Jyri Näränen

The European Comparison of Absolute Gravimeters 2011 (ECAG-2011) in Walferdange, Luxembourg: results and recommendations

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.

Small-angle goniometry for backscattering measurements in the broadband spectrum

We present experiments on spectral bidirectional reflectance distribution function (BRDF) effects at backscatter and discuss the feasibility of new methods for laboratory and field simulations of remote sensing of land surfaces. The extreme sharpness of the intensity peak allows both directional and comparative experimental spectral studies of hot spots. We demonstrate wavelength-dependent features in the hot-spot reflectance signatures that facilitate extension of spectral and directional BRDF measurements of natural targets (such as forest understories and ice surfaces) into retroreflection to exploit their hot-spot characteristics in the interpretation of spaceborne and airborne data.

CCM.G-K2 key comparison

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).

Reflectance of various snow types: measurements, modeling, and potential for snow melt monitoring

Seasonal snow covers large parts of the northern hemisphere annually. It can change the albedo of the surfaces from dark to bright overnight (and back), causing significant climate feedback (Manninnen and Stenberg, 2008; Flanner and Zender, 2006; Pirazzini, 2008; Nolin and Frei, 2001; Roesch et al., 2001). It forms large energy reservoirs which can be exploited by hydro energy power plants, and is the source of big floods when melting. It can significantly impact traffic and construction safety. It changes living and environmental conditions radically, and has major recreational value.

The Effect of Helium Emissions by a Superconducting Gravimeter on the Rubidium Frequency Standards of Absolute Gravimeters

Recently, large offsets in the frequency of rubidium oscillators of absolute gravimeters have been reported, due to contamination by helium from a superconducting gravimeter that shares the same laboratory space. Such offsets can cause large errors in measured gravity if they go unnoticed and are not corrected for in the data processing. We give an account of the calibration histories of the rubidium oscillators of the FG5-221 (2003–2012) and of the FG5X-221 (2013–2014) of the FGI from the viewpoint of helium contamination. These gravimeters have been stationed (when not in field work) in the Metsahovi gravity laboratory, about 5 m from the superconducting gravimeter GWR TT020, in a separate room. The drift of the oscillator of the FG5-221 during the 10 years has been larger than specifications but reasonably linear. Evidence for possible helium effects is indirect: when the gravimeter is away from the laboratory some relaxation of the frequency offset is seen. This could be an indication that contamination by helium contributed to the large drift. The drift of the oscillator of the FG5X-221 has been much larger and correlates with known episodic helium emissions. During the installation of a new superconducting gravimeter GWR Dual OSG 073 in January-February 2014 and the ensuing large helium emissions we monitored the response of the FG5X-221 oscillator continuously. In 5 days, the clock frequency increased by 4.7 × 10−9. Such an offset, if not corrected for in the data processing would cause an offset of −9.2 μGal in observed gravity.