More Thoughts on AG–SG Comparisons and SG Scale Factor Determinations
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We revisit a number of details that arise when doing joint AG–SG (absolute gravimeter–superconducting gravimeter) calibrations, focusing on the scale factor determination and the AG mean value that derives from the offset. When fitting SG data to AG data, the choice of which time span to use for the SG data can make a difference, as well as the inclusion of a trend that might be present in the fitting. The SG time delay has only a small effect. We review a number of options discussed recently in the literature on whether drops or sets provide the most accurate scale factor, and how to reject drops and sets to get the most consistent result. Two effects are clearly indicated by our tests, one being to smooth the raw SG 1 s (or similar sampling interval) data for times that coincide with AG drops, the other being a second pass in processing to reject residual outliers after the initial fit. Although drops can usefully provide smaller SG calibration errors compared to using set data, set values are more robust to data problems but one has to use the standard error to avoid large uncertainties. When combining scale factor determinations for the same SG at the same station, the expected gradual reduction of the error with each new experiment is consistent with the method of conflation. This is valid even when the SG data acquisition system is changed, or different AG’s are used. We also find a relationship between the AG mean values obtained from SG to AG fits with the traditional short-term AG (‘site’) measurements usually done with shorter datasets. This involves different zero levels and corrections in the AG versus SG processing. Without using the Micro-g FG5 software it is possible to use the SG-derived corrections for tides, barometric pressure, and polar motion to convert an AG–SG calibration experiment into a site measurement (and vice versa). Finally, we provide a simple method for AG users who do not have the FG5-software to find an internal FG5 parameter that allows us to convert AG values between different transfer heights when there is a change in gradient.
KeywordsSuperconducting gravimeters absolute gravimeters scale factor calibration
Two anonymous reviewers provided many insightful comments and criticisms that allowed us to rethink some of our initial results and ideas, from which we trust the reader will benefit. We also thank Bruno Meurers as editor for excellent suggestions throughout the reviewing process. We are deeply indebted to R. David Wheeler, from the National Geospatial-Intelligence Agency stationed at Holloman AFB, New Mexico, for making the difficult AG measurements in the cone room at Apache Point Observatory. DC benefitted from very useful discussions with Hartmut Wziontek (BKG, Leipzig) and Derek Van Westrum (National Geodetic Survey, Boulder). The work was done as a subcontract originating from the pioneering work of Tom Murphy (UCSD, California) on the APOLLO LLR system, and his effort to install an SG to improve the LLR; funding came from Grants 10-APRA10-0045 (NASA), PHY-1068879 and 10322410-SUB (NSF). The Strasbourg SG data is available at https://doi.org/10.5880/igets.st.l1.001.
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