  The Laser Interferometer Space Antenna (LISA) is a proposed space mission for
the detection of gravitational waves. It consists of three drag-free satellites
flying in a triangular constellation. A gravitational reference sensor is used
in conjunction with a laser interferometer to measure the distance between test
masses inside the three satellites. Other future space mission such as DECIGO
and BBO also require a gravitational reference sensor. The Modular
Gravitational Reference Sensor (MGRS) is being designed for these purposes and
consists of two different optical sensors and a UV LED charge management
system. The Differential Optical Shadow Sensor (DOSS) is one of the optical
sensors and measures the position of a spherical test mass with respect to the
surrounding satellite. This measurement is used for the drag-free feedback
control loop. This paper describes the most recent, third generation of the
experimental setup for the DOSS that uses a fiber coupled super luminescent
LED, an integrated mounting structure and lock-in amplification. The achieved
sensitivity is 10 nm/Hz^(1/2) above 300 mHz, and 20 nm/Hz^(1/2) for frequencies
above 30 mHz.
