A device carried on some spacecraft to provide a more stable downlink frequency. In normal conditions, a spacecraft receives a stable uplink frequency and then uses it to generate a stable downlink frequency. The USO is used when an uplink is impossible, as when the spacecraft passes behind the Moon or a planet. Low-mass oscillators are also used for this purpose, but they are affected by temperature changes and are not highly stable. The USO, however, is kept in a temperature-controlled unit.
Subjects: Astronomy and Astrophysics.