|Gravity Meter, often called a "Gravimeter"||- Gravimeter|
Above: InView on another mission.
Measurements of minute ( 1:200,000,000 ) changes in the gravitational field over a region can indicate the location of:
The measurement relies on precisely timed zero-crossings.
The uncertainty in the wavelength of a standards grade HeNe laser stabilised on the i (a 13 ) component of the 11-5 R(127) hyperfine transition of the 127 I 2 Iodine molecule is 2.1E-11. The error in the precision Cesium atomic clock is around 1E-12.
Note that the gravitational field strength decreases by about 3uGal with a 10 mm increase in height.
The accuracy of the FG-5 shown on the topof the opposite page is 2.1 uGal, equivalent to 6 mm change in altitude. Sander Geophysics Limited report a resolution for their airborne AIRGrav system from 150 … 300 uGal. To put these figures in context, the earth’s gravitational field strength is 980,600,000 uGal. The major challenge is to have an extremely low vibration level in the aircraft to enable accurate microgravity measurements. An alternative approach is to measure the gradient in the Earth’s gravitational field using a completely different and expensive instrument, called a “gradiometer”.