Douglas S. Robertson

Improvements in absolute gravity observations

In the absolute gravity instruments developed by the Joint Institute for Laboratory Astrophysics (JILA) (Zumberge et al., 1982; Niebauer et al., 1986), the release of the dropped object induces systematic vibrations in the floor-gravimeter system. These vibrations can cause significant errors in the observed time-distance values from which the gravitational acceleration is computed. Detailed study of the vibrations affecting the gravity observations shows them to contain random noise and site dependent systematic components, which can be modeled by decaying sinusoids in the range of 10 to 120 Hz. This paper discusses (1) a mathematical filtering method to correct the observed time-distance array by identifying and removing all non-random signals from each individual drop, (2) upgrades of the gravimeter controller, which allow the collection of the data required to implement the mathematical filtering, and (3) mechanical filtering experiments using shock dampening pads and braces to minimize the vibrations. The maximum correction to observed gravity has been 23 μGal using the mathematical filter; typical corrections are in the 2–7 μGal range. The use of the shock dampening devices alone resulted in a three-fold reduction in the amplitudes and decay times of the systematic vibrations.

Tidal variations in UT1 observed with very long baseline interferometry

Nine years of very long baseline interferometry (VLBI) observations have been analyzed to determine the magnitudes of the tidal variations in UTl for periods between 5 and 35 days. Corrections for variations in atmospheric angular momentum (AAM) significantly reduce the scatter of the measured amplitudes across both time and frequency. The AAM corrections are found to reduce the scatter in the observed tidal amplitudes by as much as 60% for periods as short as 5.6 days; in contrast, earlier studies have shown a loss of coherence between AAM and length-of-day (LOD) for periods shorter than about 10 days. The tidal amplitude measurements place bounds on both the nonequilibrium ocean and mantle anelasticity effects. The in-phase k/C determination is found to agree to better than 0.5% with the value of 0.944 from Yoder et al. (1981). The out-of-phase values are found to have a frequency dependence that can only be explained by nonequilibrium ocean effects. The observed slope is larger than the theoretical by about 3 times the expected error. This result may indicate that the ocean is farther out of equilibrium for the higher frequencies than present models permit. Significant improvements are needed in both ocean and atmosphere modeling to exploit the full capability of the VLBI observations.

Assessment of the accuracy of daily UT1 determinations by very long baseline interferometry

By comparing UT1 determinations from the International Radio Interferometric Surveying intensive program of quasi-daily, single-baseline, 1-hour very long baseline interferometry (VLBI) measurement sessions with coincident, multibaseline 24-hour VLBI determinations, we estimate the overall accuracy of the intensive UT1 results, averaged over 5.4 years of data, to be ∼52 μs, depending somewhat on the amount of error attributed to the multibaseline results. The largest error contributions are due to interpolated values for polar motion X and y coordinates (<38 μs), short-term variability in atmospheric propagation (∼48 μs during summers but much less at other times), and extended brightness structures in the radio sources (∼22 μs). Because the atmospheric effect is most pronounced during summers, the UT1 accuracy of the intensives is ∼47 μs most of the year but degrades to ∼67 μs during the third quarter of each year.

Recent Results of Radio Interferometric Determinations of a Transcontinental Baseline, Polar Motion, and Earth Rotation

Radio interferometric observations of extragalactic radio sources have been made with antennas at the Haystack Observatory in Massachusetts and the Owens Valley Radio Observatory in California during fourteen separate experiments distributed between September 1976 and May 1978. The components of the baseline vector and the coordinates of the sources were estimated from the data from each experiment separately. The root-weighted-mean-square scatter about the weighted mean (“repeatability”) of the estimates of the length of the 3900 km baseline was approximately 7 cm, and of the source coordinates, approximately 0 . ″ 015 or less, except for the declinations of low-declination sources. With the source coordinates all held fixed at the best available, a posteriori, values, and the analyses repeated for each experiment, the repeatability obtained for the estimate of baseline length was 4 cm. From analyses of the data from several experiments simultaneously, estimates were obtained of changes in the x component of pole position and in the Earth’s rotation (UT1). Comparison with the corresponding results obtained by the Bureau International de l’Heure (BIH) discloses systematic differences. In particular, the trends in the radio interferometric determinations of the changes in pole position agree more closely with those from the International Polar Motion Service (IPMS) and from the Doppler observations of satellites than with those from the BIH.

A suspended laser interferometer for determining the Newtonian constant of gravitation

Progress is reported on an experiment to measure the Newtonian constant of gravitation, G, with a suspended Fabry-Perot laser interferometer. With this technique, we measure the deflection of simple pendulums due to the gravitational attraction of tungsten masses. A result for G is expected with an accuracy of roughly 30 ppm.

High-Frequency Variations in the Rotation of the Earth

We report here observational results demonstrating that a three-station network of properly distributed VLBI observatories can routinely determine UT1 with a formal standard error of ±0.05 ms of time, in an observing period of 24 h. We also report the results of a three-month series of daily observing sessions of only 1-h duration with a single interferometer, which produced estimates of UT1 with standard errors of ±0.1 ms. The UT1 values obtained from the 1-h observing sessions track smoothly between the points of the 24-h time series, and the combined time series shows that it is not unusual for UT1 to vary by 1-2 ms in periods of several days. Preliminary results of reprocessing the 24-h observing sessions in 2-h segments suggest that variations of 0.4 ms may occur on time scales of only 6-8 h.

A suspended Fabry-Perot interferometer for determining the Newtonian constant of gravitation

Of all the fundamental constants of nature, the Newtonian constant of gravitation, G, has been one of the most difficult to measure. The current CODATA value of G has an uncertainty of 1.5 parts in 1000. Although recent experiments have produced values with uncertainties smaller than this, the adopted CODATA uncertainty reflects the fact that there is still substantial disagreement between the values from these experiments. The majority of previous measurements have used torsion pendulums or balances to convert the small gravitational attraction of a laboratory source mass into a relatively large mechanical displacement. However, our approach is to use simple pendulums, which results in a small displacement that we measure very accurately. This means that the attraction of the source masses is measured against a restoring force provided by earths gravity rather than the less well-understood torsion of a wire. Also, the shorter period of our pendulums allows us to make measurements much more rapidly than in most other experiments. In our apparatus, two mirrors, each suspended as a simple pendulum, form a Fabry-Perot cavity. A He-Ne laser locked to this cavity monitors the relative displacement of these two pendulums (through changes in its frequency) as laboratory source masses are moved, altering the gravitational pull on the mirrors.

A Suspended Laser Interferometer Determination of the Newtonian Constant of Gravitation

We are completing an experiment to measure the Newtonian constant of gravitation, G, with a suspended Fabry-Perot laser interferometer. The apparatus is operational and systematic errors have been found and eliminated. A result for G is hoped for in summer 2004 with an accuracy of roughly 30 ppm