Optical Clock Technologies for Future GNSS

Clocks are the primary payload on GNSS satellites. In the last decades, optical clock technologies evolved, where laboratory setups \nof single ion and lattice clocks have demonstrated frequency stabilities at the 10-18 level for longer integration times of a few thousand \nseconds. Setups using Doppler-free spectroscopy of molecular iodine have much lower complexity and demonstrated frequency \nstabilities at the 10-15 level for integration times up to 10,000 s. They might be an option for future GNSS, in a first step backing-up \nor replacing the currently used microwave clocks. Future concepts of GNSS employ optical technologies including optical frequency \nreferences and optical inter-satellite links. Beside long-term stability, also short-term stability of the oscillators is becoming relevant. \nLasers, frequency stabilized to an optical cavity deliver short-term stability up to several seconds and are most promising candidates \nfor such an oscillator. We present current work on the development of iodine- and cavity- based optical frequency references with \nrespect to applications in GNSS.