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At least 1,207 records · Page 67

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NASA NTRS1997

Detection of aseismic creep along the San Andreas fault near Parkfield, California with ERS-1 radar interferometry

The differential interferometric analysis of ERS data from Parkfield (CA) observations revealed the wide area distribution of creep along the moving fault segment of the San Andreas fault over a 15 month interval. The removal of the interferometric phase related to the surface topography was carried out. The fault was clearly visible in the differential interferogram. The magnitude of the tropospheric water vapor phase distortions is greater than the signal and hinders quantitative analysis beyond order of magnitude calculations.

Werner, Charles L.↗

NASA NTRS1997

Crustal deformation in southern California using SAR interferometry

The rate of slow deformation processes along faults activated during the Landers 1992 earthquake was investigated. The analysis was performed by combining pairs of ERS-1/2 synthetic aperture radar (SAR) of Southern California (U.S.). The interferograms revealed several centimeters of post-seismic rebound in step-overs of the 1992 break. The southern branches of the 1992 break experienced surface creep, producing sharp phase cuts in the interferometric maps. The same approach was used in the Los Angeles (CA) basin. The tectonic signal in the interferograms of the Los Angeles basin was intermingled with signals due to ground subsidence caused by oil and water withdrawal.

Peltzer, G.↗

NASA NTRS1998

Quantitative Interferometry in the Severe Acoustic Environment of Resonant Supersonic Jets

There is renewed interest in the study of supersonic jets due to advances in high speed jet propulsion, supersonic combustion, and jet noise suppression for the next generation supersonic commercial transport. Understanding fundamental fluid dynamic and acoustic processes for these applications requires quantitative velocity, density and temperature measurements. In this paper we present data demonstrating a new, robust interferometer that can provide accurate data even in the presence of intense acoustic fields. This novel interferometer, the Liquid Crystal Point Diffraction Interferometer (LCPDI), was developed earlier for space flight experiments and is applied here to the case of a supersonic shock-containing jet. The LCPDI is briefly described, then integrated line-of-sight density data from the LCPDI for two underexpanded free jets are presented. The experimental shock spacings agree well with theory.

Mercer, Carolyn R.↗

NASA NTRS1997

Ice velocity at the ice front of the Filchner-Ronne Ice Shelf, Antarctica, as observed with ERS interferometry

ERS images of the two ends of the ice front of the Filchner-Ronne ice shelf (Antarctica) were utilized interferometrically to study the deformation rate of the ice shelf in response to viscous creep. On the western bank of Berkner Island (BI), near Hemmen Ice Rise (HIR), a time series of ERS data acquired in February 1992, in both ascending and descending mode, and with a three-day time interval, were utilized to map the ice velocity in two dimensions. Finite-element ice-shelf flow simulations are compared with the ERS interferograms to interpret the ice motion in terms of the physical constraints on ice-shelf flow. The efforts to fit artificial interferograms generated with model velocity output suggest that the flow regime is strongly influenced by three processes. First, a void-creation process responsible for rifts at coastal margins tends to uncouple the ice shelf from the ice rise and neighboring coast of BI. Secondly, sea ice within the void space appears to act as a binding agent between discrete ice-shelf fragments, allowing rigid-body rotations of these fragments. Third, strain rates appear to be enhanced in a narrow zone adjacent to HIR, implying significant strain softening along the boundary. It is believed that synthetic aperture radar (SAR) interferogram/model intercomparison represents a powerful impetus towards the development of better, more physically realistic ice-shelf flow models.

Rignot, Eric↗

NASA NTRS1997

Radar ice motion interferometry

Tandem pairs of ERS-1/2 radar observations of the Jakobshavn glacier, in Greenland, were used to estimate the ice motion. Interferograms are made from two such pairs, separated by the 35 day repeat cycle. Motion can be estimated from these without knowledge of the local topography. Although the temporal baseline of the interferograms was only one day, the correlation from the fast moving ice was very low. Because of the resulting phase noise, a scene-dependent filter was used to help unwrap the interferograms. For the upper part of the glacier, the filter worked well. The filter and the measured ice motion are presented.

Goldstein, Richard↗

NASA NTRS1997

Atom Interferometry in a Microgravity Environment

The scientific objectives for this proposal were: (1) development of a rugged laser cooled source of atoms using DBR/DFB laser technology; (2) to use this source and these lasers to demonstrate coherent atom manipulation techniques; and (3) to incorporate these techniques into atom interferometer inertial force sensors. Our fulfillment of these objectives is detailed.

Kasevich, Mark↗

NASA NTRS1999

Quantitative Interferometry in the Severe Acoustic Environment of Resonant Supersonic Jets

Understanding fundamental fluidic dynamic and acoustic processes in high-speed jets requires quantitative velocity, density and temperature measurements. In this paper we demonstrate a new, robust Liquid Crystal Point Diffraction Interferometer (LCPDI) that includes phase stepping and can provide accurate data even in the presence of intense acoustic fields. This novel common path interferometer (LCPDI) was developed to overcome difficulties with the Mach Zehnder interferometer in vibratory environments and is applied here to the case of a supersonic shock- containing jet. The environmentally insensitive LCPDI that is easy to align and capable of measuring optical wavefronts with high accuracy is briefly described, then integrated line of sight density data from the LCPDI for two underexpanded jets are presented.

Mercer, Carolyn R.↗

NASA NTRS2000

Effect of Medium Symmetries in Limiting the Number of Parameters Estimated with Polarimetric Interferometry

The addition of interferometric backscattering pairs to the conventional polarimetric SAR data over forests and other vegetated areas increases the dimensionality of the data space, in principle enabling the estimation of a larger number of vegetation parameters. Without regard to the sensitivity of these data to vegetation scattering parameters, this paper poses the question: Will increasing the data channels as such result in a one-to-one increase in the number of parameters that can be estimated, or do vegetation and data properties inherently limit that number otherwise? In this paper, the complete polarimetric interferometric covariance matrix is considered and various symmetry properties of the scattering medium are used to study whether any of the correlation pairs can be eliminated. The number of independent pairs has direct consequences in their utility in parameter estimation schemes, since the maximum number of parameters that can be estimated cannot exceed the number of unique measurements. The independent components of the polarimetric interferometric SAR (POL/INSAR) data are derived for media with reflection, rotation, and azimuth symmetries, which are often encountered in vegetated surfaces. Similar derivations have been carried out before for simple polarimetry, i.e., zero baseline. This paper extends those to the interferometric case of general nonzero baselines. It is shown that depending on the type of symmetries present, the number of independent available measurements that can be used to estimate medium parameters will vary. In particular, whereas in the general case there are 27 mathematically independent measurements possible from a polarimetric interferometer, this number can be reduced to 15, 9, and 6 if the medium has reflection, rotation, or azimuthal symmetries, respectively. The results can be used in several ways in the interpretation of SAR data and the development of parameter estimation schemes, which will be discussed at the presentation. Recent POL/INSAR data from the JPL AIRSAR over a forested area will be used to demonstrate the results of this derivation. This work was performed by the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, under contract from the National Aeronautics and Space Administration.

Moghaddam, Mahta↗

NASA NTRS2000

Interferometry for X-Ray Astronomy

With direct imaging, the nature of distant astronomical objects and the physical mechanisms that control them can be constrained and understood. From Galileo's observations of the solar system, to Hubble Space Telescope's imaging of distant galaxies, improved astronomical imaging has always brought scientific understanding. The x-ray band of the spectrum, where exotic objects can have extremely high surface brightness, is ideally suited for high resolution imaging, but has lacked ultra-high quality telescopes. We report a practical x-ray interferometer that features high efficiency, affordable mirrors, adjustable baseline, and can be scaled to a full size observatory. Our prototype system, with just under one millimeter of baseline, created fringes at 1.25 keV with angular resolution of 100 milli-arcseconds. With a larger version of this interferometer in orbit it will be possible to resolve stars, black holes and other compact constituents of the universe. We can study the environments of pulsars, image and then model relativistic blast waves, study the space-time metric near the surface of a black hole, watch the physical formation of astrophysical jets, and study the dynamos of stellar coronae.

Cash, Webster↗

NASA NTRS2001

Holographic Interferometry and Laminar Jet Diffusion Flames in the Presence of Non-Uniform Magnetic Fields

Magnetic fields impact combustion processes in a manner analogous to that of buoyancy, i.e., as a body force. It is well known that in a terrestrial environment buoyancy is one of the principal transport mechanisms associated with diffusion flame behavior. Unfortunately, in a terrestrial environment it is difficult if not impossible to isolate flame behavior due magnetic fields from the behavior associated with buoyancy. A micro-, or reduced, gravity environment is ideally suited for studying the impact of magnetic fields on diffusion flames due to the decreased impact of buoyancy on flame behavior.

Baker, J.↗

NASA NTRS2000

Topographic Map Generation from the Shuttle Radar Topography Mission C-band SCANSAR Interferometry

A highly accurate global topographic map of the Earth's surface has been an elusive goal for at least three decades that may soon be achieved with the newly acquired Shuttle Radar Topographic Mission (SRTM) data. SRTM collected data for 99.97% of the Earth's landmass between -57 degrees and 60 degrees latitude during a 11 day mission in February, 2000. A modified version of the SIR-C radar that previously flew on the shuttle in 1994 augmented with a radar mounted on a 62 m boom was used to collect radar interferometric data at C (5.6 cm wavelength) and X (3 cm wavelength) bands. The C-band radar was operated in the SCANSAR mode in order to extend the swath width to 225 km the minimal amount required to achieve contiguous coverage at the equator. This paper presents an overview of the new algorithms and techniques used to process the SCANSAR data to digital elevation maps. First results of topographic maps generated from the SRTM data are used to illustrate the techniques described in this paper.

Hensley, Scott↗

NASA NTRS1994

BIOME: An Ecosystem Remote Sensor Based on Imaging Interferometry

Until recent times, optical remote sensing of ecosystem properties from space has been limited to broad band multispectral scanners such as Landsat and AVHRR. While these sensor data can be used to derive important information about ecosystem parameters, they are very limited for measuring key biogeochemical cycling parameters such as the chemical content of plant canopies. Such parameters, for example the lignin and nitrogen contents, are potentially amenable to measurements by very high spectral resolution instruments using a spectroscopic approach. Airborne sensors based on grating imaging spectrometers gave the first promise of such potential but the recent decision not to deploy the space version has left the community without many alternatives. In the past few years, advancements in high performance deep well digital sensor arrays coupled with a patented design for a two-beam interferometer has produced an entirely new design for acquiring imaging spectroscopic data at the signal to noise levels necessary for quantitatively estimating chemical composition (1000:1 at 2 microns). This design has been assembled as a laboratory instrument and the principles demonstrated for acquiring remote scenes. An airborne instrument is in production and spaceborne sensors being proposed. The instrument is extremely promising because of its low cost, lower power requirements, very low weight, simplicity (no moving parts), and high performance. For these reasons, we have called it the first instrument optimized for ecosystem studies as part of a Biological Imaging and Observation Mission to Earth (BIOME).

Peterson, David L.↗

NASA NTRS2003

Precision Pointing for the Laser Interferometry Space Antenna Mission

The Laser Interferometer Space Antenna (LISA) mission is a planned NASA-ESA gravitational wave detector consisting of three spacecraft in heliocentric orbit. Lasers are used to measure distance fluctuations between proof masses aboard each spacecraft to the picometer level over a 5 million kilometer separation. Each spacecraft and its two laser transmit/receive telescopes must be held stable in pointing to less than 8 nanoradians per root Hertz in the frequency band 0.1-100 mHz. The pointing error is sensed in the received beam and the spacecraft attitude is controlled with a set of micro-Newton thrusters. Requirements, sensors, actuators, control design, and simulations are described.

Hyde, T. Tupper↗

NASA NTRS2003

Precision Pointing for the Laser Interferometry Space Antenna Mission

Hyde, T. Tupper↗

NASA NTRS2003

Precision Pointing for the Laser Interferometry Space Antenna Mission

This viewgraph presentation discusses requirements for control systems in the design and production of space-based telescopes. Specific topics covered include: pointing control methods, wavefront control methods, vibration control methods and thermal control methods. Control systems on the Hubble Space Telescope and the James Webb Space Telescope are reviewed. Control system requirements for future space telescopes are also mentioned.

Hyde, T. Tupper↗

NASA NTRS2002

Two Color Interferometry with Nonlinear Refractive Properties

Using nonlinear refractive properties of salt-water solution at two wavelengths, numerical analysis has been performed to extract temperature and concentration from virtual interferometric fringe data. The theoretical study, using a commercially available equation solving tool, starts with critical fringe counting needs and the role of nonlinear refractive properties in such measurements. Finally, methodology of the analysis, developed codes, and fringe counting accuracy needs are described in detail.

Vikram, Chandra S.↗

NASA NTRS2003

Probing The Invisible Universe: The Case for Far-IR/Submillimeter Interferometry

The question "How did we get here and what will the future bring? captures the human imagination and the attention of the National Academy of Science s Astronomy and Astrophysics Survey Committee (AASC). Fulfillment of this fundamental goal requires astronomers to have sensitive, high angular and spectral resolution observations in the far-infrared submillimeter (far-IR-sub-mm) spectral region. With half the luminosity of the universe and vital information about galaxy, star and planet formation, observations in this spectral region require capabilities similar to those currently available or planned at shorter wavelengths. The scientific motivation, some mission concepts and technology requirements for far-IR-sub-mm space interferometers that can be developed in the 2010-2020 timeframe are summarized.

Leisawitz, D.↗

NASA NTRS2003

A Laboratory-Based Microwave Radio-Interferometry Testbed

The Goddard Radio lnterferometry Testbed (GRIT) is an adaptable platform for laboratory testing of Synthetic Thinned Array Radiometers (STAR). Using this testbed, we demonstrate that the Doppler radiometer can image a point source in an observed scene.

Moriarity, M.↗