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Precise Geodetic Infrastructure: National Requirements for a Shared Resource
Sea level—In general, the reference elevation of the surface of the sea from which elevations are measured. This term is used as a curtailed form of mean sea level (see Datum) (NRC, 2007b).
Sea surface height—The spatially and temporally variable height of the sea surface.
Sea surface topography—Sea surface height.
SEASAT—The first satellite designed for remote sensing of the Earth’s oceans with synthetic aperture radar.
Shoreline—The boundary line between a body of water and the land, in particular, the boundary line between the water and the line marking the extent of high water or mean high water (Datum) (NRC, 2007b).
Space weather—The environmental conditions in near-Earth space, including the ionosphere.
Stability—Reference frame and station position predictability through time.
Standard—An agreed-upon procedure in a particular industry or profession that is to be followed in producing a particular product or result (NRC, 2007b). Alternatively, a number, or set of numbers, established in an industry, a science, or a technology, setting limits on the precision or accuracy with which operations, measurements, or products are to be made.
Starlette—A passive French satellite, launched in 1975, used in Satellite Laser Ranging, predominantly to measure the gravity field.
Stick-slip—Behavior (often of a fault) characterized by periods of sticking followed by periods of slipping.
Strain—A measure of deformation that occurs (in the Earth’s crust, for example) in response to applied forces.
Subduction—The process whereby one of the Earth’s tectonic plates flows beneath another plate.
Subsidence—Downward vertical motion of land.
Surface Water Ocean Topography (SWOT)—A proposed NASA/CNES satellite mission to make the first complete survey of Earth’s oceans and freshwater bodies.
Synthetic Aperture Radar—A radar containing a moving or scanning antenna; the signals received are combined to produce a signal equivalent to that which would have been received by a larger, stationary antenna (NRC, 2007b).
TOPEX-Poseidon—Joint NASA/CNES satellite altimeter for mapping ocean surface topography.
Topography—The form of the features of the actual surface of the Earth in a particular region, considered collectively; also called terrain (Maune, 2007).
Total Electron Content (TEC)—A measure of the density of free electrons per square meter as integrated along a path traced through the ionosphere, usually measured in TEC units, where 1 TEC = 1016 electrons / m2.
Troposphere—The lowest region of the atmosphere, containing almost all the water vapor.
UNAVCO—NSF- and NASA-sponsored organization that supports geodetic research in the United States.
Universal Time 1 (UT1)—Used to represent the Earth’s rotation, this nomenclature is left over from when time was determined by the Earth’s rotation rather than by atomic clocks.
Vertical Datum—See Datum.
Very Long Baseline Interferometry (VLBI)—A geodetic technique using large, ground-based, parabolic-dish radio telescopes to observe quasars (the most distant objects in the cosmos). VLBI sites provide information on the Earth’s rotation and the direction of the Earth’s spin axis.
Wobble—See polar motion.
World Geodetic System 1984 (WGS-84)—The latest version of the DoD World Geodetic System, which is consistent with ITRF at the centimeter level (but the ITRF is more accurate).