How satellites reveal the invisible shape of our gravity field
In short: We perceive the Earth as a solid, static sphere. In reality, it is a dynamic body with a complex gravitational figure and shifting crustal motions. By using artificial satellites to measure these hidden dimensions, scientists can map the planet's evolution in real-time.
Satellite geodesy is the science of measuring Earth’s form, dimensions, gravity field, and rotation. Unlike traditional astronomical geodesy, it relies on orbital mechanics to observe how the planet behaves over time. This data provides the essential reference frame required to assign coordinates to any point on Earth, allowing us to track how objects move across the globe. The field accelerated rapidly following the 1957 launch of Sputnik. By 1958, observations of Explorer 1 and Sputnik 2 allowed for an accurate determination of Earth's flattening. Throughout the 1960s, missions like the Doppler satellite Transit-1B and balloon satellites such as Echo 1 and PAGEOS paved the way.
A major milestone occurred with ANNA-1B, a collaborative effort involving NASA and the DoD, which carried the first SECOR instruments to determine leading spherical harmonic coefficients of the geopotential. Today, these techniques support diverse applications ranging from navigation and hydrography to monitoring crustal motion and polar motion. Key technologies include Global Navigation Satellite Systems (GNSS), such as the 31-satellite GPS constellation, which uses trilateration to locate receivers within a few meters. Other specialized methods like Satellite Laser Ranging (SLR) calibrate radar altimeters, while Very Long Baseline Interferometry (VLBI) measures radio wavefronts from distant quasars to stabilize our global coordinate systems. During the late 1960s and early 1970s, Soviet military satellites performed geodesic missions specifically to aid in ICBM targeting. Furthermore, NASA hosts one of six global archives for GNSS data to support these scientific efforts. These tools also enable precise orbit determination and satellite-to-satellite tracking.
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Sources
- Summary by Intellect · source: Satellite geodesy
- Also: Space Geodesy Techniques | NASA Earthdata
- Also: www.bgs.ac.uk — Geodesy and Earth observation - British Geological Survey
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