Consistency of Earth Rotation, Gravity, and Shape Measurements

Richard Gross⁽¹⁾, David Lavalle⁽²⁾, Geoffrey Blewitt⁽³⁾ and Peter Clarke⁽²⁾

⁽¹⁾ Jet Propulsion Laboratory, Geodynamics and Space Geodesy Group, Pasadena, CA 91109, United States
⁽²⁾ University of Newcastle, School of Civil Engineering and Geosciences, Newcastle upon Tyne NE1 7RU, United Kingdom
⁽³⁾ University of Nevada, Mackay School of Earth Sciences and Engineering, Reno, NV 89557, United States

Abstract

The rearrangement of mass within the surficial fluid layers of the Earth, including the atmosphere, oceans, and water, snow and ice stored on land, causes the Earth's gravitational field to change, causes the Earth's rotation to change by changing the Earth's inertia tensor, and causes the Earth's shape to change by changing the load acting on the solid, but not rigid, Earth. Large-scale changes in the Earth's gravitational field have been measured for more than two decades by satellite tracking and more recently by the CHAMP and GRACE satellite missions. Changes in the Earth's rotation have been measured since the 1970s by the space-geodetic techniques of satellite and lunar laser ranging (SLR and LLR) and very long baseline interferometry (VLBI) and more recently by the global positioning system (GPS). GPS can also be used to measure large-scale changes in the Earth's shape by precisely positioning the sites of a global network of ground-based GPS receivers. On time scales of months to a decade, loading of the solid Earth by surface fluids dominates the observed variations in each of these three fundamental geodetic quantities (gravity, rotation, and shape). Since these three quantities all change in response to the same changes in surface mass load, observations of these quantities must be consistent with each other. This consistency is evaluated here on monthly to interannual time scales. Degree-2 spherical harmonic coefficients of the surface mass density determined from independent GRACE, SLR, GPS, and Earth rotation measurements are compared to each other and to a model of the surface mass density obtained by summing the contributions of atmospheric surface pressure, ocean-bottom pressure, land hydrology, and a mass-conserving term. In general, the independent measurements are found to be quite consistent with each other and with the model, with correlations being as high as 0.87 and with the model explaining as much as 88% of the observed variance. Of the different measurements, SLR agrees best with the modeled degree-2 zonal coefficient, Earth rotation with the modeled (2,1) sine coefficient, and GRACE with the other modeled degree-2 coefficients although GPS agrees nearly as well as GRACE with the modeled (2,1) cosine coefficient.