Air-Sea Gravity System II

Air seaThe Micro-G LaCoste Air-Sea gravity meter (Model S) consists of a highly damped, spring-type gravity sensor that is mounted on a gyro stabilized platform with associated electronics for obtaining gravity readings.

Without resetting, the Model S gravity meter has a calibrated range of 12,000 mGals. The sensor is calibrated at a test range in Cloudcroft, New Mexico which has a gravity difference of about 240 mGal (Valliant, 1991). Each gravity meter is then laboratory tested on several testing machines which subjects it to various combinations of horizontal and vertical accelerations, ranging up to 100,000 mGals.

The Model S sensor incorporates a hinged beam supported by a zero-length spring. Damping of the large, vertical accelerations due to the ship's motion is achieved through the use of air dampers.

The gravity sensor is maintained level despite the ship's motions by means of a stabilized platform. However, the vertical accelerations of the ship also makes it impractical to keep the beam constantly nulled. Therefore, it is necessary to read the gravity sensor when the beam is in motion. Mathematical analysis shows that this can be done with a spring type gravity sensor by making use of the position of the beam, its velocity and its acceleration. Furthermore, if the beam is highly damped, the acceleration term can be neglected; and if the gravity sensor has a very high sensitivity, the position term can also be neglected.

Both of these conditions exist in the Model S gravity meter which is very highly damped and has been made very sensitive by using a "zero-length-spring" to support the beam. (A zero-length-spring is a spring whose unstretched length is zero.) Accordingly, this allows for the reading of the Model S gravity meter.