Post-Newtonian reference-ellipsoid for relativistic geodesy

TL;DR

This paper develops a relativistic model of the Earth's reference ellipsoid using post-Newtonian gravity, improving the precision of geodetic measurements by incorporating general relativity into the shape and gravitational field calculations.

Contribution

It formulates a relativistic reference ellipsoid for geodesy, extending classical hydrodynamic models to include post-Newtonian effects and analyzing gauge freedom and measurable gravitational parameters.

Findings

Fractional deviation from the Maclaurin ellipsoid is smaller than previous estimates.

Derived gauge-invariant relations between mass, angular velocity, and shape parameters.

Expanded geodetic equations in eccentricity for practical applications.

Abstract

We apply general relativity to construct the post-Newtonian background manifold that serves as a reference spacetime in relativistic geodesy for conducting relativistic calculation of the geoid's undulation and the deflection of the plumb line from the vertical. We chose an axisymmetric ellipsoidal body made up of perfect homogeneous fluid uniformly rotating around a fixed axis, as a source generating the reference geometry. We, then, reformulate and extend hydrodynamic calculations of rotating fluids done by previous researchers to the realm of relativistic geodesy to set up algebraic equations defining the shape of the post-Newtonian reference ellipsoid. To complete this task, we explicitly perform all integrals characterizing gravitational field inside the fluid body and represent them in terms of the elementary functions depending on its eccentricity. We fully explore the coordinate freedom of the equations describing the post-Newtonian ellipsoid and demonstrate that the fractional deviation of the post-Newtonian level surface from the Maclaurin ellipsoid can be made much smaller than the previously anticipated estimate based on the coordinate gauge advocated by Bardeen and Chandrasekhar. We also derive the gauge-invariant relations of the post-Newtonian mass and the angular velocity of the rotating fluid with the parameters characterizing the shape of the post-Newtonian ellipsoid. We formulate the post-Newtonian theorems of Pizzetti and Clairaut that are used in geodesy to connect the geometric parameters of the reference ellipsoid to the physically measurable force of gravity at the pole and equator. Finally, we expand the post-Newtonian geodetic equations to the Taylor series with respect to the eccentricity of the ellipsoid and discuss their practical applications.