Satellite and Space Geodesy

Satellites have transformed geodesy, delivering global reference frames, millimeter positioning, and direct measurements of the Earth's gravity field and its changes over time.

Definition

Satellite and space geodesy is the determination of the Earth's geometry, gravity field, orientation, and their temporal variations using observations of and from artificial satellites and extragalactic radio sources, including positioning systems, ranging, altimetry, and gravity-mapping missions.

Scope

This topic covers the space techniques of modern geodesy: global navigation satellite systems for precise positioning, satellite laser ranging and very long baseline interferometry for reference frames and Earth rotation, satellite altimetry for sea-surface and geoid measurement, and dedicated gravity missions that recover the time-varying gravity field. It treats the definition and realization of terrestrial reference frames, and the geophysical signals these methods detect, from plate motion to mass redistribution. The emphasis is on how observations from space determine position, shape, rotation, and gravity.

Core questions

• How do satellite navigation systems achieve precise global positioning?
• How are terrestrial reference frames and Earth rotation realized from space?
• How does satellite altimetry measure the sea surface and the marine geoid?
• How do dedicated missions recover the time-varying gravity field?

Key concepts

• Global navigation satellite systems and precise positioning
• Satellite laser ranging and very long baseline interferometry
• Terrestrial reference frames and Earth orientation
• Satellite altimetry
• Time-variable gravity from satellite missions

Key theories

Space-geodetic reference frames and positioning

Combining satellite navigation, laser ranging, and radio interferometry yields a stable global reference frame and millimeter-level positions, enabling direct measurement of plate motions, crustal deformation, and Earth rotation.

Time-variable gravity from satellites

Tracking the changing distance between or perturbations of orbiting satellites recovers monthly maps of the gravity field, whose variations record the redistribution of water, ice, and other mass at and below the surface.

Mechanisms

Precise timing of signals from navigation satellites, two-way laser ranging, and interferometric observation of distant quasars fix positions and the orientation of the Earth in space; for gravity, the orbit of a low satellite, or the varying separation of a satellite pair, responds to the underlying mass distribution, so measuring these orbital perturbations over time yields the static and changing gravity field.

Clinical relevance

Space geodesy provides the global positioning and reference frames underlying navigation, mapping, and timing infrastructure, measures tectonic and volcanic deformation for hazard monitoring, and tracks ice-sheet melting, groundwater depletion, and sea-level rise central to climate science.

History

Satellite geodesy began with tracking of early artificial satellites that refined the Earth's flattening, matured with the global navigation satellite systems and laser ranging of the late twentieth century, and reached gravimetry with dedicated missions such as GRACE, launched in 2002, and GOCE.

Key figures

• Byron Tapley
• Reiner Rummel
• Günter Seeber

Seminal works

• seeber2003
• tapley2004
• torge2012

Frequently asked questions

How can satellites measure the Earth's gravity field?

A satellite's orbit is perturbed by the uneven gravity of the Earth below it; by precisely tracking these perturbations, or the tiny changes in distance between two satellites flying in formation, missions can map the gravity field and even watch it change as water and ice move.

What does space geodesy contribute to studying climate change?

Time-variable gravity missions detect the loss of mass from ice sheets and glaciers and changes in groundwater storage, while satellite altimetry measures sea-level rise, providing direct global measurements of how mass and water are redistributing.