Mark W. Werner

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.

Performance verification of the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) on-board blackbody calibration system

The NASA New Millennium Programs Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) instrument was designed to provide 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 the onboard calibration approach used by GIFTS that employs two internal blackbody sources (290 K and 255 K) plus a space view sequenced at regular programmable intervals, this instrument level requirement places tight requirements on the blackbody temperature uncertainty (0.1 K) and emissivity uncertainty (0.001). The blackbody references are cavities that follow the UW Atmospheric Emitted Radiance Interferometer (AERI) design, scaled to the GIFTS beam size. The engineering model blackbody system was completed and fully calibrated at the University of Wisconsin and delivered for integration into the GIFTS Engineering Development Unit (EDU) at the Utah State Space Dynamics Laboratory. This paper presents a detailed description of the methodology used to establish the required temperature and emissivity performance, with emphasis on the traceability to NIST standards. In addition, blackbody temperature data are presented from the GIFTS EDU thermal vacuum tests that indicate excellent temperature stability. The delivered on-board blackbody calibration system exceeds performance goals - the cavity spectral emissivity is better than 0.998 with an absolute uncertainty of less than 0.001, and the absolute blackbody temperature uncertainty is better than 0.06 K.

Project status of the Robert Stobie spectrograph near infrared instrument (RSS-NIR) for SALT

The Robert Stobie Spectrograph Near Infrared Instrument (RSS-NIR), a prime focus facility instrument for the 11-meter Southern African Large Telescope (SALT), is well into its laboratory integration and testing phase. RSS-NIR will initially provide imaging and single or multi-object medium resolution spectroscopy in an 8 arcmin field of view at wavelengths of 0.9 - 1.7 μm. Future modes, including tunable Fabry-Perot spectral imaging and polarimetry, have been designed in and can be easily added later. RSS-NIR will mate to the existing visible wavelength RSS-VIS via a dichroic beamsplitter, allowing simultaneous operation of the two instruments in all modes. Multi-object spectroscopy covering a wavelength range of 0.32 - 1.7 μm on 10-meter class telescopes is a rare capability and once all the existing VIS modes are incorporated into the NIR, the combined RSS will provide observational modes that are completely unique. The VIS and NIR instruments share a common telescope focal plane, and slit mask for spectroscopic modes, and collimator optics that operate at ambient observatory temperature. Beyond the dichroic beamsplitter, RSS-NIR is enclosed in a pre-dewar box operating at -40 °C, and within that is a cryogenic dewar operating at 120 K housing the detector and final camera optics and filters. This semi-warm configuration with compartments at multiple operating temperatures poses a number of design and implementation challenges. In this paper we present overviews of the RSSNIR instrument design and solutions to design challenges, measured performance of optical components, detector system optimization results, and an update on the overall project status.

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.

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.

Vibration-induced tilt error model for aircraft interferometer data: sample position errors

The Scanning High-resolution Interferometer Sounder (S-HIS) instrument is a cross track scanning Fourier-transform interferometer with 0.5 wavenumber resolution. It is comprised of three detectors, which are the shortwave (SW), midwave (MW), and longwave (LW). Vibration experienced during flight can cause a significant level of spectrally correlated noise in the calibrated spectra. The S-HIS instrument has a wavefront tilt measurement system that monitors vibration induced optical tilts, which both reduces the interferometric noise and makes it possible to remove it with post processing. This two-axis tilt measurement system records small changes in the wavefront angles during the data collection of both scene and blackbody interferograms. The amplitude-modulation errors dominate the SW band while sample-position errors are found in the LW and MW bands. Here we show that the sample-position errors can be removed from the final calibrated radiances.

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).