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Why must scientists determine both satellite orbits and Earth’s gravity field when using satellite measurements?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

Scientists must determine both satellite orbits and Earth’s gravity field simultaneously because the two are interdependent in satellite geodesy.

A satellite’s path is determined by the gravitational potential of Earth; however, irregularities in the planet's mass distribution create variations in this field, which in turn cause perturbations in the satellite's orbit. While precise tracking of a satellite allows scientists to map these gravitational anomalies, the accuracy of that tracking depends on having a well-defined orbital model.1

This creates an apparent paradox: the orbit is needed to measure the gravity field, but the gravity field is required to determine the orbit. To resolve this, researchers use a large volume of observational data to simultaneously calculate both the coordinates of ground stations and the orbital parameters. By solving for both, scientists can accurately define the geoid—the irregular equipotential surface that represents Earth's true figure—and account for the gravitational forces that influence satellite motion.123