Operating LISA as a Sagnac interferometer

A phase-locking configuration for LISA is proposed that provides a significantly simpler mode of operation. The scheme provides one Sagnac signal readout inherently insensitive to laser frequency noise and optical bench motion for a nonrotating LISA array. This Sagnac output is also insensitive to clock noise, requires no time shifting of data, nor absolute arm length knowledge. As all measurements are made at one spacecraft, neither clock synchronization nor exchange of phase information between spacecraft is required. The phase-locking configuration provides these advantages for only one Sagnac variable yet retains compatibility with the baseline approach for obtaining the other TDI combinations. The orbital motion of the LISA constellation is shown to produce a 14-km path length difference between the counterpropagating beams in the Sagnac interferometer. With this length difference a laser frequency noise spectral density of $1\mathrm{Hz}/\sqrt{\mathrm{Hz}}$ would consume the entire optical path noise budget of the Sagnac variables. A significant improvement of laser frequency stability (currently at $30\mathrm{Hz}/\sqrt{\mathrm{Hz}})$ would be needed for full-sensitivity LISA operation in the Sagnac mode. Alternatively, an additional level of time-delay processing could be applied to remove the laser frequency noise. The new time-delayed combinations of the phase measurements are presented.