Satellite altimetry

Satellite altimetry estimates the height of Earth's surface by timing radar or laser pulses sent from an orbiting spacecraft and reflected back.

Radar altimeter: An instrument that measures distance to a surface by timing reflected radio pulses. Its pulse round-trip time supplies the range used in most satellite altimetry.
Sea surface height: The height of the ocean surface relative to a defined reference surface. Satellite measurements map its regional and global variations.
Range measurement: The determination of distance between an instrument and a target. Satellite altimetry is a specialized range-measurement method.
Skylab: The United States' first space station, operated from 1973 to 1974. Its experimental radar altimeter demonstrated space-based ocean-height measurement.
Satellite laser ranging: A technique that measures the distance to satellites by timing laser pulses from ground stations. It times pulses in the opposite direction and typically measures spacecraft rather than Earth's surface.
Laser altimeter: An instrument that measures distance by timing reflected laser pulses. Laser pulses provide altimetry measurements over ice, land, and vegetation.
Ocean topography: The spatial pattern of ocean-surface height, including slopes and broad dynamic features. Altimetry reveals currents and circulation through small sea-surface slopes.
Geodesy: The science of measuring Earth's shape, gravity field, and rotation. Geodetic reference systems define the positions and surfaces used in height calculations.
GEOS-3: A NASA geodetic satellite launched in 1975 to measure Earth's gravity field and ocean topography. It provided early satellite-altimetry observations of ocean-surface shape.
Interferometric synthetic-aperture radar: A method that uses phase differences between radar observations to map surface displacement or elevation. It derives height or change from image-phase comparisons rather than pulse travel time alone.

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