The overall instrument design bears much heritage from Cluster. A dual redundant routing bus is used by the DPU to communicate with the individual instrument sub-units. While the DC-DC converter, analogue to digital converter and spacecraft interface cards are copies of the Cluster design, the Double Star instrument is significantly different from Cluster with respect to the sensors. The Double Star FGM utilises two non-identical sensors and electronics. The outboard (OB) sensor on both spacecraft is provided by Imperial College and the inboard (IB) by the Space Research Institute (Institut für Weltraumforschung, IWF) of the Austrian Academy of Sciences in Graz (in cooperation with the Institute for Geophysics and Extraterrestrial Physics (IGeP) of the Technical University of Braunschweig). The principal reason for this arrangement was one of expediency due to the very stringent requirements with respect to the instrument delivery schedules. Due to reasons of export control it was also necessary to redesign the instrument DPU. The newly designed Double Star sub-units are indicated in shaded colours on the Block Diagram.