The Tri-Directional Helicopter Gradiometer was the first 4 sensor gradiometer developed in the world.
This special gradiometer is a magnetic measuring system deployed at 30m below the helicopter. It consists of a three-armed towed bird configured with 4 sensors for calculation of Tri-Directional magnetic gradients. A special configuration of sensors is used to achieve vertical gradient spacing of 3m, and horizontal across and along track gradient separations of 3.46 and 3.76, respectively.
The sensors measure three “true” gradients based on simultaneous measurements from each of 4 sensors. Additional components include GPS (<1.5m or higher resolution) for accurate positioning of the bird during flight.