Each of these techniques determines the relative positions of the same type of instruments on the ground, e.g., GNSS-to-GNSS, with an accuracy of typically a few millimeters (e.g., Herring et al. VLBI is a technique that measures the difference in arrival times, at two antennas, of the radio signals from distant objects in the Universe. SLR operates on the same principles but the signals are pulses of light. GNSS and DORIS rely on measuring the distances to Earth-orbiting satellites using radio waves. The four techniques contributing to the ITRF are currently the global navigation satellite system (GNSS), satellite laser ranging (SLR), very-long-baseline interferometry (VLBI), and Doppler orbitography and radiopositioning integrated by satellite (DORIS). Construction of the ITRF incorporates extraordinarily precise measurements by instruments both on the surface of the Earth and in space. 2016), is fundamental to both the understanding of the Earth’s structure and the functioning of society. These results have significant implications for the accuracy of global reference frames that require accurate local ties between geodetic instruments, such as the International Terrestrial Reference Frame (ITRF).Ī quantitative description of the shape of the Earth and how it changes with time, as provided by the International Terrestrial Reference Frame (ITRF, see, e.g., Altamimi et al. These results bring into focus the importance of (1) correcting to a common reference temperature the measurements of the reference points of all geodetic instruments within a site, (2) obtaining measurements of the gravitational deformation of all antennas, and (3) monitoring local motions of the geodetic instruments. Furthermore, possible tilting of the 12 m antenna increases the uncertainties in the differences in the horizontal components to 1.0 mm. We also estimate that the Up component of the baseline may suffer from systematic errors due to gravitational deformation and uncalibrated instrumental delay variations at the 20 m antenna that may reach ± 10 and −2 mm, respectively, resulting in an accuracy uncertainty on the order of 10 mm for the relative heights of the antennas. The difference between the VLBI and survey results are 0.2 ± 0.4 mm, −1.3 ± 0.4 mm, and 0.8 ± 0.8 mm in the East, North, and Up topocentric components, respectively. We applied corrections to the measured positions for the varying thermal deformation of the antennas on the different days of the VLBI and survey measurements, which can amount to 1 mm, bringing all results to a common reference temperature. The uncertainties of the latter survey were 0.3 and 0.7 mm in the horizontal and vertical components of the baseline, respectively. Independent estimates of the vector between the two antennas were obtained by the National Geodetic Survey (NGS) using standard optical surveys in 20. The two KPGO antennas are the 20 m legacy VLBI antenna and the 12 m VLBI Global Observing System (VGOS) antenna. It is presently the only directly imaged L,T,Y-dwarf known to produce an RV trend around a solar-type star.We measured the components of the 31-m-long vector between the two very-long-baseline interferometry (VLBI) antennas at the Kokee Park Geophysical Observatory (KPGO), Hawaii, with approximately 1 mm precision using phase delay observables from dedicated VLBI observations in 20. These measurements of the companion mass are independent of its brightness and spectrum and establish HR7672B as a rare and precious "benchmark" brown dwarf with a well-determined mass, age, and metallicity essential for testing theoretical evolutionary models and synthetic spectral models. HR7672B thus resides near the substellar boundary, just below the hydrogen-fusing limit. We find that HR7672B has a highly eccentric, $e=0.50^M_J$ at the 68.2% confidence level. The mass of the host star is determined using a direct radius measurement from CHARA interferometry in combination with high resolution spectroscopic modeling. In this paper, we use jointly-fitted Doppler and astrometric models to calculate the three-dimensional orbit and dynamical mass of the companion. We have also obtained a recent image of HR7672B with NIRC2 at Keck. The radial velocity variations show significant curvature (change in the acceleration) including an inflection point. Originally targeted with adaptive optics because it showed a long-term radial velocity acceleration (trend), we have monitored this star with precise Doppler measurements and have now established a 24 year time baseline. (2002) has moved measurably along its orbit since the discovery epoch, making it possible to determine its dynamical properties. The companion to the G0V star HR7672 directly imaged by Liu et al.
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