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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1992

Mission operations system for Russian space Very-Long-Baseline Interferometry mission

A mission operations system developed to meet the unique requirements of the prospective Russian space VLBI mission Radioastron is described. The challenges associated with the Radioastron operations are mainly due to the interrelationship between the space-based telescope and the ground observatories, and the mission's international character. The Radioastron project includes radio observatories located in 17 different countries, tracking facilities in possibly seven different countries, orbit determination centers in Russia and USA, and widely located data processing facilities. In addition, unique scheduling constraints arise from the competing demands placed on the ground observatory time by earth-based VLBI experiments, and the tracking station time by a Japanese space interferometry mission VSOP, which is expected to be in orbit concurrently with Radioastron. A diagram illustrating the organizational structure of the Radioastron project during operations is presented.

Altunin, V. I.↗
NASA NTRS1993

Improved Estimation Of Delays In Radio Interferometry

Report describes status of mathematical model of delays in propagation of radio signals that originate at extra-galactic or other distant sources and received at widely separated terrestrial antennas engaged in very-long-baseline interferometry. Implemented in multiparameter estimation computer program MODEST. Needed, in applications as geodynamics and astronomy, to extract significant parameters from observed signal delays. Program and present report are updated versions of MASTERFIT program and accompanying report.

Sovers, Ojars J.↗
NASA NTRS1992

The Mount Wilson optical interferometer: The first automated instrument and the prospects for lunar interferometry

Before contemplating an optical interferometer on the Moon one must first review the accomplishments achieved by this technology in scientific applications for astronomy. This will be done by presenting the technical status of optical interferometry as achieved by the Mount Wilson Optical Interferometer. The further developments needed for a future lunar-based interferometer are discussed.

Johnston, Ken J.↗
NASA NTRS1992

Science objectives for ground- and space-based optical/IR interferometry

Ground-based interferometry will make spectacular strides in the next decade. However, it will always be limited by the turbulence of the terrestrial atmosphere. Some of the most exciting and subtle problems may only be addressed from a stable platform above the atmosphere. The lunar surface offers such a platform, nearly ideal in many respects. Once built, such a telescope array will not only resolve key fundamental problems, but will revolutionize virtually every topic in observational astronomy. Estimates of the possible performance of lunar and ground-based interferometers of the 21st century shows that the lunar interferometer reaches the faintest sources of all wavelengths, but has the most significant advantage in the infrared.

Ridgway, Stephen T.↗
NASA NTRS1992

Precise tracking of the Magellan and Pioneer Venusorbiters by same-beam interferometry. Part 1: Dataaccuracy analysis

Simultaneous tracking of two spacecraft in orbit about a distant planet by two widely separated Earth-based radio antennas provides more-accurate positioning information than can be obtained by tracking each spacecraft separately. A demonstration of this tracking technique, referred to as same-beam interferometry (SBI), is currently being done using the Magellan and Pioneer 12 orbiters at Venus. Signals from both spacecraft fall within the same beamwidth of the Deep Space Station antennas. The plane-of-sky position difference between spacecraft is precisely determined by doubly differenced phase measurements. This radio metric measurement naturally complements line-of-sight Doppler. Data was first collected from Magellan and Pioneer 12 on August 11-12, 1990, shortly after Magellan was inserted into Venus orbit. Data were subsequently acquired in February and April 1991, providing a total of 34 hours of same-beam radio metric observables. Same-beam radio metric residuals have been analyzed and compared with model measurement error predictions. The predicted error is dominated by solar plasma fluctuations. The rms of the residuals is less than predicted by about 25 percent for 5-min averages. The shape of the spectrum computed from residuals is consistent with that derived from a model of solar plasma fluctuations. This data type can greatly aid navigation of a second spacecraft when the first is well-known in its orbit.

Border, J. S.↗
NASA NTRS1993

Precision optical interferometry in space

POINTS, an astrometric Optical interferometer with a nominal measurement accuracy of 5 microarcseconds for the angle between a pair of stars separated by about 90 deg, is presently under consideration by two divisions of NASA-OSSA. It will be a powerful new multi-disciplinary tool for astronomical research. If chosen as the TOPS-1 (Toward Other Planetary Systems) instrument by the Solar-System Exploration Division, it will perform a definitive search for extra-solar planetary systems, either finding and characterizing a large number of them or showing that they are far less numerous than now believed. If chosen as the AIM (Astrometric Interferometry Mission) by the Astrophysics Division, POINTS will open new areas of astrophysical research and change the nature of the questions being asked in some old areas. In either case. it will be the first of a new class of powerful instruments in space and will prove the technology for the larger members of that class to follow. Based on a preliminary indication of the observational needs of the two missions, we find that a single POINTS mission will meet the science objectives of both TOPS-1 and AIM. The instrument detects dispersed fringe (channel led spectrum) and therefore can tolerate large pointing errors.

Reasenberg, Robert D.↗
NASA NTRS1993

Precise Tracking of the Magellan and Pioneer Venus Orbiters by Same-Beam Interferometry. Part 2: Orbit Determination Analysis

A new radio metric positioning technique has demonstrated improved orbit determination accuracy for the Magellan and Pioneer Venus Orbiter orbiters. The new technique, known as Same-Beam Interferometry (SBI), is applicable to the positioning of multiple planetary rovers, landers, and orbiters which may simultaneously be observed in the same beamwidth of Earth-based radio antennas. Measurements of carrier phase are differenced between spacecraft and between receiving stations to determine the plane-of-sky components of the separation vector(s) between the spacecraft. The SBI measurements complement the information contained in line-of-sight Doppler measurements, leading to improved orbit determination accuracy. Orbit determination solutions have been obtained for a number of 48-hour data arcs using combinations of Doppler, differenced-Doppler, and SBI data acquired in the spring of 1991. Orbit determination accuracy is assessed by comparing orbit solutions from adjacent data arcs. The orbit solution differences are shown to agree with expected orbit determination uncertainties. The results from this demonstration show that the orbit determination accuracy for Magellan obtained by using Doppler plus SBI data is better than the accuracy achieved using Doppler plus differenced-Doppler by a factor of four and better than the accuracy achieved using only Doppler by a factor of eighteen. The orbit determination accuracy for Pioneer Venus Orbiter using Doppler plus SBI data is better than the accuracy using only Doppler data by 30 percent.

Folkner, W. M.↗
NASA NTRS1992

Space-based astrometric optical interferometry with POINTS

POINTS, a dual space-based astrometric optical interferometer with nominal baseline length of 2 meters and measurement accuracy of 5 microarcseconds, which could be a powerful new multidisciplinary research tool is introduced. The instrument, including the laser gauges which are a central aspect of the instrument, are described. The design of the spacecraft, and the mission operations, including the bias determination and correction, are discussed. As a candidate for the Astrometric Interferometry Mission (AIM) mission, POINTS could open new areas of astrophysical research and change the nature of the questions being asked in some old areas. As a candidate for the TOPS-1 (Towards Other Planetary Systems) mission, it could be used to perform a definitive search for extra solar planetary systems, either finding and characterizing a large number of them or showing that they are far less numerous than now believed. POINTS, which is small, agile, and mechanically simple, would be the first of a new class of powerful instruments in space and would prove the technology for the larger members of that class to follow.

Reasenberg, R. D.↗
NASA NTRS1991

Variable sensitivity moire interferometry

This paper demonstrates that optical microlithography can be used to produce a crossed grating which diffracts light into multiple orders sufficient to record moire interferograms with sensitivities ranging from 2.0 to 0.285 micron/fringe. The grating profile produced by the method is analyzed to establish the diffraction efficiency in each diffraction order, and generalized expressions are given for variable sensitivity moire interferometry. Experimental tests are conducted to verify analytical arguments. In one of these tests, two different diffraction order pairs are used simultaneously to verify that surface displacement can be measured at different sensitivities.

Johnson-Cole, Helen↗
NASA NTRS1992

Determination of refractive properties of fluids for dual-wavelength interferometry

Methods to calculate the refractive properties of solutions at different wavelengths are described by using experimental data at just two wavelengths. The properties are the refractive index and its gradients with temperature and concentration. Cauchy's equation is used to determine the refractive indices. The gradients versus temperature and concentration are then determined by using the Murphy-Alpert and the Lorentz-Lorenz equation, respectively. Finally, the particular case of a triglycine sulfate aqueous solution is considered as an example. The approach should provide the desired information for fringe analysis when dual-wavelength holographic or other interferometry is used for solving heat and mass transfer problems in fluids during crystal-growth experiments.

Vikram, Chandra S.↗
NASA NTRS1993

Temporal averaging of phase measurements in the presence of spurious phase drift - Application to phase-stepped real-time holographic interferometry

A technique that compensates for low spatial frequency spurious phase changes during an interference experiment is developed; it permits temporal averaging of multiple-phase measurements, made before and after object displacement. The method is tested with phase-stepped real-time holographic interferometry applied to cantilever bending of a piezoelectric bimorph ceramic. Results indicate that temporal averaging of the corrected data significantly reduces the white noise in a phase measurement without incurring systematic errors or sacrificing spatial resolution. White noise is reduced from 3 deg to less than 1 deg using these methods.

Ovryn, B.↗
NASA NTRS1992

Temporal averaging in a turbulent environment - Compensation for phase drifts in phase shifting interferometry

In an effort to improve the signal to noise in an interference experiment, we have developed a method to remove systematic phase drift between data sets acquired over long time intervals. Using this technique, it is possible to average repeatedly acquired phase measurements and improve the phase estimate without sacrificing spatial resolution. Results from tests using real-time phase stepping holographic interferometry applied to cantilever bending of a piezoelectric bimorph indicate that white noise has been reduced from 3 to less than 1 deg (lambda/360) by averaging 36 phase compensated data sets before object bending and 36 data sets after bending.

Ovryn, B.↗
NASA NTRS1991

Displacement analysis of the interior walls of a pipe using panoramic holo-interferometry

Standard phase-displacement equations, used for both double exposure and real time holo-interferometry, are combined with the geometrical mapping and optical characteristics of panoramic annular lenses to accurately measure surface displacement on the interior wall of a pipe. In this approach, one panoramic annular lens is used to illuminate the surface with coherent light, and the resulting intensity distribution is holographically recorded through a second panoramic annular lens. Interference fringes are obtained in real-time by comparing holograms recorded before and after the pipe is displaced. The annular images and the holographic fringes are acquired and stored digitally in a computer system. Image transformation algorithms are applied to remove optical distortions in the holographic pattern and fringe loci are compared to those predicted on the basis of theory.

Gilbert, John A.↗
NASA NTRS1992

Temporal decorrelation in repeat pass-radar interferometry

Correlation in pass-to-pass, interferometric radar can be degraded by thermal noise, lack of parallelism between the radar flight tracks, spatial baseline noise, and surficial change. The effects of decorrelation due to thermal noise can be easily evaluated and removed, while those due slight angular changes between flight tracks are negligible for data acquired using near-repeat orbits. Empirical results obtained using images of Death Valley confirm that as the baseline increases, the overall correlation decreases due to spatial baseline noise. It is shown that areas of Cottonball Basin in Death Valley remained unchanged over the three-week period for which data was obtained, while a heavily forested area in Oregon exhibited significant temporal decorrelation. Lava in central Oregon also appeared to decorrelate. The results demonstrate that generation of height maps of heavily vegetated areas using pass-to-pass interferometry is practical, provided that the time between passes is at most several weeks.

Villasenor, John↗
NASA NTRS1993

Techniques and Applications of SAR interferometry for ERS-1: Topographic Mapping, Change Detection, and Slope Measurement

SAR (Synthetic Aperture Radar) interferometry allows the measurement of high resolution topography of terrain from repeat orbit SAR data sets obtained by the ERS-1 AMI (Active Microwave Instrument). The system parameters which affect the accuracy of the topographic maps are errors in determination of the interferometric baseline, non parallel orbit tracks, decorrelation caused by baseline length, thermal noise, and surface change. Surface change can be observed in the interferograms either through decorrelation of the interferometric phase, or coherent phase shifts caused by locally uniform surface displacements. Phase gradient maps can be derived directly from the complex interferograms and can be transformed into surface slopes mapped onto a geometrically corrected grid.

Charles L Werner↗
NASA NTRS1993

Earth-based measurement of lunar topography using delayed radar interferometry

Radar interferometry has been applied to radar mapping of the surface of Venus to resolve the ambiguity in the backscatter from the areas with the same delay-Doppler coordinates. For observations of the Moon these points are usually isolated by the small extent of the area illuminated by the radar beam so that interferometric techniques can be used to determine a third dimension to the location of the radar backscatter. Recent observations of Sinus Iridum (41.5 deg N, 31.0 deg W) using the Arecibo Observatory S-Band radar system (12.6 cm wavelength) in Apr. and Aug. 1992 have yielded a pair of images viewed with a very small 0.0420 deg difference in incidence angle. These images can be used as a delay interferometric pair and have been correlated to generate phase fringes that are related to topography. The spatial resolution of the images are 18 m in delay and 33 m in cross range (Doppler). The anticipated topographic resolution once the phase fringes have been unwrapped is approximately +/- 10 m.

Stacy, N. J. S.↗
NASA NTRS1993

Techniques and applications of SAR interferometry for ERS-1

The ERS-1 Synthetic Aperture Radar (SAR) demonstrated that it is possible to measure high resolution topography over large areas using interferometry with repeat orbit data sets. Given that the ERS-1 orbit is known with high precision, an automatic system for generation of interferograms is presented. Least squares estimation using ground control points provides an accurate method for determining the precise interferometric baseline. The system parameters which affect the accuracy of the Digital Elevation Models (DEM's) are errors in the interferometric baseline, decorrelation caused by baseline separation, thermal noise, and surface change. An adaptive filter based on the local interferometric coherence was developed to improve phase unwrapping of the interferogram. Final accuracies of the DEM's generated for single scene pairs in Alaska (U.S.) and Bonn (Germany) are on the order of 5 to 10 m.

Werner, Charles L.↗
NASA NTRS1994

International data transfer for space very long baseline interferometry

Space very long baseline interferometry (SVLBI) experiments using a TDRSS satellite have successfully demonstrated the capability of using spacecraft to extend the effective baseline length of VLBI observations beyond the diameter of the Earth, thereby improving the resolution for imaging of active galactic nuclei at centimeter wavelengths. As a result, two spacecraft dedicated to SVLBI, VSOP (Japan) and RadioAstron (Russia), are scheduled to be launched into high Earth orbit in 1996 and 1997. The success of these missions depends on the cooperation of the international community in providing support from ground tracking stations, ground radio telescopes, and correlation facilities. The timely exchange and monitoring of data among the participants requires a well-designed and automated international data transfer system. In this paper, we will discuss the design requirements, data types and flows, and the operational responsibilities associated with the SVLBI data transfer system.

Wiercigroch, Alexandria B.↗