering lessons learned. Also discussed is the ability to accurately validate high spectral resolution IR radiance measurements from space using comparisons with aircraft spectrometer observations. Aircraft comparisons of this type provide a mechanism for periodically verifying expected absolute calibration of spacecraft instruments with instrumentation for which the calibration can be carefully maintained on the ground. This capability is especially valuable for achieving the long-term consistency and accuracy of climate observations, including those from the NASA EOS spacecrafts (Terra, Aqua, Aura).