M.G. Mlynczak1, R.P. Cageao1, H. Latvakoski2, D. Kratz1, D. Johnson1, J. Mast3
1 NASA Langley Research Center, Hampton, Va.; 2 Space Dynamics Laboratory, Logan, Utah; 3 Science Systems and Applications Inc., Hampton, Va.
The energetically significant portion of the Earth’s infrared energy flow occurs between five and 100 microns (mm; 1 mm = 10-6 meter) in wavelength. Wave number, or reciprocal wavelength in inverse centimeters (cm-1), is often used as a measure of frequency in infrared, and the relevant wave numbers are 2,000 to 100 cm-1. Figure 1 shows calculated clear-sky radiance spectra (W m-2 sr-1 (cm-1)-1) that would be observed by an ideal instrument on a satellite in low Earth orbit. The figure is divided into two parts, the mid-IR wavelengths between 15.5 and 5 mm, and far-IR wavelengths between 100 and 15.5 mm. Current infrared sensors in orbit observe only the mid-IR spectrum, or they observe the entire integrated spectrum from 100 to 5 mm. There are no direct, spectrally resolved observations of the far-IR spectrum from orbiting instruments.
The far-IR is rich in spectral structure and in significance to Earth’s climate. Nearly one-half of the Earth’s infrared radiant energy rejected to space occurs in the far-IR. It is modulated primarily by water vapor, the main natural greenhouse gas in Earth’s atmosphere. The entire free troposphere, that is, the atmosphere above the Earth’s boundary layer, cools to space in the far-IR. Cirrus clouds also play a key role in climate in the far-IR due to the low temperatures at which they are found in the high atmosphere. In addition, the far-IR region near 25 mm is uniquely sensitive to the characteristics of these high-altitude ice clouds and can be used to diagnose both their ice crystal “habits” and radiative properties. Water vapor feedback, an anticipated positive feedback to climate forcing by carbon dioxide, occurs primarily in the far-IR. Recently, it has been shown that changes in the far-IR spectrum contain the “fingerprints” from which the magnitude and pace of climate change can be diagnosed. These are all critical components in Earth’s complex climate system into which the FIRST instrument is providing pioneering insights.
To meet the need for a space-based far-IR observatory, the FIRST instrument was developed by NASA Langley and the Space Dynamics Laboratory (SDL) of Utah State University under the NASA ESTO Instrument Incubator Program beginning in late 2001. FIRST is a Fourier transform spectrometer with 0.643 cm-1 spectral resolution, and was designed for a 100 to 10 mm (100 to 1000 cm-1) spectral range. FIRST’s observed response is from 200 to 4.5 mm (50 to 2200 cm-1). The instrument has a large aperture designed to illuminate a focal plane populated with 10 detectors coupled to Winston cones as shown in Figure 2. The Winston cones maximize the amount of light from the atmosphere that the FIRST instrument can collect and detect.
The FIRST instrument (Figure 3) has three sections separated by vacuum windows: the scene select assembly, the interferometer section, and the detector dewar, which contains liquid-helium-cooled silicon bolometers and cryogen tanks. The interferometer section contains the interferometer and the aft-optics that focus the beam from the interferometer onto the detectors. There are no imaging fore-optics. The scene select assembly can be rotated to allow FIRST to operate either in a ground-based mode (viewing the zenith direction) or from a high-altitude balloon (looking down at the Earth from aloft). The scene select assembly also contains a rotating mirror that allows FIRST to view two calibration ports. During balloon operations, radiometric calibration is maintained by viewing an ambient temperature calibration blackbody and an open port for a space view between observations of the atmosphere, ensuring continuous radiometric calibration of all observed atmospheric spectra.
The FIRST instrument conducted an instrument technology validation flight on a high altitude balloon launched from Fort Sumner, New Mexico, in June 2005, recording five hours of upwelling Earth radiance from 30 kilometers (km). Figure 4 shows the FIRST instrument in the gondola just prior to launch on an 11 million cubic foot balloon. A second balloon flight was conducted at Fort Sumner in September 2006 in conjunction with an overpass of the NASA “A-Train” of satellites. Comparisons of mid-IR spectra recorded by FIRST and the Atmospheric Infrared Sounder (AIRS) instrument on the Aqua satellite showed excellent agreement.
In 2007, FIRST’s capability was expanded to operate as a ground-based instrument in addition to its established high-altitude balloon capability. This involved adding a second calibration blackbody so that standard radiometric calibration techniques could be used. FIRST has since participated in two ground-based campaigns: the Radiative Heating in the Under-explored Bands Campaign (RHUBC-II) in Cerro Toco, Chile, in 2009; and in a radiative transfer validation campaign at Table Mountain, California in 2012. The RHUBC-II campaign was staged at an altitude of 17,500 feet (5.4 km) above sea level in the Atacama Desert of Chile, and the Table Mountain campaign is at 7,700 feet (2.3 km) above sea level. The operational environment in Chile is shown in Figure 5. From these high venues, FIRST can investigate climate processes in the middle and upper atmosphere that are hidden from ground observations due to opaque water vapor spectral absorption.
Figure 6 shows FIRST spectra from the balloon and ground-based operations. The low radiance values from Cerro Toco (red curve) are indicative of the very low water vapor amounts in the Atacama Desert in Chile. The spectral and radiometric quality of the data are excellent. The FIRST instrument has recently been calibrated against known radiometric standards traceable to those at the National Institute of Standards and Technology (NIST). FIRST data from Cerro Toco and Table Mountain are now calibrated against these standards and are being used in scientific studies of the far-IR spectrum, the least well-known portion of Earth’s infrared radiant energy system.
The next step in the technology development process leading to space observatory deployment is to replace the liquid-helium-cooled silicon bolometers with a passively cooled pyroelectric detector that can operate at ambient temperature. This change will substantially lower the mass, power, complexity, and cost of an instrument designed to fly on an extended spacecraft mission. The FIRST team has proposed this next step to NASA that would culminate in a technology validation demonstration in a two-week, high-altitude balloon flight in Antarctica in late 2015. The advantage of an Antarctic flight is that the infrared spectrum occurs largely in the far-IR at the temperatures found above the southern continent, and polar orbiting satellites providing correlative data pass over every 15 minutes.
FIRST represents a remarkable success story in NASA’s technology development arena. Starting literally from scratch, FIRST has developed and demonstrated, through measurement of atmospheric spectra in the field, from high altitude balloons and from the ground, the interferometer, beamsplitter, and focal plane technology necessary for development of a space-based far-IR observatory. The FIRST instrument demonstration and development are continuing in support of the NASA Climate Absolute Radiance and Refractivity Observatory (CLARREO) satellite project, currently in pre-formulation. CLARREO IR and far-IR observations will lead to a more accurate and complete record of climate change.
Acknowledgement: The FIRST team at NASA Langley and SDL acknowledge continued support from the NASA ESTO since 2001 via the Instrument Incubator and Advanced Component Technology Programs. The team also acknowledges support from the NASA Langley Research Center, the NASA Earth Science Directorate Radiation Sciences Program, and the Department of Energy Atmospheric Radiation Measurement (ARM) program.