H. F. Hinteregger

Precision Geodesy Using the Mark-III Very-Long-Baseline Interferometer System

Very-long-baseline interferometry (VLBI) has been used to make precise measurements of the vector separation between widely separated antennas. The system for acquiring and processing VLBI data known as Mark-III is described. Tests of the system show it to have millimeter-level accuracy on short baselines; measurements of baselines longer than a few hundred kilometers suggest that accuracy is limited by the uncertainty in the calibration of tropospheric path delay to the level of a few centimeters. VLBI experiments conducted between 1976 and 1983 have demonstrated the stability of the North American plate by showing that there is no change in the distance between easternl-California and Massachusetts at the level of a few millimeters per year or greater. Experiments made from 1980 to 1984 indicate that the distance from Massachusetts to Sweden is increasing by 1.7 ± 1 cm/year where the quoted standard deviation includes the estimated effects of systematic atic errors

Very-Long-Baseline Radio Interferometry: The Mark III System for Geodesy, Astrometry, and Aperture Synthesis

The Mark III very-long-baseline interferometry (VLBI) system allows recording and later processing of up to 112 megabits per second from each radio telescope of an interferometer array. For astrometric and geodetic measurements, signals from two radio-frequency bands (2.2 to 2.3 and 8.2 to 8.6 gigahertz) are sampled and recorded simultaneously at all antenna sites. From these dual-band recordings the relative group delays of signals arriving at each pair of sites can be corrected for the contributions due to the ionosphere. For many radio sources for which the signals are sufficiently intense, these group delays can be determined with uncertainties under 50 picoseconds. Relative positions of widely separated antennas and celestial coordinates of radio sources have been determined from such measurements with 1 standard deviation uncertainties of about 5 centimeters and 3 milliseconds of arc, respectively. Sample results are given for the lengths of baselines between three antennas in the United States and three in Europe as well as for the arc lengths between the positions of six extragalactic radio sources. There is no significant evidence of change in any of these quantities. For mapping the brightness distribution of such compact radio sources, signals of a given polarization, or of pairs of orthogonal polarizations, can be recorded in up to 28 contiguous bands each nearly 2 megahertz wide. The ability to record large bandwidths and to link together many large radio telescopes allows detection and study of compact sources with flux densities under 1 millijansky.

Precision Geodesy via Radio Interferometry

Very-long-baseline interferometry experiments, involving observations of extragalactic radio sources, were performed in 1969 to determine the vector separations between antenna sites in Massachusetts and West Virginia. The 845.130-kilometer baseline was estimated from two separate experiments. The results agreed with each other to within 2 meters in all three components and with a special geodetic survey to within 2 meters in length; the differences in baseline direction as determined by the survey and by interferometry corresponded to discrepancies of about 5 meters. The experiments also yielded positions for nine extragalactic radio sources, most to within 1 arc second, and allowed the hydrogen maser clocks at the two sites to be synchronized a posteriori with an uncertainty of only a few nanoseconds.

Astronomical Applications of Differential Interferometry

Intercomparison of radio signals received simultaneously at several sites from several sources with small mutual angular separation provides a powerful astrometric tool. Applications include tracking the Lunar Rover relative to the Lunar Module, determining the moons libration, measuring winds in Venuss lower atmosphere, mapping Mars radiometrically, and locating the planetary system in an inertial frame.

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

Nine separate very-long-baseline interferometry (VLBI) experiments, carried out in 1972 and 1973 with radio telescopes 3900 kilometers apart, yielded values for the baseline length with a root-mean-square deviation about the mean of less than 20 centitneters. The corresponding fractional spread is about five parts in 108. Changes in universal time and in polar motion were also detertnined accurately from these data; the root-mean-square scatter of these results with respect to those based on optical methods were 2.9 milliseconds and 1.3 meters, respectively. Solid-earth tides were apparently detected, but no useful estimate of their amplituide was extracted.

Interferometric Observations of an Artificial Satellite

Very-long-baseline interferometric observations of radio signals from the TACSAT synchronous satellite, even though extending over only 7 hours, have enabled an excellent orbit to be deduced. Precision in differenced delay and delay-rate measurements reached 0.15 nanosecond (∼ 5 centimeters in equivalent differenced distance) and 0.05 picosecond per second (∼ 0.002 centimeter per second in equivalent differenced velocity), respectively. The results from this initial three-station experiment demonstrate the feasibility of using the method for accurate satellite tracking and for geodesy. Comparisons are made with other techniques.

A high data rate recorder for astronomy

A magnetic tape recorder developed for the special requirements of radio astronomy and geodesy is described. These requirements include a high bit packing density and long record times. The current version of this longitudinal recorder used by the Very Long Baseline Array (VLBA) records 5.5 Terabits on a 14-in diameter reel of inch-wide tape. A maximum record rate of 256 Mb/s is achieved in the VLBA configuration with one recorder operating at 4 ms and utilizing 32 of the heads in a single stack. The VLBA recorders have been tested using a longitudinal density of 2.25 fr/ mu m (57.15 kfrpi); 448 data +56 system tracks are recorded in 14 passes, each lasting 50 min, for a total record time (at 128 Mb/s) of 12 h on 14-in diameter reel of inch-wide 13- mu -thick D1-equivalent tape. >

The Self-Acting, Subambient Foil Bearing in High Speed, Contact Tape Recording with a Flat Head

The mechanics and tribology of a flat head for high-speed, contact tape recording is presented. Experiments performed on a “row-bar” of thin-film disk heads where the tape is wrapped only on the edge opposite to the heads showed very stable contact for a wide range of tape speeds and very low wear. A model of the interface showed that a self-acting, subambient air bearing forms near the leading wrapped corner. This suction is caused by the expansion of air into the diverging gap on the upstream side of the head-tape interface, which is unique to this wrap geometry, and it is responsible for the stability and low contact pressures. A bidirectional version of a flat head geometry is analyzed via modeling and suggestions are made for that design. This work also showed strong evidence of a threshold of contact pressure below which wear becomes negligible.

Thermal considerations for the edge guiding of thin magnetic tape in a longitudinal tape transport

Abstract High data rate recording using a longitudinal instrumentation tape transport requires high tape speeds. The need for high volumetric capacity requires thin tapes. The running of thin magnetic tape at high speeds can result in edge damage by excessive heating. In this paper, we have studied flash temperature dependence on tape speed, edge contact pressure, and edge guide material characteristics. For an instrumentation transport, some redesign was found to be needed in order to handle the new generation of thin tapes. In this transport the tape is edge guided to ensure good tracking of narrow data tracks. The edge guiding mechanism was optimized by reducing the guiding forces as much as possible and utilizing a guide with low friction, high thermal conductivity, and its ability to prevent build up of tape (polymeric) deposits. The role of the environment on the tape edge damage is also discussed.

Measurement of Magnetic Tape Abrasivity by Interchanging Tape Thickness

Magnetic tape abrasivity can be measured by observing the short wavelength signal recovery rate following an interchange of tape thickness. The method is very sensitive and can be used to measure head wear rates as low as one nanometer/hour in a few hours. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Trlbology Conference In San Diego, California, October 19–21, 1992