Review: Requirements for a Ground Rotation Sensor to Improve Advanced LIGO

The goal of the Laser Interferometer Gravitational-Wave Observatory (LIGO) project is to measure gravitational waves from astrophysical sources. The project has recently begun the construction of Advanced LIGO, a major upgrade to the existing detectors. The performance of the new detectors requires the optical component to be mounted on high performance seismic isolation and alignment systems. The performance and reliability of these isolation systems could be improved with accurate measurements of the tip-tilt rotations of the ground at the level of 3 x 10(-10) rad/root Hz at 10 mHz to 6 x 10(-9) rad/root Hz at 200 mHz. We briefly describe the LIGO project and explain the desire for rotation sensing of the ground. We then derive the performance requirements for a rotation sensor and show that it would improve the Advanced LIGO detector.