Superconducting gravity gradiometers on STEP and GEM

The superconducting gravity gradiometers for the European STEP and GEM missions have common design features. Both gradiometers have their test masses magnetically levitated, stiff against all unwanted degrees of freedom. The sensitive axes of the component accelerometers are aligned to with respect to each other, by adjusting persistent currents in the alignment coils, to improve the common-mode rejection ratio to . The axial displacements of the two test masses in each gradiometer are coupled through two superconducting circuits to two DC SQUIDs. Persistent currents are stored in the circuits such that the acceleration signals are summed and differenced at the respective SQUID inputs. This signal differencing before detection reduces the linearity and dynamic range requirements of the electronics by several orders of magnitude. The STEP gradiometer will be a single-axis device with a baseline of about 60 cm and with its sensitive axis oriented along the orbit normal. Its intrinsic noise is expected to be above . Below this frequency 1/f power noise should appear. A compact three-axis superconducting gravity gradiometer with a baseline of 12 cm is proposed for GEM. This gradiometer will have an intrinsic noise of above . Below this frequency 1/f noise will dominate.