gravimetry

Gravimetry is the measurement of the strength of a gravitational field. This can be accomplished using a gravimeter, an instrument designed to measure gravitational force at a given mass (usually characterized as acceleration, and in some such instruments, measured by measuring acceleration) or through precise tracking of the movement of an object passing through the field, such as tracking the position of an object orbiting the body whose gravitational field is of interest. In the latter case, position is measured through timing of EMR signals sent between the probe and other probes or stations at known locations, and using triangulation (which is the same method as used by GPSes). Gravity sounding implies use of an orbit specifically for this purpose.

A gravimeter (sometimes referred to as a gravitometer) may work by timing the motion of something falling, or (analogous to a bathroom scale) by measuring the deformation of a spring. A gravimeter is designed to be extremely sensitive because the purpose is to measure the minute variations across a body's surface, i.e., the gravity anomalies. It is basically an accelerometer, a device measuring acceleration, devices widely used in consumer devices for detecting motion, but a gravimeter requires a means of isolating the acceleration due to gravity from that due to motion and everyday vibrations.

A gravity traverse is a series of measurements in a line across some landscape, carefully calibrated so as to allow comparison. The comparison of values along the way reveals the position of subsurface mass along the direction of the line.

Gravimeters are candidate instruments for inclusion on probes landing on planets, and two (the Lunar Surface Gravimeter, LSG, and the Traverse Gravimeter Experiment, TGE) were used on the Moon during the Apollo missions.

A technology used for making small instruments, e.g., for portability on Earth or for space missions is MEMS (for microelectromechanical systems), essentially electronic chips that include mechanical elements as well. A MEMS instrument to measure acceleration is termed a MEMS accelerometer, or if suitable for gravimetry, a MEMS gravimeter.

Note that the term gravimetry is also used in chemistry, for something entirely different: essentially, weighing some material undergoing a chemical reaction before and after, to determine the mass-difference. This is also termed gravimetric analysis.