Highly stable frequency references are needed for position, naviation, and timing applications (PNT). They have been used for exoplanet exploration, precision LADAR, and precision spectroscopy for Earth and planetary science. Frequency combs can be of interest to the NASA SCaN office to set up precise free-space timing networks for navigation, and improve data transmission rates. Free-space timing networks can be employed using NASA's Deep Space Atomic Clock (DSAC) for space craft navigation and to improve the GPS system, and for sensitive tests of General Relativity. Coherent optical frequency combs provide an absolute frequency ruler that covers the visible to near-IR range that can be used for laser stabilization and absolute referencing, and precise distance metrology and mapping using LIDAR. Comb-based spectroscopy including cavity-enhanced spectroscopy provides a tremendous spectral coverage with accurate wavelength position in a relatively compact optical system.