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How does radar altimetry measure the height of the ocean surface from a satellite?

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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.

Radar altimetry measures the height of the ocean surface by using a satellite equipped with a radar altimeter to sound the distance from the satellite’s instantaneous position to the sea surface.1

The process functions as follows:

Signal Transmission: The satellite emits radar waves that reflect off the ocean surface, which acts as a reflector for these signals.1
Distance Calculation: By measuring the time it takes for the radar pulse to travel to the ocean and back, the instrument determines the precise distance between the satellite and the sea surface.12
Data Refinement: To accurately reconstruct the ocean surface, the satellite's exact orbital position must be known. Because the sea surface is dynamic, researchers must account for factors like waves, tides, and currents by averaging height measurements over several days from successive passes over identical points.1

These measurements are critical for mapping the geoid, as the ocean surface closely coincides with it once dynamic effects are removed. Furthermore, these observations reveal how seafloor features, such as seamounts and deep-sea trenches, warp the ocean surface through gravitational attraction.132