LISA is required to reduce two important noise sources by post-processing on ground using time-delay interferometry (TDI): phase-noise of the on-board reference clocks and laser frequency noise. To achieve the desired suppression, the TDI algorithm needs measurements of the differential clock noise between any two spacecraft and inter-spacecraft ranging measurements with at least 1 m accuracy, which is beyond the precision of ground-based measurements for deep space missions. Therefore, we need on-board measurements by transmitting clock noise and ranging information between the spacecraft as auxiliary functions of the laser link. This paper reports our current experimental results in clock noise transfer and ranging for noise subtraction via postprocessing as well as additional data transfer.

Interferometric gravitational wave detectors with an unequal and time-varying arm length configuration like the Laser Interferometer Space Antenna rely on time-delay interferometry (TDI) for laser frequency noise subtraction. However, the TDI algorithm requires a laser ranging scheme with meter accuracy over a five million kilometer arm length. At the end of each arm only about 100 pW of light power will be detected for gravitational wave measurements and only 1% of this power can be used for laser ranging in order to avoid degradation in the phase stability of the science measurements. Here, we present the first experimental demonstration of such a ranging scheme at 1 pW power levels using a Direct Sequence Spread Spectrum (DS/SS) modulation. This type of modulation also enables optical communication by encoding data with ranging signals and provides significant noise reduction against spurious interfering signals for bidirectional ranging. Experimental results show ranging measurements of 42 cm at 3 Hz and the viability of highly reliable data transfer at several kilobits per second.

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Ranging measurements in the radio band have been extensively used in space-based applications, for example in GNSS for navigation and GRACE for mapping the Earth's gravity field. However, the increasing demand for high-bandwidth communication and precision ranging will make optical systems ideal for these applications. Our investigations are focused on inter-spacecraft laser ranging and data communication for the LISA mission using Direct Sequence Spread Spectrum (DS/SS) modulation onto the laser links. We present the setup of an optical experiment to test the levels of performance achievable with a single laser link as well as a new hardware prototype based on FPGA (Field Programmable Gate Array) processing. This prototype performs the phase readout of the interferometric signal at microcycle sensitivity, ranging measurements at submeter accuracy, data communication at rates of several kilobits per seconds and is compatible with inter-spacecraft clock noise transmission and digital laser offset-phase locking.