NDACC MLO FTIR

Solar viewing Fourier Transform Interferometer (FTIR). This is a ground based instrument stationed at the NOAA Mauna Loa Observatory (MLO). It operates daily in an autonomous mode taking middle infrared solar spectra of the terrestrial atmosphere. It began operation in 1995 and has run continuously since. The data are used for long term studies of many trace species in the atmosphere. Its operated as part of the Network for the Detection for Atmospheric Composition Change (NDACC www.ndacc.org). See https://www2.acom.ucar.edu/irwg for information on the network and https://www2.acom.ucar.edu/irwg for info on PI J. Hannigan. Data are publicly available at www.ndacc.org. Data products consist of retrievals from the remote sensing spectra of vertical profiles of CO, CH4, ClONO2, HCOOH, C2H6, HCN, HCl, HF, HNO3, H2O, HDO, OCS, N2O, O3, H2CO. Other species are available.

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Airborne Emission Spectrometer

Targeting Fourier Transform Spectrometer (FTS) measuring infrared spectra from 4.5 to 13.4 µm. AES was the airborne testbed for the EOS/Aura TES instrument and operated ~1994-2000.

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Utah State University Airglow Study

The 1999 Leonid MAC campaign consisted of five consecutive nighttime flights including stops in the United States, England, Israel, and the Azores. The Space Dynamics Laboratory of Utah State University operated several instruments in the visible and infrared spectral bands. One system obtained high-resolution (4 cm-1) measurements of the night sky emission spectra in the 1 to 1.65-micrometer band. Measurements were obtained above the clouds providing exceptional viewing conditions. The OH airglow emission layer originates at an altitude of ~87 km and has a half-width of typically 8–10 km. Its behavior during the storm night of 17/18 November 1999 was of particular interest because the OH airglow emission may be affected by the Leonid meteor ablation products that can penetrate to altitudes as low as 80 to 90 km altitudes. Typical Leonid meteor end-heights are much higher above ~100 km. Variability of the OH emission was measured to investigate any changes that may result from meteor interactions with the atmosphere that could cause changes in the natural airglow emission via excitation caused by the meteor ablation products. It is also possible that organic materials in the meteors could be broken down into simpler products that include the OH hydroxyl radical.

To search for these effects, airglow data were collected by a Bomem Michelson M-150 interferometer. This interferometer operates at 4 cm-1 resolution (apodized) with a scan rate of about 1 scan every 3 seconds. The interferometer field of view is 1.5° and it is sensitive from 1 to 1.65 micrometers. An intensified Xibion camera recorded the instrument field of view during the flight, providing information on the pointing elevation and azimuth. This sensor operated almost continuously during the entire 1999 Leonid MAC campaign and collected an extensive set of night airglow spectra.

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Fabry-Perot Interferometer

A Fabry-Perot interferometer is constructed of two very flat, partially reflecting mirrors held parallel to one another at a fixed distance. Interference occurs among the multiple reflections leading to the condition that wavelengths that exactly divide the spacing between the mirrors by an integer are transmitted very efficiently and all other wavelengths are reflected. Thus if the plates are held fixed at a separation of 10 μm, then radiation at 10, 5, 3.333, … μm will be transmitted. Note that these wavelengths are equally spaced in energy according to the relationship E=hc/l, where l is the wavelength of the light and h and c are Planck’s constant and the speed of light, respectively. This particular FPI technique makes use of these multiple passbands to increase the measurement signal and the resulting signal to noise ratio.

A Fabry-Perot can be tuned to transmit different wavelengths by changing the (optical) spacing between the mirrors. This is commonly done by employing piezo-electric transducers to translate the mirrors by very small distances, while maintaining the very precise parallelism between them. Fixed gap Fabry-Perots can be tuned by tilting, which changes the effective path length between the plates; by using the thermal expansion and contraction of the spacers between the mirrors; and by changing the composition or pressure of the gas that fills the space between the plates, which alters the index of refraction thereby changing the optical separation. Finally, Fabry-Perots can be constructed using a solid substrate of fused silica or optical quality glass onto which reflective coatings are deposited. These devices can be angle tuned or temperature tuned.

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Scanning High-Resolution Interferometer Sounder

The Scanning High-resolution Interferometer Sounder (S-HIS) is a scanning interferometer which measures emitted thermal radiation at high spectral resolution between 3.3 and 18 microns The measured emitted radiance is used to obtain temperature and water vapor profiles of the Earth's atmosphere in clear-sky conditions. S-HIS produces sounding data with 2 kilometer resolution (at nadir) across a 40 kilometer ground swath from a nominal altitude of 20 kilometers onboard a NASA ER-2 or Global Hawk.

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National Airborne Sounder Testbed - Interferometer

The National Airborne Sounder Testbed-Interferometer (NAST-I) is a high spectral resolution (0.25 cm-1) and high spatial resolution (0.13 km linear resolution per km of aircraft flight altitude, at nadir) scanning (2.3 km ground cross-track swath width per km of aircraft flight altitude) passive infrared (IR) Michelson interferometer sounding system that was developed to be flown on high-altitude aircraft to provide experimental observations needed to finalize the specifications and to test proposed designs and data processing algorithms for the Cross-track Infrared Sounder (CrIS) flying on the Suomi NPP (SNPP) and Joint Polar Satellite System (JPSS) platforms. Because the NAST-I infrared spectral radiance and temperature, humidity, trace species, cloud and surface property soundings have unprecedented spectral and high spatial resolution, respectively, the data can be used to support a variety of satellite sensor calibration / validation and atmospheric research programs. The NAST-I covers a spectral range from ~ 600-2900 cm-1 (3.5-16 microns) with 0.25 cm-1 spectral resolution, yielding more than 9000 spectral channels of radiance emission/absorption information. The NAST-I instrument has flown numerous science missions on the ER-2, WB-57, and Proteus aircraft, and the team has evaluated efforts needed to become operational on the DC-8. Most recently, NAST-I was part of the ER-2 science payload for the FIREX-AQ field campaign conducted during August, 2019 (https://www.esrl.noaa.gov/csl/projects/firex-aq/). Additional information can be obtained from Anna Noe (anna.m.noe@nasa.gov, 757-864-6466), Dr. Daniel Zhou (daniel.k.zhou@nasa.gov, 757-864-5663), or Dr. Allen Larar (Allen.M.Larar@nasa.gov, 757-864-5328).

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Anna Noe (Mgr)

JPL Mark IV Balloon Interferometer

The MkIV interferometer operates in solar absorption mode, meaning that direct sunlight is spectrally analyzed and the amount of various gases at different heights in the Earth's atmosphere is derived from the shapes and depths of their absorption lines. The optical design of the MkIV interferometer is based largely on that of the ATMOS instrument, which has flown four times on the Space Shuttle. The first three mirrors in the optical path comprise the suntracker. Two of these mirrors are servo-controlled in order to compensate for any angular motion of the observation platform. The subsequent wedged KBr plates, flats, and cube-corner retro-reflectors comprise a double-passed Michelson interferometer, whose function is to impart a wavelength-dependent modulation to the solar beam. This is achieved by sliding one of the retro-reflectors at a uniform velocity so that the recombining beams interfere with each other. A paraboloid then focusses the solar beam onto infrared detectors, which measure the interferometrically modulated solar signal. Finally, Fourier transformation of the recorded detector outputs yields the solar spectrum. An important advantage of the MkIV Interferometer is that by employing a dichroic to feed two detectors in parallel, a HgCdTe photoconductor for the low frequencies (650-1850 cm-1) and a InSb photodiode for the high frequencies (1850-5650 cm-1), the entire mid-infrared region can be observed simultaneously with good linearity and signal-to-noise ratio. In this region over 30 different gases have identifiable spectral signatures including H2O, O3, N2O, CO, CH4, NO, NO2, HNO3, HNO4, N2O5, H2O2, ClNO3, HOCl, HCl, HF, COF2, CF4, SF6, CF2ClCFCl2, CHF2Cl, CF2Cl2, CFCl3, CCl4, CH3Cl, C2H2, C2H6, OCS, HCN, N2, O2, CO2 and many isotopic variants. The last three named gases, having well known atmospheric abundances, are important in establishing the observation geometry of each spectrum, which otherwise can be a major source of uncertainty. Similarly, from analysis of T-sensitive CO2 lines, the temperature profile can be accurately determined. The simultaneity of the observations of all these gases greatly simplifies the interpretation of the results, which are used for testing computer models of atmospheric transport and chemistry, validation of satellite data, and trend determination.

Although the MkIV can measure gas column abundances at any time during the day, the highest sensitivity to atmospheric trace gases is obtained by observing sunrise or sunset from a balloon. The very long (~ 400 km) atmospheric paths traversed by incoming rays in this observation geometry also make this so-called solar occultation technique insensitive to local contamination.

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Balloon, DC-8 - AFRC
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Lightweight Balloon Interferometer

The LWBI includes an optical head, containing an interferometer, detectors, translation mechanism, and metrology laser, using light weight technologies and a more mass-conscious approach. The lightweight balloon interferometer (LWBI) will have a performance similar to that of the JPL MkIV, yet will weigh just one fifth of the mass. The reduced payload size and mass will make the LWBI much easier to launch, facilitating close co-location with observations from space-borne sensors such as TES and HIRDLS on board AURA.

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In-situ Net Flux within the Atmosphere of the Earth

INFLAME provides a direct measurement of the net flux, and requires high stability with much less stringent requirements on absolute accuracy, thereby removing the major impediment of existing measurements. The INFLAME instruments, deployed on small aircraft, will provide a direct measurement of the net flux radiative, flux divergences, and heating rates in clear sky, in the presence of aerosols, and in the presence of cloud particles.

The INFLAME instrument measures the net flux by using a low-resolution Fourier transform spectrometer (FTS) to observe the upward and downward flux simultaneously using the two inputs of the same instrument. The two complementary outputs of the FTS can be transformed to produce spectra proportional to the difference between the two inputs.

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Airborne Glacier and Land Ice Surface Topography Interferometer

Critical to progress in understanding and modeling ice sheets are a better characterization of what ice sheets are doing at present, how fast they are changing, what are the driving processes controlling these changes, and how we can better represent these processes in numerical models to derive more realistic predictions of the evolution of glaciers and ice sheets in the future. Chief among these measurements, are detailed, enhanced and sustained measurements of ice sheet elevation, at high spatial resolution, with high vertical accuracy, over the entire ice sheets. These measurements provide critical information about long-term ice sheet dynamics (mass balance trends) and short-term variability (precipitation, ablation events, surface lowering of an accelerating glacier, etc.).

Ideally suited to making these measurements is GLISTIN, a Ka-band single pass interferometric synthetic aperture radar (InSAR). Proposed also as a spaceborne mission concept [1], the airborne GLISTIN-A serves as a proof-of-concept demonstration and science sensor. Key features include:

1. The Ka-band center frequency maximizes the single-pass interferometric accuracy (which is proportional to the wavelength), reduces snow penetration (when compared with lower frequencies), and remains relatively impervious to atmospheric attenuation.

2. Imaging capabilities that are important for mapping large areas. Imaging allows features to be tracked with time for estimation of ice motion and reduces data noise when measuring topographic changes over rough surfaces of glaciers and coastal regions of ice sheets.

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