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Why did the Lageos satellite carry hundreds of laser reflectors?

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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.

The Lageos (Laser Geodynamic Satellite) was designed for geodesy, the scientific study of the Earth's size, shape, and gravity field. To achieve high-precision tracking, the satellite carried 426 laser reflectors, which allowed ground stations to beam laser pulses at the satellite and measure the time taken for the light to travel to the satellite and back.1

This technique, known as satellite trilateration, allows scientists to determine the satellite's instantaneous distance from ground stations with extreme accuracy. By analyzing these precise orbital measurements, researchers can define the global geoid and gravity patterns, as well as calculate the drift rates of continents and the relative movement of tectonic plates. The high number of reflectors ensures that the satellite can reliably reflect these laser beams back to their source, facilitating the simultaneous determination of both station coordinates and orbital parameters.21