LISA Gravitational Reference Sensors

We review state of the art of the gravitational reference sensor (GRS) for the Laser Interferometer Space Antenna (LISA). LISA consists of three identical spacecraft placed at the corners of an equilateral triangle with a 5 million kilometer baseline. In the LISA baseline design, the spacecraft at each corner will have two optical assemblies subtending an angle of 60 degrees. A proof mass (PM) is housed in a GRS located at the center of each assembly. LISA measures the distance variation between PMs separated by 5 million kilometers to a precision of 40 pm/Hz. The GRS must shield the PM from external disturbances such as solar wind and functions as a drag-free sensor for spacecraft control. The GRS must minimize the back action and cross talk exerted by measurements themselves. Significant progress has been made in the design, fabrication and testing of the GRS. LISA Pathfinder will fly a test GRS system scheduled around 2009. In addition, there have also been new architectures proposed to simplify the LISA payloads by using a single PM and therefore only one GRS per spacecraft. Further a modular GRS (MGRS) structure is proposed to reduce complexity. Optical sensing and large gap size between the PM and the MGRS housing are proposed to lower the disturbance level. Many experimental, engineering design, and trade off studies are underway.