Daniel D. LaPorte

Radiometric calibration of IR Fourier transform spectrometers: solution to a problem with the High-Resolution Interferometer Sounder

A calibrated Fourier transform spectrometer, known as the High-Resolution Interferometer Sounder (HIS), has been flown on the NASA U-2 research aircraft to measure the infrared emission spectrum of the earth. The primary use-atmospheric temperature and humidity sounding-requires high radiometric precision and accuracy (of the order of 0.1 and 1 degrees C, respectively). To meet these requirements, the HIS instrument performs inflight radiometric calibration, using observations of hot and cold blackbody reference sources as the basis for two-point calibrations at each wavenumber. Initially, laboratory tests revealed a calibration problem with brightness temperature errors as large as 15 degrees C between 600 and 900 cm(-1). The symptom of the problem, which occurred in one of the three spectral bands of HIS, was a source-dependent phase response. Minor changes to the calibration equations completely eliminated the anomalous errors. The new analysis properly accounts for the situation in which the phase response for radiance from the instrument itself differs from that for radiance from an external source. The mechanism responsible for the dual phase response of the HIS instrument is identified as emission from the interferometer beam splitter.

GHIS—The GOES High-Resolution Interferometer Sounder

Abstract A high spectral resolution interferometer sounder (GHIS) has been designed for flight on future geostationary meteorological satellites. It incorporates the measurement principles of an aircraft prototype instrument, which has demonstrated the capability to observe the earth-emitted radiance spectrum with high accuracy. The aircraft results indicate that the theoretical expectation of 1°C temperature and 2°–3°C dewpoint retrieval accuracy will be achieved. The vertical resolution of the water vapor profile appears good enough to enable moisture tracking in numerous vertical layers thereby providing wind profile information as well as thermodynamic profiles of temperature and water vapor.

High-altitude aircraft measurements of upwelling IR radiance: Prelude to FTIR from geosynchronous satellite

An aircraft version of the high-resolution interferometer sounder (HIS), a Fourier transform spectrometer designed for meteorological applications, has been used to measure the upwelling infrared emission of the earth with a resolving power on the order of 1000. HIS measurements from high-altitude NASA research aircraft have demonstrated that the high radiometric accuracies required for atmospheric temperature and humidity sounding (1°C absolute brightness temperature and 0.1° C RMS reproducibility) can be achieved. Calibration is accomplished using periodic views of two onboard high-emissivity blackbodies, servo controlled to 300 K and 240 K. For an interferometer, this approach relies on careful optical design and alignment to avoid unknown dependence of the responsivity on optical path difference. The aircraft model is a successful prototype for spacecraft versions for weather and climate monitoring.

Calibration of the Geostationary Imaging Fourier Transform Spectrometer (GIFTS)

The NASA New Millennium Programs Geostationary Imaging Fourier Transform Spectrometer (GIFTS) requires highly accurate radiometric and spectral calibration in order to carry out its mission to provide water vapor, wind, temperature, and trace gas profiling from geostationary orbit. A calibration concept has bene developed for the GIFTS Phase A instrument design. The in-flight calibration is performed using views of two on-board blackbody sources along with cold space. A radiometric calibration uncertainty analysis has been developed and used to show that the expected performance for GIFTS exceeds its top level requirement to measure brightness temperature to better than 1 K. For the Phase A GIFTS design, the spectral calibration is established by the highly stable diode laser used as the reference for interferogram sampling, and verified with comparisons to atmospheric calculations.

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.

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.

Blackbody emissivity considerations for radiometric calibration of the MODIS Airborne Simulator (MAS) thermal channels

The impact of non-unit calibration blackbody emissivity on MODIS airborne simulator (MAS) absolute thermal calibration accuracy is investigated. Estimates of blackbody effective emissivity were produced for MAS infrared channels using laboratory observations of a thermally controlled external source in a stable ambient environment. Results are consistent for spectrally close atmospheric window channels. SWIR channels show an effective emissivity of about 0.98; LWIR channels show an effective emissivity of about 0.94. Using non-unit blackbody effective emissivity reduces MAS warm scene brightness temperatures by about 1 degree Celsius and increases cold scene brightness temperatures by more than 5 degrees Celsius as compared to those inferred from assuming a unit emissivity blackbody. To test the MAS non- unit effective emissivity calibration, MAS and high- resolution interferometer sounder (HIS) LWIR data from a January 1995 ER-2 flight over the Gulf of Mexico were compared. Results show that including MAS blackbody effective emissivity decreases LWIR absolute calibration biases between the instruments to less than 0.5 degrees Celsius for all scene temperatures, and removes scene temperature dependence from the bias.

The High Resolution Interferometer Sounder (HIS)

The High Resolution Interferometer Sounder (HIS) is a device designed to measure the infrared spectrum (3.5–17μm) with very high spectral resolution (<0.1% v) and radiometric accuracy (errors <1% absolute and <0.2% relative). Measurements are currently being obtained from a NASA U2 aircraft flying at 65,000 feet. This paper describes the HIS aircraft system and provides a few examples of the remote sensing capabilities of the instrument.

The University of Wisconsin Space Science and Engineering Center Absolute Radiance Interferometer (ARI)

A summary of the development of the Absolute Radiance Interferometer (ARI) at the University of Wisconsin Space Science and Engineering Center (UW-SSEC) will be presented. At the heart of the sensor is the ABB CLARREO Interferometer Test-Bed (CITB), based directly on the ABB Generic Flight Interferometer (GFI). This effort is funded under the NASA Instrument Incubator Program (IIP).

Radiometric evaluation of MODIS emissive bands through comparison to ER-2-based MAS data

The calibration accuracy of the Moderate resolution Imaging Spectro-radiometer (MODIS) on Terra near its one year anniversary of first light has been assessed using ER-2 aircraft underflights during the Terra eXperiment (TX-2001) in the spring, 2001. The ER-2, equipped with the MAS and SHIS instruments, underflew Terra several times viewing clear sky earth scenes of the Gulf of Mexico. MAS and SHIS form a powerful tandem, combining high spatial resolution imaging with high spectral resolution sampling in the midwave to longwave infrared region. The assessment is based on co-located MODIS and MAS fields of view with matching viewing geometry and removes spatial and spectral dependencies. The MAS L1B calibration accuracy is improved by transferring the SHIS calibration accuracy (conservatively 0.5 K) to MAS. The early results of two days from TX-2001 indicate that MODIS bands are performing well, but not optimally. The MODIS MWIR window bands appear to be close to the 0.75 - 1% radiometric accuracy specification for the uniform warm, low reflectance scenes assessed, perhaps suggesting that known electronic crosstalk in MODIS SWIR and MWIR bands is small for such scenes. MODIS LWIR window bands show residuals of about 0.5 K to 0.6 K, larger than the 0.5% radiometric accuracy specification. However with the 0.5 K (window bands) to 1 K (atmospheric bands) uncertainties associated with the current assessment, it is not possible to definitively state whether these MODIS bands are or are not within specification. MODIS LWIR atmospheric CO2 bands appear to perform near the 1% accuracy specification with the exception of bands 35 and 36, the upper tropospheric CO2 bands at 13.9micrometers and 14.1micrometers . Different MODIS viewing geometry on the two days seems to suggest that scan mirror reflectance dependence on mirror angles may be influencing the MODIS L1B calibration for some bands, most notably the 8.6micrometers and LWIR CO2 bands; however this assessment is dependent upon the accuracy of the spectral correction (a function of atmospheric conditions), which will be further investigated in coming months. It was surprising to find large MODIS residuals for several bands when the mirror angle to the earth scene closely matched that of when MODIS views its onboard blackbody.