|Airborne full tensor Gradiometer||- Gradiometer|
The 3D Full Tensor Gradiometer (FTG) System,was developed and manufactured by Lockheed Martin Federal Systems. Bell Geospace purchased two systems for use in marine applications. Both units have been upgraded for airborne surveys but still maintain the capability to acquire high quality Marine-FTG® data.
- above and below from http://www.bellgeo.com/tech/measurements.html
The 3D FTG system consists of 3 Gravity Gradiometer Instruments (GGI's). Internal to each GGI is a rotating disk with 4 accelerometers. The opposing pairs of matched accelerometers are mounted 10 cm apart. The disk rotates at a commanded rate, usually 0.5 Hz. Gradients are measured by the difference in readings between opposing pairs of accelerometers. The output of all 4 accelerometers is summed together, to measure the gradient accelerations, with linear accelerations removed. Data is sampled at 128 Hz.
The GGI's are mounted on a 3 gimbaled stabilized platform, oriented at 120 degrees from each other and 54.74 degrees from vertical. The azimuth gimbal is set to carousel at a commanded rate, usually 300 degrees /hour. Rotation of the GGI's through all the planes, allows for improved noise reduction.
Historical note. In 1982 the USAF Geophysical Laboratory started a program to develop a Gravity Gradiometer Survey System (GGSS). With funding from the Defense Threat Reduction Agency, Lockheed Martin further improved the gradiometer, creating the basis for both the Bell Geospace Air-FTG system and the BHP Billiton Falcon Gradiometer.