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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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At least 55 records · Page 3
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NASA NTRS1997

Spacecraft Tests of General Relativity

Current spacecraft tests of general relativity depend on coherent radio tracking referred to atomic frequency standards at the ground stations. This paper addresses the possibility of improved tests using essentially the current system, but with the added possibility of a space-borne atomic clock. Outside of the obvious measurement of the gravitational frequency shift of the spacecraft clock, a successor to the suborbital flight of a Scout D rocket in 1976 (GP-A Project), other metric tests would benefit most directly by a possible improved sensitivity for the reduced coherent data. For purposes of illustration, two possible missions are discussed. The first is a highly eccentric Earth orbiter, and the second a solar-conjunction experiment to measure the Shapiro time delay using coherent Doppler data instead of the conventional ranging modulation.

Anderson, John D.↗
NASA NTRS2004

Space Weather Workshop

This workshop will focus on what space weather is about and its impact on society. An overall picture will be "painted" describing the Sun's influence through the solar wind on the near-Earth space environment, including the aurora, killer electrons at geosynchronous orbit, million ampere electric currents through the ionosphere and along magnetic field lines, and the generation of giga-Watts of natural radio waves. Reference material in the form of Internet sites will be provided so that teachers can discuss space weather in the classroom and enable students to learn more about this topic.

Gallagher, D. L.↗
NASA NTRS1982

Earth rotation information derived from MERIT and POLARIS VLBI observations

Advanced methods involving observations of extraterrestrial objects, such as artificial satellites or quasars, may make it possible to monitor geometrical variations in survey networks of regional, continental, and global scale with a spatial resolution of a few centimeters and a temporal resolution of better than one day. However, in connection with a realization of this potential, it is necessary to account for variations in the orientation of the earth in space. The accuracy provided by the conventional approaches for determining the orientation of the earth is not sufficient in this case. However, an appropriate method could be based on the utilization of independent clock radio interferometry (commonly referred to as VLBI) observations of extragalactic radio sources. The project POLARIS (Polar-motion Analysis by Radio Interferometric Sampling) is concerned with an implementation of this method. The MERIT (Monitor Earth Rotation and Intercompare the Techniques of observation and analysis) observations represent an aid to project POLARIS. Attention is given to details regarding these programs and the results obtained thus far.

Robertson, D. S.↗
NASA NTRS1998

Telemetry Data Collection from Oscar Satellite

This paper discusses the design, configuration, and operation of a satellite station built for the Center for Space Telemetering and Telecommunications Laboratory in the Klipsch School of Electrical and Computer Engineering Engineering at New Mexico State University (NMSU). This satellite station consists of a computer-controlled antenna tracking system, 2m/70cm transceiver, satellite tracking software, and a demodulator. The satellite station receives satellite,telemetry, allows for voice communications, and will be used in future classes. Currently this satellite station is receiving telemetry from an amateur radio satellite, UoSAT-OSCAR-11. Amateur radio satellites are referred to as Orbiting Satellites Carrying Amateur Radio (OSCAR) satellites as discussed in the next section.

Haddock, Paul C.↗
NASA NTRS1984

A catalog of selected compact radio sources for the construction of an extragalactic radio/optical reference frame

A catalog of 234 strong compact extragalactic radio sources that display optical counterparts is presented. This catalog identifies proposed sources for establishing an almost inertial reference frame against which the motions of the earth, solar system, galactic objects, and spacecraft may be measured. This catalog also defines those sources for which precise optical positions should be determined in order to relate this reference frame with the optical FK 5 fundamental system. The accuracy of the radio source positions of these sources is not greater than 0.1 arcsec with the majority not greater than 0.01 arcsec. Further refinement to not greater than 0.005 arcsec by future observations is expected soon.

Argue, A. N.↗
NASA NTRS1990

A determination of the radio-planetary frame tie and the DSN tracking station locations

The orientation of the reference frame of radio source catalogs relative to that of planetary ephemerides is uncertain by 30 mas (150 nrad). At this level of uncertainty this orientation offset, or 'frame tie', can be a major systematic error source for interplanetary spacecraft orbit determination. This work presents a method of determining the radio-planetary frame tie from a comparison of Very Long Baseline Interferometry (VLBI) and Lunar Laser Ranging (LLR) station coordinate and earth orientation parameter estimates. Preliminary results are presented which indicate that accuracies of 5 mas or better may be achieved with this method. An important by-product of this method of frame tie determination is a set of Deep Space Network (DSN) station locations with 10 cm per component accuracy. This station set is in a geocentric coordinate system with known orientation relative to the radio and planetary frames.

Finger, Mark H.↗
NASA NTRS1992

A determination of the radio-planetary frame tie from comparison of Earth orientation parameters

The orientation of the reference frame of radio source catalogs relative to that of planetary ephemerides, or 'frame tie,' can be a major systematic error source for interplanetary spacecraft orbit determination. This work presents a method of determining the radio-planetary frame tie from a comparison of very long baseline interferometry (VLBI) and lunar laser ranging (LLR) station coordinate and earth orientation parameter estimates. A frame tie result is presented with an accuracy of 25 nrad.

Finger, M. H.↗
NASA NTRS1991

Astrometry of southern radio sources

An overview is presented of a number of astrometry and astrophysics programs based on radio sources from the Parkes 2.7 GHz catalogs. The programs cover the optical identification and spectroscopy of flat-spectrum Parkes sources and the determination of their milliarcsecond radio structures and positions. Work is also in progress to tie together the radio and Hipparcos positional reference frames. A parallel program of radio and optical astrometry of southern radio stars is also under way.

White, Graeme L.↗
NASA NTRS1992

Position and morphology of the compact non-thermal radio source at the Galactic Center

We have determined with VLBI the position of the compact nonthermal radio source at the Galactic Center, commonly referred to as SgrA*, in the J2000.0 reference frame of extragalactic radio sources. We have also determined the size of SgrA* at 1.3, 3.6, and 13 cm wavelengths and found that the apparent size of the source increases proportionally to the observing wavelength squared, as expected from source size broadening by interstellar scattering and as reported previously by other authors. We have also established an upper limit of about 8 mJy at 3.6 cm wavelength for any ultracompact component. The actual size of the source is less than 15 AU. Fourier analysis of our very sensitive 3.6 cm observations of this source shows no significant variations of correlated flux density on time scales from 12 to 700 s.

Marcaide, J. M.↗
NASA NTRS1990

Inertial coordinate system on the sky; Proceedings of the 141st Symposium of the IAU, Leningrad, USSR, Oct. 17-21, 1989

The present conference on the inertial coordinate system on the sky encompasses the legacy of Pulkovo for inertial systems and reference frames, Pulkovo today, concepts and models, and the realization and comparison of reference frames. Specific issues addressed include the longest geodetic arc, the origin of celestial coordinates, the photographic vertical catalog, a program for determining the connection between radio and optical reference frames, positional observations of triple stars for the HST mission, and a general relativistic description of the celestial reference system. Also addressed are proposals for an improved nutation formula, dynamical reference frames in the planetary and earth-moon systems, the construction of a frame of homogeneous accuracy based on space observations, astrometric interferometry, and the definition and stability of local inertial reference frames.

Lieske, Jay H.↗
NASA NTRS1988

Source and event selection for radio-planetary frame-tie measurements using the Phobos Landers

The Soviet Phobos Lander mission will place two spacecraft on the Martian moon Phobos in 1989. Measurements of the range from Earth-based stations to the landers will allow an accurate determination of the ephemerides of Phobos and Mars. Delta Very Long Base Interferometry (VLBI) between the landers and compact radio sources nearby on the sky will be used to obtain precise estimates of the angular offset between the radio and planetary reference frames. The accuracy of this frame-tie estimate is expected to be in the vicinity of 10 mrad, depending on how well several error sources can be controlled (calibrated or reduced). Many candidate radio sources for VLBI measurements were identified, but additional work is necessary to select those sources which have characteristics appropriate to the present application. Strategies for performing the source selection are described.

Linfield, R.↗
NASA NTRS1992

High-precision VLBI astrometry of the radio-emitting star Sigma CrB - A step in linking the Hipparcos and extragalactic reference frames

VLBI observations of the optically bright radio-emitting star Sigma Coronae Borealis (Sigma CrB) have yielded its position relative to an angularly nearby quasar, its annual proper motion and its trigonometric parallax with formal uncertainties slightly better than 0.2 milliarcsec. This is the first result of the VLBI astrometric program we are conducting on 11 radio stars similar to Sigma CrB. We plan to obtain comparable results for all stars when enough epochs of observations are acquired. These stars will be used to astrometrically link the Hipparcos and VLBI extragalactic reference frames.

Lestrade, Jean-Francois↗
NASA NTRS1990

The definition and stability of local inertial reference frames

VLBI measurements of extragalactic radio sources are utilized to develop a definition system for local inertial reference frames. Significant astrometric errors are identified and minimized by means of a parameter-estimation technique. A reference frame is measured for 30 min to observe the radio emission of the object to be located in the frame as well as the emission from about five additional sources which define the frame. Because the structures of the objects are unknown and tropospheric fluctuations exist, limiting errors exist for both single-epoch position determination and epoch-to-epoch differential position measurements. Data are presented regarding relativistic gravitational deflection by Jupiter which show that the local reference frame is stable at 240 microarcseconds over 12 days.

Treuhaft, R. N.↗
NASA NTRS1983

Radio stars - A possible link between the Hipparcos optical reference frame and an extra-galactic very long baseline interferometry reference frame

The concept of typing the Hipparcos optical and the JPL VLBI frames of reference by means of VLBI measurements of the positions and proper motions of the radio components of some bright stars is considered. The properties of the thermal and non-thermal radio-stars are discussed and 22 candidate stars are selected to achieve this tie. A description is given of the first VLBI attempt to detect these stars on the intercontinental baselines of the Deep Space Network with the Mark II recording system.

Lestrade, J.-F.↗
NASA NTRS1978

A high declination search at 8 GHz for compact radio sources

With the Haystack-NRAO interferometer (baseline length of 20 million wavelengths at 3.8 cm) 37 sources were observed whose declinations were above 50 deg. Seven of these sources have compact cores with diameters smaller than 5 milliarcsec and with correlated flux densities greater than about 0.5 Jy; the remaining sources have no cores with flux densities above about 0.3 Jy, the sensitivity limit of the interferometer. Two of the sources with detected compact cores, 4C 67.05 and 3C 418, were also observed with longer-baseline interferometers; the diameter of the core of 4C 67.05 was estimated to be smaller than 1 milliarcsec and that of 3C 418 to be smaller than 0.4 milliarcsec. All diameter estimates were based on an assumed circular Gaussian distribution of radio brightness and refer to the contour with brightness density e to the -1/2 power times that of the center. Positions for the detected sources were also obtained from the interferometric data, the uncertainty in these coordinate estimates ranging from 0.04 to 0.6 arcsec. The compact core detected in 3C 390.3 was found to lie near the center of this extended (approximately 4 arcmin in diameter) double radio source and to be coincident to within 1 arcsec with an N galaxy previously identified with 3C 390.3.

Wittels, J. J.↗
NASA NTRS1988

Method for determining artillery position

A method is disclosed for determining the position of cannon from measurement sites whose distance from each other lies in the same order of magnitude as the distance between the cannons -- that distance being in the kilometer range -- with the help of the travel time evaluation of muzzle blasts received at the measurement sites. There are at least two measurement sites, consisting of a cruciform of four microphones each positioned so that one axis is oriented to an arbitrarily chosen reference direction with the microphones spaced closely together. In this arrangement of diametrically opposed microphones, the respective travel times are determined and placed in a relationship whose arctangent is a radio bearing to the reference direction in which radio bearings are determined with consideration of their position and their opposing distance from the cannon position.

Fischer, Johannes↗