Smith, Victoria Anne; (2018) Extracting geodetically useful information from absolute gravimetry at a fundamental geodetic station. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Measuring the acceleration due to gravity at the NERC Space Geodesy Facility (SGF), Herstmonceux, provides a complimentary geodetic technique to the long established satellite laser ranging and global navigation satellite system measurements. The gravimetry measurements at the SGF were added to conform to the gravity field objective of the Global Geodetic Observing System and the European Combined Geodetic Network. Since both SLR and GNSS measurements are used in the computation of the International Terrestrial Reference Frame any un-modelled movements in the ground stations are undesirable. Gravity measurements can be used in the identification of such signals. This thesis reports the establishment of the technique at the SGF and contains a description of the installation experiences, a maintenance guide for the instrument and a comprehensive analysis of the gravity measurements over the ten years of operation to date, from 2006- 2016. Environmentally driven influences on the gravity measurements are investigated. The precision of the measurements are shown to be seasonally dependent and of varying magnitude. The hydrological influence on the gravity data from groundwater variation is calculated to be approximately 3.14 µGal, determined from temporal measurement of groundwater depth and an estimation of soil properties. A maximum influence from soil moisture content is estimated resulting in an influence of less than a microgal, which confuses correlation studies between local tide gauge data and an intermittent periodic signal seen in the gravity data. The high frequency data taken at the SGF highlights bias corrections two explainable and one of unknown origin. The bias corrections, of magnitude +1.5, -2 and -7.33 µGal, are shown to be critical to the interpretation of the time series, and, simulated campaign style measurements, using one set of measurements on an annual basis, prove that the data would be easily misinterpreted if the bias offsets found are not applied.

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