Interferometric sensing for drag-free satellites

We discuss a polarisation based homodyne interferometer that demonstrates a promising sensitivity of approximately 3 x10(-12) m/HZ(1/2). This performance figure is limited above 10Hz by the resolution of the current analogue-to-digital converter (ADC). Sensitivity below 10Hz is influenced by environmental factors and / or noise inherent in the laser. We then describe the development of a compact interferometric sensor, undertaken at The University of Birmingham, discussing its application as a zero-stiffness sensor for drag-free satellites and suggest a geometry of electrostatic actuator also with zero-stiffness.