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 te st masses magnetically levitated, stiff against all unwanted degrees o f freedom. The sensitive axes of the component accelerometers are alig ned to 10(-7) rad with respect to each other, by adjusting persistent currents in the alignment coils, to improve the common-mode rejection ratio to 10(7). The axial displacements of the two test masses in each gradiometer are coupled through two superconducting circuits to two D C SQUIDs. Persistent currents are stored in the circuits such that the acceleration signals are summed and differenced at the respective SQU ID inputs. This signal differencing before detection reduces the linea rity and dynamic range requirements of the electronics by several orde rs of magnitude. The STEP gradiometer win 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 3 x 10(-5) E H z(-1/2) above 10(-3) Hz. Below this frequency 1/f power noise should a ppear. 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 4 x 10(-4) E Hz(-1/2) above 3 x 10(-3) Hz. Below th is frequency 1/f noise will dominate.