Ryan Billson’s scientific contributions

A corner-cube retroreflector has the property that the optical path length for a reflected laser beam is insensitive to rotations about a mathematical point called its optical center (OC). This property is exploited in ballistic absolute gravity meters in which a proof mass containing a corner-cube retroreflector is dropped in a vacuum, and its position is accurately determined with a laser interferometer. In order to avoid vertical position errors when the proof mass rotates during free fall, it is important to collocate its center of mass (COM) with the OC of the retroreflector. This is commonly done using a mechanical scale-based balancing procedure, which has limited accuracy due to the difficulty in finding the exact position of the COM and the OC. This paper describes a novel way to achieve the collocation by incorporating the proof mass into a pendulum and using a quadrature interferometer to interrogate its apparent translation in its twist mode. The mismatch between the COM and OC generates a signal in a quiet part of the spectrum where no mechanical resonance exists. This allows us to tune the position of the COM relative to the OC to an accuracy of about 1 μm in all three axes. This provides a way to directly demonstrate that a rotation of the proof mass by several degrees causes an apparent translation in the direction of the laser beam of less than 1 nm. This technique allows an order of magnitude improvement over traditional methods of balancing.

In November 2013 an International Key Comparison, CCM.G-K2, was organized in the Underground Laboratory for Geodynamics in Walferdange. The comparison has assembled 25 participants coming from 19 countries and four different continents. The comparison was divided into two parts: the key comparison that included 10 NMIs or DIs, and the pilot study including all participants. The global result given by the pilot study confirms that all instruments are absolutely coherent to each other. The results obtained for the key comparison confirm a good agreement between the NMI instruments. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

We present the results of the third European Comparison of Absolute Gravimeters held in Walferdange, Grand Duchy of Luxembourg, in November 2011. Twenty-two gravimeters from both metrological and non-metrological institutes are compared. For the first time, corrections for the laser beam diffraction and the self-attraction of the gravimeters are implemented. The gravity observations are also corrected for geophysical gravity changes that occurred during the comparison using the observations of a superconducting gravimeter. We show that these corrections improve the degree of equivalence between the gravimeters. We present the results for two different combinations of data. In the first one, we use only the observations from the metrological institutes. In the second solution, we include all the data from both metrological and non-metrological institutes. Those solutions are then compared with the official result of the comparison published previously and based on the observations of the metrological institutes and the gravity differences at the different sites as measured by non-metrological institutes. Overall, the absolute gravity meters agree with one another with a standard deviation of 3.1 µGal. Finally, the results of this comparison are linked to previous ones. We conclude with some important recommendations for future comparisons.

The measured value of gravity from a ballistic absolute gravity meter depends upon a stable clock reference. Typically, a commercial rubidium atomic clock is employed for the time reference. These clocks are very robust and have a low drift rate. However, they are adversely affected by the presence of helium, which is a principle component of the superconducting gravity meter, which is often used with absolute gravity meters to control their drift. We have made measurements of the diffusion rate of helium from contaminated rubidium clocks from FG5 instruments and have found a characteristic time constant that can be employed to correct past values once contamination is identified. We also present frequency measurements from clocks exposed to a controlled helium environment. Finally, we discuss strategies to avoid helium contamination in rubidium clocks. Precautions can be taken while servicing superconducting gravity meters to avoid helium contamination.

The gravitational attraction of the body of a gravity meter upon its own proof mass is sometimes called the self-attraction. The self-attraction is a source of systematic error for absolute measurements of g, the acceleration of an object due to Earth's gravity. While the effect is typically small—of the order of one part per billion of the Earth's gravitational attraction—it is significant at the current level of accuracy of absolute gravity meters. In the past, a self-attraction correction for the FG5 gravity meter has been estimated by considering a rather coarse description of the instrument using simple geometrical shapes (spheres and cylinders). This paper describes a more complete calculation using a CAD-based digitized model of the newest FG5X instrument. We have also included the attraction of the co-moving drag-free chamber as well as the self-attraction of the counterweights used in the FG5X to reduce recoil. The results are also applicable to older style FG5 instruments with a fibre-optic interferometer base. The correction found with this new approach agrees with previous estimates but is now based upon a more complete and accurate model.

The European Comparison of Absolute Gravimeters, ECAG 11, was held in the Underground Laboratory for Geodynamics in Walferdange, Luxembourg in November 2011. The ECAG-2011 is registered as EURAMET project 1186 as well as Key Comparison EURAMET.M.G-K1. METAS was the Pilot Laboratory under the leadership of Dr. Henri Baumann. Prof. Dr. Olivier Francis, Dr. Christian Rothleitner and Ing. Gilbert Klein from the University of Luxembourg are the members of the local organizing committee. Before the comparison, the Technical Protocol (TP) based on the document of the ICAG-2009 was presented to the participants. This important document includes the list of the reg-istered participants, a description of the comparison site, the timetable of the measurements, and a standardized excel table to express the uncertainty of the gravimeters. The TP also specifies the data processing as well as the reporting of the results. The final version of the TP was approved by all the participants the 15th of September 2011. The report is based on the EURMATE.M.GK1 Draft B report the has been approved by all participants. It includes a description of the Walferdange Underground Laboratory for Geo-dynamics where the comparison took place, the list of the participants, the absolute gravity measurements, the measurement strategy, a section on the self-attraction and laser beam diffraction corrections, the data processing, the results, and links with previous comparisons. In the conclusion, we also propose some recommendations for future comparisons.

The European Comparison of Absolute Gravimeters, ECAG 11, was held in the Underground Laboratory for Geodynamics in Walferdange, Luxembourg in November 2011. The ECAG�2011 is registered as EURAMET project 1186 as well as Key Comparison EURAMET.M.G�K1. METAS was the Pilot Laboratory under the leadership of Dr. Henri Baumann. Prof. Dr. Olivier Francis, Dr. Christian Rothleitner and Ing. Gilbert Klein from the University of Luxembourg are the members of the local organizing committee. Before the comparison, the Technical Protocol (TP) based on the document of the ICAG�2009 was presented to the participants. This important document includes the list of the reg�istered participants, a description of the comparison site, the timetable of the easurements, and a standardized excel table to express the uncertainty of the gravimeters. The TP also specifies the data processing as well as the reporting of the results. The final version of the TP was approved by all the participants the 15th of September 2011. The report is based on the EURMATE.M.GK1 Draft B report the has been approved by all participants. It includes a description of the Walferdange Underground Laboratory for Geo�dynamics where the comparison took place, the list of the participants, the absolute gravity measurements, the measurement strategy, a section on the self-attraction and laser beam diffraction corrections, the data processing, the results, and links with previous comparisons. In the conclusion, we also propose some recommendations for future comparisons. Dowload @ https://www.bipm.org/kcdb/comparison?id=695

This paper discusses simultaneous gravity and vertical gravity gradient measurements obtained with a newly designed recoil-compensated dropping chamber adapted to an FG5 absolute gravimeter. The new dropping chamber incorporates counterweights to compensate recoil effects. It has the same physical length as the standard FG5 dropping chamber but the free-fall distance was increased from 20 cm to 25 cm. The new drive train pulls on the centre of the system to reduce unwanted horizontal velocity and rotation of the free-falling test mass. The test-mass material was chosen to reduce possible magnetic eddy-current damping caused by external magnetic field gradients. External lead masses were used to change the gravity and vertical gravity gradient. The measurements agree well with the theoretical gravity field changes derived from the position of the external weights. The experiment clearly demonstrates the efficacy of using an absolute gravity meter to measure both the gravity and the gravity gradient signals caused by variations in the external gravity field. This technique shows promise for passive gravity-monitoring applications.

The results of an international comparison of absolute gravimeters held in Walferdange Luxembourg in November 2003 are presented. The absolute meters agreed with one another with a standard deviation of less than 2 µGal (1 Gal = 1 cm/s2) (if we exclude one prototype instrument from the analysis). For the first time, the ability of the operators was put to the test. The comparison indicates that the errors due to the operator are less than 1 µGal, i.e. within the observational errors.