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NASA NTRS2006

The Development Of Enabling Technologies For Submillimeter-Wave Remote Sensing of Ice Clouds From Space

This paper presents the chronological development of technologies and techniques that have led to a satellite mission concept aimed at quantifying the temporal and spatial distributions of upper tropospheric ice clouds. The Submillimeter-wave and Infrared Ice Cloud Experiment (SIRICE) is an Earth System Science Pathfinder mission concept designed to improve our understanding of the upper tropospheric water cycle and its coupling to the Earth s radiation budget. Ice outflow from convective storm systems is known to play an important role in regional energy budgets; however, ice generation and subsequent precipitation and sublimation are poorly quantified. SIRICE will provide measurements of ice cloud distributions and microphysical properties which are needed for understanding the crucial link between the hydrologic and energy cycles. The SIRICE measurement platform is comprised of two integrated instruments, the Submillimeter/millimeter-wave radiometer (SM4) and the Infrared Cloud Ice Radiometer (IRCIR). The primary instrument is the SM4, a conical scanner that provides a 1600 km swath of the Earth's surface at 53 degree incidence. The SM4 has 6 linearly polarized receivers measuring 12 spectral bands centered at 183 GHz, 325 GHz, 448 GHz, 643 GHz and 874 GHz; two receivers at 643 GHz measure horizontal and vertical polarizations. Submillimeter-wavelengths are well suited to the remote sensing of ice clouds due to the relative size of the wavelengths to particle sizes. Upwelling emission from lower tropospheric water vapor is scattered by the ice clouds thus causing a brightness temperature depression at submillimeter wavelengths. The IRCIR is a push broom imager with approximately 1500 km swath and spectral channels at 11 and 12 micrometers. This combination of coincident infrared and submillimeter-wavelength measurements were chosen because of its ability to provide retrieval of ice water path and median particle size for a wide range of ice clouds from thin cirrus to thick anvil structures. Over the past decade there has been a parallel development of submillimeter-wave technologies, demonstration instruments, and remote sensing techniques that have led to the present SIRICE mission concept. Mapping of these developmental paths reveals the origins, rational and maturity of features of the SIRICE payload such as its channel selection, compact design, and multipoint calibration. This presentation traces the evolution of the SIRICE mission concept from the early 1990's to its present status.

Racette, Paul↗
NASA NTRS1999

Submillimeter-Wave Cloud Ice Radiometry

Submillimeter-wave cloud ice radiometry is a new and innovative technique for characterizing cirrus ice clouds. Cirrus clouds affect Earth's climate and hydrological cycle by reflecting incoming solar energy, trapping outgoing IR radiation, sublimating into vapor, and influencing atmospheric circulation. Since uncertainties in the global distribution of cloud ice restrict the accuracy of both climate and weather models, successful development of this technique could provide a valuable tool for investigating how clouds affect climate and weather. Cloud ice radiometry could fill an important gap in the observational capabilities of existing and planned Earth-observing systems. Using submillimeter-wave radiometry to retrieve properties of ice clouds can be understood with a simple model. There are a number of submillimeter-wavelength spectral regions where the upper troposphere is transparent. At lower tropospheric altitudes water vapor emits a relatively uniform flux of thermal radiation. When cirrus clouds are present, they scatter a portion of the upwelling flux of submillimeter-wavelength radiation back towards the Earth as shown in the diagram, thus reducing the upward flux o f energy. Hence, the power received by a down-looking radiometer decreases when a cirrus cloud passes through the field of view causing the cirrus cloud to appear radiatively cool against the warm lower atmospheric thermal emissions. The reduction in upwelling thermal flux is a function of both the total cloud ice content and mean crystal size. Radiometric measurements made at multiple widely spaced frequencies permit flux variations caused by changes in crystal size to be distinguished from changes in ice content, and polarized measurements can be used to constrain mean crystal shape. The goal of the cloud ice radiometry program is to further develop and validate this technique of characterizing cirrus. A multi-frequency radiometer is being designed to support airborne science and spacecraft validation missions. This program has already extended the initial millimeter-wave modeling studies to submillimeter-wavelengths and has improved the realism of the cloud scattering models. Additionally a proof-of-concept airborne submillimeter-wave radiometer was constructed and fielded. It measured a radiometric signal from cirrus confirming the basic technical feasibility of this technique. This program is a cooperative effort of the University of Colorado, Colorado State University, Swales Aerospace, and Jet Propulsion Laboratory. Additional information is contained in the original.

Walter, Steven J.↗
NASA NTRS1991

International Symposium on Space Terahertz Technology, 1st, University of Michigan, Ann Arbor, Mar. 5, 6, 1990, Proceedings

Problems, proposed solutions, and recent results related to the development of solid-state oscillators, detectors, mixers, diode multipliers, and antennas at THz frequencies are addressed. Individual topics considered include: submillimeter heterodyne remote sensing of upper atmospheric gases, planar dipole array antenna with an elliptical lens, twin slot antenna structures integrated with microbolometer detectors for 94-GHz imaging. Also discussed are: quantum-well and quantum-barrier diodes for generating submillimeter wave power, planar Schottky barrier mixer diodes for space applications at submillimeter wavelengths, GaAs Schottky barrier varactor diodes for submillimeter wavelength power generation, planar doped barrier devices for subharmonic mixers.

Ulaby, Fawwaz↗
NASA NTRS1991

SALSA - A Synthesis Array for Lunar Submillimeter Astronomy

A conceptual design is described for a lunar submillimeter wavelength interferometer called SALSA, a Synthesis Array for Lunar Submillimeter Astronomy. Its design is greatly simplified over conventional submillimeter wavelength arrays because it takes advantage of a beam waveguide to relay signals to a centrally-located receiver system. The array is optimized to synthesize images at wavelengths between 60 and 300 microns, and it has an angular resolution of 10 milliarcsecs at 60 microns. Observations in this region provide unique insights into fundamental issues in astrophysics such as star formation, but are impossible from the earth's surface because of atmospheric absorption. The baseline design for SALSA consists of twelve, 3.5-meter diameter antennas arranged in a Y-shaped configuration consisting of three 0.5-km long arms, each with four antennas distributed according to a power-law function of distance. Such a beam-waveguide approach has significant advantages, since all the high-power-consumption items (active cryogenics, correlator) are at a central location and could share a single power source. In addition, the antennas can be quite simple and maintenance is greatly reduced. The primary technology challenges for developing such an array are the coherent detectors (mixers and local oscillator sources), and ensuring the dimensional stability of the optical elements under the extreme lunar thermal environment (85-385 K).

Mahoney, M. J.↗
NASA NTRS2014

Transition-Edge Hot-Electron Microbolometers for Millimeter and Submillimeter Astrophysics

The millimeter and the submillimeter wavelengths of the electromagnetic spectrum hold a wealth of information about the evolution of the universe. In particular, cosmic microwave background (CMB) radiation and its polarization carry the oldest information in the universe, and provide the best test of the inflationary paradigm available to astronomy today. Detecting gravity waves through their imprint on the CMB polarization would have extraordinary repercussions for cosmology and physics. A transition-edge hot-electron micro - bolometer (THM) consists of a superconducting bilayer transition-edge sensor (TES) with a thin-film absorber. Unlike traditional monolithic bolometers that make use of micromachined structures, the THM em ploys the decoupling between electrons and phonons at millikelvin temperatures to provide thermal isolation. The devices are fabricated photolithographically and are easily integrated with antennas via microstrip transmission lines, and with SQUID (superconducting quantum interference device) readouts. The small volume of the absorber and TES produces a short thermal time constant that facilitates rapid sky scanning. The THM consists of a thin-film metal absorber overlapping a superconducting TES. The absorber forms the termination of a superconducting microstripline that carries RF power from an antenna. The purpose of forming a separate absorber and TES is to allow flexibility in the optimization of the two components. In particular, the absorbing film's impedance can be chosen to match the antenna, while the TES impedance can be chosen to match to the readout SQUID amplifier. This scheme combines the advantages of the TES with the advantages of planar millimeter-wave transmission line circuits. Antenna-coupling to the detectors via planar transmission lines allows the detector dimensions to be much smaller than a wavelength, so the technique can be extended across the entire microwave, millimeter, and submillimeter wavelength ranges. The circuits are fabricated using standard microlithographic techniques and are compatible with uniform, large array formats. Unlike traditional monolithic bolometers that make use of micromachined structures, the THM employs the decoupling between electrons and phonons at millikelvin temperatures to provide thermal isolation. There is no fragile membrane in the structure for thermal isolation, which improves the fabrication yield.

Hsieh, Wen-Ting↗
NASA NTRS2006

Fundamental Limits to Wavefront Sensing in the Submillimeter

With the advent of large-format submillimeter wavelength detector arrays, and a new 25 m diameter submillimeter telescope under consideration, the question of optimal wavefront sensing methods is timely. Indeed, not only should bolometric array detectors allow the use of a variety of wavefront sensing techniques already in use in the optical/infrared, but in some cases it should actually be easier to apply these techniques because of the more benign temporal properties of the atmosphere at long wavelengths. This paper thus addresses the fundamental limits to wavefront sensing at submillimeter wavelengths, in order to determine how well a telescope surface can be measured in the submillimeter band. First several potential measurement approaches are discussed and compared. Next the theoretical accuracy of a fringe phase measurement in the submillimeter is discussed. It is concluded that with Mars as the source, wavefront sensing at the micron level should be achievable at submillimeter wavelengths in quite reasonable integration times.

wavefront sensing↗
NASA NTRS1992

Barrier/n/n+ Varactor Frequency Multipliers

Barrier/n/n+ (BNN+) varactor diodes developed as frequency multipliers at millimeter and submillimeter wavelengths. Devices required to serve as frequency triplers or quintuplers to provide powers of order of milliwatts at frequencies from 0.1 THz to about 1 THz. Feature Mott or heterojunction barriers and back-to-back diode configuration, which make it possible to obtain symmetrical capacitance-versus-voltage characteristics with high ratio between maximum and minimum capacitances. Extension of barrier/intrinsic/n+ (BIN+) concept described in "BIN Diode for Submillimeter Wavelengths" (NPO-17258).

Lieneweg, Udo↗
NASA NTRS1982

Throughput of diffraction-limited field optics systems for infrared and millimetric telescopes

Telescopes for submillimeter wavelengths have point spread functions some millimeters or centimeters in diameter, but the detectors may be only fractions of a millimeter in size. Thus a field aperture and collecting optics are needed. Optimizing the aperture by a calculation of the effects of diffraction on signal and resolution as a function of size of the collecting aperture is shown. Calculations are compared to experimental results from observations of Mars at submillimeter wavelengths.

Hildebrand, R. H.↗
NASA NTRS1987

High-Rydberg Xenon Submillimeter-Wave Detector

Proposed detector for infrared and submillimeter-wavelength radiation uses excited xenon atoms as Rydberg sensors instead of customary beams of sodium, potassium, or cesium. Chemically inert xenon easily stored in pressurized containers, whereas beams of dangerously reactive alkali metals must be generated in cumbersome, unreliable ovens. Xenon-based detector potential for infrared astronomy and for Earth-orbiter detection of terrestrial radiation sources. Xenon atoms excited to high energy states in two stages. Doubly excited atoms sensitive to photons in submillimeter wavelength range, further excited by these photons, then ionized and counted.

Chutjian, Ara↗
NASA NTRS1993

An Improved Solution for Integrated Array Optics in Quasi-Optical mm and Submm Receivers: the Hybrid Antenna

The hybrid antenna discussed here is defined as a dielectric lens-antenna as a special case of an extended hemi-spherical dielectric lens that is operated in the diffraction limited regime. It is a modified version of the planar antenna on a lens scheme developed by Rutledge. The dielectric lens-antenna is fed by a planar-structure antenna, which is mounted on the flat side of the dielectric lens-antenna using it as a substrate, and the combination is termed a hybrid antenna. Beam pattern and aperture efficiency measurements were made at millimeter and submillimeter wavelengths as a function of extension of the hemi- spherical lens and different lens sizes. An optimum extension distance is found experimentally and numerically for which excellent beam patterns and simultaneously high aperture efficiencies can be achieved. At 115 GHz the aperture efficiency was measured to be (76 4 +/- 6) % for a diffraction limited beam with sidelobes below -17 dB. Results of a single hybrid antenna with an integrated Superconductor-Insulator-Superconductor (SIS) detector and a broad-band matching structure at submillimeter wavelengths are presented. The hybrid antenna is diffraction limited, space efficient in an array due to its high aperture efficiency, and is easily mass produced, thus being well suited for focal plane heterodyne receiver arrays.

Buttgenbach, Thomas H.↗
NASA NTRS1990

Fabrication and optimization of a whiskerless Schottky barrier diode for submillimeter wave applications

The following accomplishments were made towards the goal of an optimized whiskerless diode chip for submillimeter wavelength applications. (1) Surface channel whiskerless diode structure was developed which offers excellent DC and RF characteristics, reduced shunt capacitance and simplified fabrication compared to mesa and proton isolated structures. (2) Reliable fabrication technology was developed for the surface channel structure. The new anode plating technology is a major improvement. (3) DC and RF characterization of the surface channel diode was compared with whisker contacted diodes. This data indicates electrical performance as good as the best reported for similar whisker contacted devices. (4) Additional batches of surface channel diodes were fabricated with excellent I-V and reduced shunt capacitance. (5) Large scale capacitance modelinng was done for the planar diode structure. This work revealed the importance of removing the substrate gallium arsenide for absolute minimum pad capacitance. (6) A surface channel diode was developed on quartz substrate and this substrate was completely removed after diode mounting for minimum parasitic capacitance. This work continues with the goal of producing excellent quality submillimeter wavelength planar diodes which satisfy the requirements of easy handling and robustness. These devices will allow the routine implementation of Schottky receivers into space-based applications at frequencies as high as 1 THz, and, in the future, beyond.

Bishop, W.↗
NASA NTRS1984

A 150 Micron Heterodyne Spectrometer for Airborne Astronomy

The advantages of heterodyne detection for high resolution spectroscopy were long known to radio astronomers. Heterodyne spectroscopy was productive at submillimeter wavelengths, but its extension to shorter submillimeter and far-infrared (FIR) wavelengths was hampered by inadequate mixers and local oscillators. A significant advance in FIR receiver technology was demonstrated using an optically-pumped FIR laser and a Schottky-diode mixer. Any similar spectrometer intended for airborne observations at shorter wavelengths must be compatible with the more restrictive aircraft environment. The effectiveness of the design of a FIR heterodyne spectrometer was verified in ground-based observations with a prototype version. The airborne successor is even more compact, and has a much broader spectral coverage. Flight tests are scheduled for the new instrument.

Betz, A.↗
NASA NTRS2006

Observing Ice in Clouds from Space

There are many satellite observations of cloud top properties and the liquid and rain content of clouds, however, we do not yet quantitatively understand the processes that control the water budget of the upper troposphere where ice is the predominant phase, and how these processes are linked to precipitation processes and the radiative energy budget. The ice in clouds in the upper troposphere either melts into rain or is detrained, and persists, as cirrus clouds affecting the hydrological and energy cycle, respectively. Fully modeling the Earth's climate and improving weather and climate forecasts requires accurate satellite measurements of various cloud properties at the temporal and spatial scales of cloud processes. These properties include cloud horizontal and vertical structure, cloud water content and some measure of particle sizes and shapes. The uncertainty in knowledge of these ice characteristics is reflected in the large discrepancies in model simulations of the upper tropospheric water budget. Model simulations are sensitive to the partition of ice between precipitation and outflow processes, i.e., to the parameterization of ice clouds and ice processes. One barrier to achieving accurate global ice cloud properties is the lack of adequate observations at millimeter and submillimeter wavelengths (183-874 GHz). Recent advances in instrumentation have allowed for the development and implementation of an airborne submillimeter-wave radiometer. The brightness temperatures at these frequencies are especially sensitive to cirrus ice particle sizes (because they are comparable to the wavelength). This allows for more accurate ice water path estimates when multiple channels are used to probe into the cloud layers. Further, submillimeter wavelengths offer simplicity in the retrieval algorithms because they do not probe into the liquid and near surface portions of clouds, thus requiring only one term of the radiative transfer equation (ice scattering) to relate brightness temperatures to ice. The next step is a satellite mission designed to acquire global Earth radiance measurements in the submillimeter-wave region, thus bridging the measurement gap between microwave sounders and shorter-wavelength infrared and visible sensors. This presentation provides scientific justification and an approach to measuring ice water path and particle size from a satellite platform that spans a range encompassing both the hydrologically active and radiatively active components of cloud systems.

Ackerman, S.↗
NASA NTRS2012

Subaru Imaging of Asymmetric Features in a Transitional Disk in Upper Scorpius

We report high-resolution (0.07 arcsec) near-infrared polarized intensity images of the circumstellar disk around the star 2MASS J16042165.2130284 obtained with HiCIAO mounted on the Subaru 8.2 m telescope. We present our H-band data, which clearly exhibit a resolved, face-on disk with a large inner hole for the first time at infrared wavelengths. We detect the centrosymmetric polarization pattern in the circumstellar material as has been observed in other disks. Elliptical fitting gives the semi-major axis, semi-minor axis, and position angle (P.A.) of the disk as 63 AU, 62 AU, and -14deg, respectively. The disk is asymmetric, with one dip located at P.A.s of approx. 85deg. Our observed disk size agrees well with a previous study of dust and CO emission at submillimeter wavelength with Submillimeter Array. Hence, the near-infrared light is interpreted as scattered light reflected from the inner edge of the disk. Our observations also detect an elongated arc (50 AU) extending over the disk inner hole. It emanates at the inner edge of the western side of the disk, extending inward first, then curving to the northeast. We discuss the possibility that the inner hole, the dip, and the arc that we have observed may be related to the existence of unseen bodies within the disk.

Mayama, S.↗
NASA NTRS1991

Thin-film power-density meter for millimeter wavelengths

A quasi-optical power density meter for millimeter and submillimeter wavelengths has been developed. The device is a 2-cm2 thin-film bismuth bolometer deposited on a mylar membrane. The resistance responsivity is 150 Ohms/W, and the time constant is 1 min. The meter is calibrated at DC. The bolometer is much thinner than a wavelength, and can thus be modeled as a lumped resistance in a transmission-line equivalent circuit. The absorption coefficient is 0.5 for 189-Ohms/square film. The power-density meter has been used to measure absolute power densities for millimeter-wave antenna efficiency measurements. Absolute power densities of 0.5 mW/sq cm have been measured to an estimated accuracy of 5 percent.

Lee, Karen A.↗
NASA NTRS1983

The large scale dust distribution in the inner galaxy

Initial results are presented from a new large-scale survey of the first quadrant of the galactic plane at wavelengths of 160, 260, and 300 microns. The submillimeter wavelength emission, interpreted as thermal radiation by dust grains, reveals an optically thin disk of angular width about 0.09 deg (FWHM) with a mean dust temperature of 23 K and significant variation of the dust mass column density. Comparison of the dust column density with the gas column density inferred from CO survey data shows a striking spatial correlation. The mean luminosity per hydrogen atom is found to be 2.5 x 10 to the -30th W/H, implying a radiant energy density in the vicinity of the dust an order of magnitude larger than in the solar neighborhood. The data favor dust in molecular clouds as the dominant submillimeter radiation source.

Hauser, M. G.↗