Engineering topics

Shapiro, I. I.

Publications and source records attributed to Shapiro, I. I..

At least 127 records · Page 7

NASA NTRS1976

An unusually strong radio outburst in Algol - VLBI observations

For 8 hours during a strong radio flare on Jan. 15, 16, 1975, the close binary system Beta Persei (Algol) was observed with a three-station VLBI array operating at 7850 MHz. The size of the radio source was estimated to have been about 1.7 milliarcsec (0.05 AU), based on a model of a uniformly bright disk. The corresponding brightness temperature was nearly 10 billion K, indicating that the emission was probably nonthermal. There was no evidence for expansion of the source; the upper limit on the rate of any expansion was 100 km/s. The position of the Algol radio source with respect to an extragalactic reference frame was also determined from these observations with an uncertainty of about 0.1 arcsec in each coordinate.

Clark, T. A.↗

NASA NTRS1976

A very-long-baseline interferometer system for geodetic applications

A very-long-baseline interferometer system was designed and built for geodetic applications. Each interferometer terminal records a 360-kHz spectral band of noise from a compact extragalactic radio source. The center frequency of the spectral band can be selected to sample sequentially bands covering a much wider frequency range to obtain subnanosecond accuracy in group-delay measurements. A tunnel-diode pulse generator is used to calibrate the delays in the receiver. The necessary sets of algorithms and computer programs have been developed to analyze the data and have allowed the system to be employed to make accurate determinations of vector baselines, radio-source positions, polar motion, and universal time.

Whitney, A. R.↗

NASA NTRS1976

70-cm radar observations of 433 Eros

Radar observations of 433 Eros were made at the Arecibo Observatory using a wavelength of 70 cm during the close approach of Eros to earth in mid-January, 1975. A peak radar cross section of plus or minus 15 sq km was observed. The spectral broadening obtained was approximately 30 Hz, which is consistent with a value of 16 km for the maximum radius of the asteroid. The surface of Eros appears to be relatively rough at the scale of a wavelength as compared to the surfaces of the terrestrial planets and the moon. The composition of the surface is not well determined, except that it cannot be a highly conducting metal. A single measurement each of round-trip echo times delay and Doppler shift was made.

Campbell, D. B.↗

NASA NTRS1976

Verification of the principle of equivalence for massive bodies

Analysis of 1389 measurements, accumulated between 1970 and 1974, of echo delays of laser signals transmitted from earth and reflected from cube corners on the moon shows gravitational binding energy to contribute equally to earth's inertial and passive gravitational masses to within the estimated uncertainty of 1.5%. The corresponding restriction on the Eddington-Robertson parameters is 4(beta) - gamma 3 = -0.001 plus or minus 0.015. Combination with other results, as if independent, yields values for beta and gamma in accord with general relativity.

Shapiro, I. I.↗

NASA NTRS1975

Analysis of the Capability and Limitations of Relativistic Gravity Measurements Using Radio Astronomy Methods

The uses of radar observations of planets and very-long-baseline radio interferometric observations of extragalactic objects to test theories of gravitation are described in detail with special emphasis on sources of error. The accuracy achievable in these tests with data already obtained, can be summarized in terms of: retardation of signal propagation (radar), deflection of radio waves (interferometry), advance of planetary perihelia (radar), gravitational quadrupole moment of sun (radar), and time variation of gravitational constant (radar). The analyses completed to date have yielded no significant disagreement with the predictions of general relativity.

Shapiro, I. I.↗

NASA NTRS1975

The gravity field of Mars

A complete sixth-degree spherical-harmonic model of the gravitational potential of Mars has been produced from an analysis of Doppler data obtained from the radio tracking of the Mariner-9 spacecraft for over 200 orbital revolutions. The equipotential surface defined by this model is estimated to be accurate to within 100 m below 60 deg north latitude and to within 300 m above. The dominant feature of this surface is a Tharsis-related bulge which rises nearly two kilometers above the surrounding areas. Gravitational 'lows' are found to correspond to the Hellas depression and the Valles Marineris.

Reasenberg, R. D.↗

NASA NTRS1975

Fine structure of 25 extragalactic radio sources

Interferometric observations taken at 7.8 GHz (gamma approximately = 3.8 cm) with five pairings of antennae of 25 extragalactic radio sources between April, 1972 and May, 1973 are reported. These sources exhibit a broad variety of fine structure from very simple to complex. The total flux and the correlated flux of some of the sources underwent large changes in a few weeks, while the structure and total power of others remained constant during the entire period of observation. Some aspects of the data processing and a discussion of errors are presented. Numerous figures are provided and explained. The individual radio sources are described in detail.

Wittels, J. J.↗

NASA NTRS1975

Upper limit on the radar cross section of the comet Kohoutek

On January 12, 1974, an attempt was made to observe radar echoes from the comet Kohoutek. The experiment involved the transmission of a signal at a radio frequency of 7840 MHz. The procedures used in the evaluation analyses were tested thoroughly by observing strong echoes from the planet Venus. No echo was apparent in any of the spectra obtained from the observations of the comet. An upper limit is computed regarding the radar cross section of the comet.

Chaisson, E. J.↗

NASA NTRS1974

Solar gravitational deflection of radio waves measured by very-long-baseline interferometry

Utilizing a four-antenna technique, simultaneous observations were made, at each end of an 845-km baseline, of the radio sources 3C279 and 3C273B, which are 10 deg apart in the sky. Differences in interferometric phases at 3.7-cm wavelength monitored near the time of the 1972 occultation of 3C279 by the sun, yielded a gravitational deflection of 0.99 plus or minus 0.03 times the value predicted by general relativity, corresponding to gamma = 0.98 plus or minus 0.06 (standard error).

Counselman, C. C., III↗

NASA NTRS1974

Transcontinental baselines and the rotation of the earth measured by radio interferometry

Nine separate very-long-baseline interferometry experiments, carried out in 1972 and 1973 with radio telescopes 3900 kilometers apart, yielded values for the baseline length with an rms deviation about the mean of less than 20 centimeters. The corresponding fractional spread is about five parts in 100,000,000. Changes in universal time and in polar motion were also determined accurately from these data. The rms scatter of these results with respect to those based on optical methods was 2.9 msec and 1.3 m, respectively. Solid-earth tides were apparently detected, but no useful estimate of their amplitude was extracted.

Shapiro, I. I.↗

NASA NTRS1974

The structure of radio sources 3C 273B and 3C 84 deduced from the 'closure' phases and visibility amplitudes observed with three-element interferometers

The derived 'closure' phase relation for a three-element interferometer is used in a presented analysis of data obtained from observations at 7.8 GHz of the radio sources 3C 273B and 3C 84 by antennas in Massachusetts, California, Alaska, and Sweden (the first two antennas were used in combination with each of the last two separately to form two three-element interferometers). The brightness distribution is found for each source by expansion of both the fringe amplitude and the fringe phase in separate Fourier series.

Rogers, A. E. E.↗

NASA NTRS1974

Analysis of Mariner 5 Radio Tracking Data

Mariner 5 radio tracking data was analyzed to support the following goals: (1) the determination of the mass of Venus, (2) the determination of, or the placement of a stringent upper bound on, the second order terms in the harmonic expansion of Venus' gravity field; and (3) an independent estimate of the locations of the Deep Space Net tracking stations relative to the coordinate system defined by the orbits of the planets. The rotation vector of Venus was also studied using radar observations of the planet.

Shapiro, I. I.↗

NASA NTRS1974

Radar studies of the planets

The radar measurements phase of the lunar studies involving reflectivity and topographic mapping of the visible lunar surface was ended in December 1972, but studies of the data and production of maps have continued. This work was supported by Manned Spacecraft Center, Houston. Topographic mapping of the equatorial regions of Mars has been carried out during the period of each opposition since that of 1967. The method comprised extended precise traveling time measurements to a small area centered on the subradar point. As measurements continued, planetary motions caused this point to sweep out extensive areas in both latitude and longitude permitting the development of a fairly extensive topographical map in the equatorial region. Radar observations of Mercury and Venus have also been made over the past few years. Refinements of planetary motions, reflectivity maps and determinations of rotation rates have resulted.

Ingalls, R. P.↗

NASA NTRS1974

Application of very long baseline interferometry to Astrometry and Geodesy: effects of frequency standard instability on accuracy

The accuracy of geodetic and astrometric information obtained from very long baseline interferometry (VLBI) observations is dependent upon the stability of the frequency standard, or clock, used at each site of VLBI array. The sensitivities of two hydrogen maser frequency standards of different design to pressure, temperature, and magnetic field variations were measured; and, for one of the standards, sensitivity was found to be severe enough to degrade the information content of VLBI measurements. However, the effect on the geometric and astrometric information of such clock instabilities, with time scales of hours or greater, can be sharply reduced through the use of differencing techniques.

Clark, T. A.↗

NASA NTRS1973

Extragalactic radio sources - Accurate positions from very-long-baseline interferometry observations

Relative positions for 12 extragalactic radio sources have been determined via wide-band very-long-baseline interferometry (wavelength of about 3.8 cm). The standard error, based on consistency between results from widely separated periods of observation, appears to be no more than 0.1 sec for each coordinate of the seven sources that were well observed during two or more periods. The uncertainties in the coordinates determined for the other five sources are larger, but in no case exceed 0.5 sec.

Rogers, A. E. E.↗

NASA NTRS1973

Lunar baselines and libration from differential VLBI observations of ALSEPs

Differential very-long-baseline interferometric observations of signals from Apollo Lunar Surface Experiment Package telemetry transmitters will yield the relative projected positions of the transmitters with uncertainty of only 1-3 m, set mainly by uncertainty of the lunar ephemeris. Noise and systematic instrumental errors which in the past affected similar observations have been reduced to the equivalent of a few centimeters on the lunar surface by the development of a new type of differential receiver. Continued observations should yield a determination of the motion of the moon about its center of mass with uncertainty less than 1 sec of selenocentric arc. Improvements (by other means) in our knowledge of the moon's orbital motion would allow a further order-of-magnitude refinement in the libration and relative position results obtainable by differential VLBI.

Counselman, C. C., III↗

NASA NTRS1973

Precision selenodesy via differential interferometry.

Review of the first results obtained from the development and use of a new type of differential receiver designed to determine the relative locations on the lunar surface of two Apollo-deposited transmitters. This receiver has made it possible to reduce random and systematic instrumental errors to nearly negligible levels (displacement uncertainties equivalent to a few centimeters).

Counselman, C. C., III↗

NASA NTRS1973

Celestial mechanics experiment

The efforts and accomplishments of the CME Team are summarized. The objectives and experiment status, gravity field of Mars, test of general relativity, and the generation of normal points are discussed.

Lorell, J.↗