Space rubidium clocks and the light-shift effect

Rubidium atomic frequency standards are currently the most common atomic clocks in global navigation satellite systems. Their frequency is affected by the light-shift effect due to the lamplight used for the optical pumping process, so that lamplight instabilities turn into frequency instabilities setting a limit to the navigation performance. Here we present the outcome of a study covering years of data from 10 GPS Rubidium clocks. We characterized the lamps' behavior; we found correlations between lamplight and frequency instabilities, like jumps and long-term variations, and used them to estimate the in-orbit light-shift coefficient. We found that the clock's long-term stability is limited by the lamp's one, and that the random walk frequency noise seems to be driven by a compound Poisson process by modelling lamplight jumps. Different solutions are proposed to overcome these problems and improve the performances of rubidium clocks and, thus, of global navigation satellite systems.