  The Drag-Free and Attitude Control System is a central element of LISA
technology, ensuring the very high dynamic stability of spacecraft and test
masses required in order to reach the sensitivity that gravitational wave
astronomy in space requires. Applying electrostatic forces on test-masses is
unavoidable but should be restricted to the minimum necessary to keep the
spacecraft-test masses system in place, while granting the optimal quality of
test-mass free-fall. To realise this, we propose a new test-mass suspension
scheme that applies forces and torques only in proportion to any differential
test mass motion observed, and we demonstrate that the new scheme significantly
mitigates the amount of suspension forces and torques needed to control the
whole system. The mathematical method involved allows us to derive a new
observable measuring the differential acceleration of test masses projected on
the relevant sensitive axes, which will have important consequences for LISA
data calibration, processing and analysis.
