Precision alignment of the LIGO 4 km arms using the dual-frequency differential global positioning system

The alignment of the Laser Interferometer Gravitational-Wave Observatory (LIGO) using the global positioning system (GPS) is described. The LIGO project is designed to detect gravitational waves from astrophysical sources by laser interferometry. There are two sites separated by 3002 km that will be operated in coincidence. At each site, laser beams propagate in two orthogonal 4 km long evacuated beam lines 1.2 m in diameter. The subject of this article is the alignment of the 16 km of beam tubes using dual-frequency differential GPS. A maximum deviation from straightness in inertial space of 5 mm root mean square and an orthogonality between arm pairs of better than 5 mu rad is reported. Analysis of the as-built alignment data allows determination of the geodetic coordinates for the vertices and the arm orientations at both sites. From this information, the baseline distance between the vertices of the Hanford, Washington and Livingston, Louisiana sites was determined to be 3001.8 km. (C) 2001 American Institute of Physics.