with the precise location of the spacecraft makes it possible to determine sea-surface height to within a few centimeters (about one inch). The strength and shape of the returning signal also provides information on wind speed and the height of ocean waves. These data are used in ocean models to calculate the speed and direction of ocean currents and the amount and location of heat stored in the ocean, which in turn reveals global climate variations. Laser altimetry A laser altimeter uses the round-trip flight-time of a beam of light at optical or infrared wavelengths to determine the spacecraft's altitude or, conversely, the ground topography. Examples: ICESat, MOLA. Radar altimetry A radar altimeter uses the round-trip flight-time of a microwave pulse between the satellite and the Earth's surface to determine the distance between the spacecraft and the surface. From this distance or height, the local surface effects such as tides, winds and currents are removed to obtain the satellite height above the geoid. With a precise ephemeris available for the satellite, the geocentric position and ellipsoidal height of the satellite are available for any given observation time. It is then possible to compute the geoid height by subtracting the measured altitude from the ellipsoidal height. This allows direct measurement of the geoid, since the ocean surface closely follows the geoid. The difference between the ocean surface and the actual geoid gives ocean surface topography. Examples: Seasat, Geosat, TOPEX/Poseidon, ERS-1, ERS-2, Jason-1, Jason-2, Envisat, SWOT (satellite) Interferometric synthetic aperture radar (InSAR) Interferometric synthetic aperture radar (InSAR) is a radar technique used in geodesy and remote sensing. This geodetic method uses two or more synthetic aperture radar (SAR) images to generate maps of surface deformation or digital elevation, using differences in the phase of the waves returning to the satellite. The technique can potentially measure centimetre-scale changes in deformation over timespans of days to years. It has applications for geophysical monitoring of natural hazards, for example earthquakes, volcanoes and landslides, and also in structural engineering, in particular monitoring of subsidence and structural stability. Example: Seasat, TerraSAR-X Space-to-space methods Gravity gradiometry A gravity gradiometer can independently determine the components of the gravity vector on a real-time basis. A gravity gradient is simply the spatial derivative of the gravity vector. The gradient can be thought of as the rate of change of a component of the gravity vector as measured over a small distance.