Spacecraft to spacecraft coherent laser tracking as a xylophone interferometer detector of gravitational radiation

Searches for gravitational radiation can be performed in space with two spacecraft tracking each other with coherent laser light. Four observables can be measured: two one-way Doppler data (each spacecraft transmits a light beam and the other reads out the Doppler data with an onboard laser), and two two-way Doppler data (light is coherently transponded by the other spacecraft and the Doppler effect is extracted by comparing the phases of transmitted and transponded lights). One-way and two-way tracking data recorded on board the two spacecraft are time tagged and telemetered back to Earth for data analysis. By linearly combining the four data sets, we derive a method for reducing by several orders of magnitude, at selected Fourier components, the frequency fluctuations due to the lasers. The gravitational wave signal remaining at these frequencies makes this spacecraft to spacecraft coherent laser tracking technique the equivalent of a xylophone interferometer detector of gravitational radiation. Estimates for the strain sensitivities achievable with these experiments are presented for gravitational wave bursts, monochromatic signals, and a stochastic background of gravitational radiation. This experimental technique could be implemented with two spacecraft carrying an appropriate optical payload, or with the proposed broadband, space-based laser interferometer detectors of gravitational waves operated in this non-interferometric mode.