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NASA NTRS1990

Numerical simulation of solar wind density fluctuations and their effects on VLF radio interferometry

A mission consisting of an array of small satellites has been proposed to investigate radio sources at frequencies from approximately 2 to 20 MHz, a range unobservable from the ground because of ionospheric absorption and aberrations. Such a mission seems feasible and relatively inexpensive, but further investigation is still necessary. The solar wind is a major concern because it contains turbulence-induced random density variations that cause a fluctuating phase shift in any electromagnetic signal passing through, sometimes making radio source imaging by interferometry impossible. A numerical simulation has been developed which attempts to accurately characterize solar wind density fluctuations. Results have allowed more concrete conclusions to be drawn about observing constraints and have shown that interferometric imaging at radio frequencies below 10 MHz with baselines up to 100 km is possible at solar elongation angles of about 90 deg or more.

Williamson, Robert S., III↗

NASA NTRS1990

Differential spacecraft tracking by interferometry

This study estimates measurement system errors for two space vehicles on the surface of Mars, and for two Mars orbiting spacecraft, which are being tracked by differential interferometry. In these examples, signals from all spacecraft lie within the same beamwidth of an earth-based radio antenna. The measurements of all spacecraft signals are made simultaneously; errors that scale with angular source separation or with temporal separation between measurement epochs are practically removed. It is shown that errors due to system thermal noise and to systematic effects within ground receiver electronics dominate, except for geometries when signals pass close to the sun, when solar plasma becomes the dominant error source. The instantaneous relative position of two orbiters may be measured to within ten meters, leading to 50-meter three-dimensional orbital accuracy.

Border, James S.↗

NASA NTRS1991

A study of dynamic stall using real time interferometry

Dynamic stall over an oscillating airfoil in compressible flow was studied using a real-time interferometry technique. Instantaneous flow field data was obtained for various unsteady as well as steady flow conditions. Comparison of steady flow interferograms with those taken in unsteady flow reveal a significant delay in the development of leading edge suction peaks in the unsteady case. The interferograms permit detailed analysis of the leading edge pressure field; as many as 13 pressure values have been obtained around the leading edge in the first 1 percent of the airfoil chord. The results offer a significant new insight into the character of the dynamic stall vortex, and the stall delay that is observed during dynamic motions.

Carr, L. W.↗

NASA NTRS1991

Applications of dark central ground interferometry

The application of dark central ground interferometry in a supersonic wind tunnel is presented. Examples of this method as a quantitative flow visualization technique for several types of supersonic flowfields are presented. Theoretical work is presented regarding the effects of vibrations and filter size. The method is shown to be an inexpensive, easy to use tool for situations requiring interferograms for flow visualization.

Loomis, Mark P.↗

NASA NTRS1991

Fiber optic phase stepping system for interferometry

A closed loop phase control system using an all-fiber optical configuration has been developed for use in phase-stepping interferometry. This system drives the relative phase of two interfering beams through a sequence of pi/2 rad increments so that the initial relative phase of these beams can be determined. This phase-stepping system uses optical fibers to provide spatially uniform phase steps from a flexible, easily aligned optical configuration. In addition, this system uses phase feedback to eliminate phase modulator errors and to compensate for phase drifts caused by environmental disturbances.

Mercer, Carolyn R.↗

NASA NTRS1991

Speckle interferometry of Hipparcos link stars. III

A third list of stars is presented which have been tested by speckle interferometry for use in the Hubble Space Telescope link between the Hipparcos astrometric reference frame and the extragalactic VLBI reference frame. Structural information on angular scales of 0.15-1.2 arcsec for 34 Southern Hemisphere stars is reported from observations made with the Imperial College Speckle Interferometer mounted on the Mount Stromlo 1.9-m telescope. Twenty-four percent of the stars (8 out of the 34) show evidence of multiplicity, in agreement with previous observations in this program.

White, Graeme L.↗

NASA NTRS1991

Orbital station-keeping for multiple spacecraft interferometry

This paper presents a three-satellite station-keeping strategy that is applicable to very long (1-100 km) baseline optical interferometry in space using the 'free-flyer' approach. The relative positions of the satellites in an arbitrarily oriented inertial coordinate system are described parametrically, and the continuous thrust vectors required to follow the paths are derived. The paths, which are constrained to be in the U-V plane of an astronomical source (a plane perpendicular to the direction of the source), are always chosen to maximize the use of gravity gradient forces and minimize the use of thrusting which can easily be provided by ion thrusters. Two different thrust programs are presented. One allows the interferometer to take data over an elliptic-shaped area of the U-V plane of arbitrary size for astronomical sources in any direction except close to the orbit plane of the system. The second thrust program reorients the U-V plane from one arbitrary source direction to another.

Decou, Anthony B.↗

NASA NTRS1990

Strain measurements using Fizzeau interferometry and photo-diode sensor in composite structures

A system is presented which allows the measurement of very small changes of strain in real time and can be applied to the design of stable space structures. The Fizzeau interferometry technique is employed in conjunction with a tube dilatometer to generate a fringe pattern that expands and contracts with temperature variations. A photodiode array transforms the bulls eye fringes to video signals which can be analyzed to determine the strain in structural elements such as tubes.

Syed, Hazari I.↗

NASA NTRS1992

Orbiter-orbiter and orbiter-lander tracking using same-beam interferometry

Two spacecraft orbiting Mars will subtend a small angle as viewed from Earth. This angle will usually be smaller than the beam width of a single radio antenna. Thus the two spacecraft may be tracked simultaneously by a single Earth-based antenna. The same-beam interferometry (SBI) technique involves using two widely separated antennas, each observing the two spacecraft, to produce a measurement of the angular separation of the two spacecraft in the plane of the sky. The information content of SBI data is thus complementary to the line-of-sight information provided by conventional Doppler data. The inclusion of SBI data with the Doppler data in a joint orbit estimation procedure can desensitize the solution to gravity mismodeling and result in improved orbit determination accuracy. This article presents an overview of the SBI technique, a measurement error analysis, and an error covariance analysis of some examples of the application of SBI to orbit determination. For hypothetical scenarios involving the Mars Observer and the Russian Mars '94 spacecraft, orbit determination accuracy improvements of up to an order of magnitude are predicted, relative to the accuracy that can be obtained by using only Doppler data acquired separately from each spacecraft. Relative tracking between a Mars orbiter and a lander fixed on the surface of Mars is also studied. Results indicate that the lander location may be determined to a few meters, while the orbiter ephemeris may be determined with accuracy similar to the orbiter-orbiter case.

Folkner, W. M.↗

NASA NTRS1992

Submillimeter horizontal position determination using very long baseline interferometry

An analysis of interferometric phase delays from 15 years of Mark I and Mark III very long baseline interferometry (VLBI) experiments carried out with two radio telescopes in Westford, Massachusetts, about 1.24 km apart, yields weighted root-mean-square (WRMS) scatters about the mean locally horizontal coordinates of 1.0 and 2.0 mm in the north and east directions, respectively. It is concluded that VLBI antennas of at least of the structural quality of the pair in Westford satisfy a necessary but not sufficient condition for being able to maintain a global reference system with submillimeter per year accuracy for intervals in excess of a decade. These data are also used to determine an error model for the VLBI group delay measurements, and, for this particular pair of telescopes, they indicate that the WRMS difference between group and phase delays is composed of a constant part (5.4 mm, for the most recent data) and a SNR term which is about 10 percent larger than that computed theoretically.

Herring, T. A.↗

NASA NTRS1992

Critical needs of fringe-order accuracies in two-color holographic interferometry

Requirements for the fringe order accuracy in two-color holographic interferometry are discussed with reference to crystal growth. A simple test cell (rectangular parallelepiped) containing a fluid is considered. The temperature and concentration variations are related to the fringe orders from the two interference patterns, and the uncertainties in the fringe orders are related to errors in the temperature and concentration determination. The formulation developed here is applied to the particular case of an aqueous solution of triglycerine sulfate as an example.

Vikram, C. S.↗

NASA NTRS1992

Orbit determination for Magellan and Pioneer 12 using same-beam interferometry

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 in progress using the Magellan and Pioneer 12 orbiters at Venus. Signals from both spacecraft fall within the same beamwidth of the earth-based tracking antennas. The plane-of-sky position difference between spacecraft is precisely determined by double-differenced phase measurements. This data type complements line-of-sight Doppler. Data were collected from Magellan and Pioneer 12 on Aug. 11-12, 1990, shortly after Magellan was inserted into Venus orbit. Orbits for both spacecraft were fit with one day data arc using Doppler and SBI data and compared to orbits fit to only Doppler data. The Doppler plus SBI orbits show improved orbit-to-orbit consistency over the Doppler-only orbits.

Folkner, W. M.↗

NASA NTRS1992

A demonstration of real-time connected element interferometry for spacecraft navigation

Connected element interferometry is a technique of observing a celestial radio source at two spatially separated antennas, and then interfering the received signals to extract the relative phase of the signal at the two antennas. The high precision of the resulting phase delay data type can provide an accurate determination of the angular position of the radio source relative to the baseline vector between the two stations. A connected element interferometer on a 21-km baseline between two antennas at the Deep Space Network's Goldstone, CA tracking complex is developed. Fiber optic links are used to transmit the data at 112 Mbit/sec to a common site for processing. A real-time correlator to process these data in real-time is implemented. The architecture of the system is described, and observational data is presented to characterize the potential performance of such a system. The real-time processing capability offers potential advantages in terms of increased reliability and improved delivery of navigational data for time-critical operations. Angular accuracies of 50-100 nrad are achievable on this baseline.

Edwards, C.↗

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.↗