Rubidium standards are used for very long baseline interferometry below 1 GHz, where the ionosphere dominates system stability. At higher frequencies, the use of rubidium standards degrades performance. Cesium clocks have poor short term stability (precision) so they are not used as very long baseline interferometry standards despite their excellent long term stability (accuracy). Hydrogen masers are used as VLBI standards because of their high precision. Their long-term accuracy, however, is subject to drift. Therefore, a maser is often used to drive the local oscillator, while a cesium clock is used as the long-term time standard.
