A gravity meter is little more than a very sensitive accelerometer with which the accelerations caused by density contrasts between air, water, ice and rock are measured. Gravity meters are entirely passive devices that can be designed to work in vehicles, on sledges, on ships, or in aircraft. The technical challenge of doing this concentrates mainly on the isolation from or compensation for the accelerations experienced by the meter as part of the vehicle's motion. All of AWI's gravity meters are relative devices, which means they do not measure the absoute value of the acceleration due to gravity (g). Their measurements need to be calibrated and adjusted with reference to a measurement taken at some location where the absolute value of g has been measured. AWI uses any of three hand-held gravity meters from LaCoste and Romberg for the purpose of these so-called tie measurements. On board Polarstern, underway measurements are made using a KSS-32 gravimeter from Bodenseewerke. Airborne measurements in Polar 5 or Polar 6 are achieved with another LaCoste and Romberg gravity meter, specially adapted for use on moving platforms, or since 2014 with the state-of-the-art GT-2A gravity meter from the Russian firm Gravimetric Technologies. The GT-2A can reliably measure the variation in g to around 0.6 mGal accuracy, or around one part in six million.