GNSS-grade Space Atomic Frequency Standards: Current Status and Ongoing Developments

Abstract We present an overview on the current state of Global Navigation Satellite Systems (GNSS)-grade or better space atomic frequency standards’ (SAFS) technologies and discuss their applications. We estimate that a total of more than 1000 such standards were sent to space so far, the vast majority consisting of Rubidium-cell frequency standards, Cs atomic beam frequency standards, and passive hydrogen masers. Finally, we review a variety of ongoing developments in view of future new generations of GNSS-grade SAFSs.