Long distance measurement with sub-femtosecond timing resolution for formation-flying satellite missions

Formation-flying satellites will perform future space observation missions such as the detection of the background glow of X-rays and search for Earth-like planets. This concept of using multiple satellites for synthetic aperture imaging requires controlling the position and orientation of each satellite precisely to a reference satellite. For the purpose, a suitable means of absolute distance measurement at long ranges is crucial, which should be not only precise but also fast enough to feedback measured distances for realtime control. In this paper we demonstrate a nonlinear intensity cross-correlation technique which enables one to perform absolute distance measurements based on the time-of-flight principle. The timing difference between ultrashort pulses is traced with precision less than 1 fs, which corresponds to a sub-micrometer resolution in distance. The ambiguity range is extended to -30 km by tuning the repetition rate of the femtosecond laser with reference to an atomic clock of time standard with a fast sampling rate of several ms.