|Glossary of GGP-Related Terms|
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Accuracy: the closeness by which a set of measurements approaches the true value; if the true value is not known the accuracy is taken to be the realistic estimation of that closeness.
Annual Wobble: the forced variation of latitude caused by the atmospheric mass fluctuations acting on the Earth's figure axis. In the mantle frame it has a period of about 1 solar year and, at mid latitudes, an amplitude in gravity of a few µgal
Chandler Wobble: the variation of latitude caused by the gyroscopic torques acting on the Earth's figure axis when displaced from its axis of instantaneous rotation. In the mantle frame it has a period of about 14 months and, at mid latitudes, an amplitude in gravity of 5-10 ugal.
Dates differ between the European and US conventions. To avoid confusion, for the abbreviated form it is safer to use the convention adopted by most scientists: yyyy/mm/dd.
Drift: Usually refers to instrument drift, the presumed secular change in gravity caused by changes in the instrument sensor and other internal components. It is usually modeled as an exponential, linear or quadratic function of time.
Eulerian Nutation: Identical motion to the Chandler Wobble, but as seen in the space-fixed (quasar) frame of reference.
Gravity: For GGP, it usually refers to the vertical derivative of the geopotential measured at the Earth's material surface, in microgal (µgal), nanogal (ngal), or nm s-2 (SI units)
Offsets: Small jumps, or tares, in a signal due to either instrumental effects or rapid changes due to real external variations (of gravity). Amplitudes varies from about 100 ngal to 100 µgal (e.g. power supply problems near a station)
Polar Motion: The departure of the Earth's axis of figure from its instantaneous axis of rotation. In the frame of reference attached to the Earth's mantle, the motion is seen as a wobble of the rotation axis. Seen from an external space frame the rotation axis is fixed and the figure axis undergoes a nutation. Polar motion is generally restricted to a combination of an annual motion, forced by the atmosphere, and the Chandler Wobble.
Precision: the consistency of repeated measurement; expressed by the number of decimal places. If a voltmeter were to make repeated estimates of an unvarying test voltage signal, precision is the standard error of these estimates.
Pressure: Atmospheric pressure at the Earth's material surface, measured in millibar (mbar), or hecto Pascal (hPa) in SI
Raw Gravity: The gravity signal as recorded before any data processing.
Residual Gravity: The gravity signal remaining when corrections of one sort or another (.e.g. tides or atmospheric pressure fluctuations) are subtracted from gravity observations. Depending on the processing involved the residual gravity could refer to a number of different quantities and must be specified explicitly.
SG: The abbreviation (in English) for the Superconducting Gravimeter, because the word superconducting in physics is now regarded as one word.