To make use of the Doppler effect for navigation and Radio Science, it is necessary to know the frequency of the radio signal transmitted from the spacecraft. Then the received frequency can be analyzed to calculate the speed of the spacecraft. The Cassini-Huygens spacecraft, like many other interplanetary spacecraft, carries an electronic assembly called an Ultra-Stable Oscillator, or USO. The USO employs a temperature-stabilized "oven" to keep its output frequency as stable as possible. During Cassini's flight, telecommunications engineers calibrate the USO by observing it for a number of hours every few months. By maintaining records and formulating a model of how the USO's output frequency changes over time, the USO can provide for moderately accurate Doppler data for navigation and Radio Science.