Next articleThe Cassini mission to Saturn is an international venture with participation from NASA, the European Space Agency, and the Italian Space Agency. The Cassini spacecraft was launched from Cape Canaveral in October 1997 and is scheduled to arrive at Saturn in July 2004. After arrival, the spacecraft will orbit Saturn about 60 times over a period of 4 years. During this time, the Cassini Radio Science Subsystem will be used to investigate the atmosphere and rings of Saturn and the atmosphere of its largest moon, Titan--which is larger than Mercury and is the only moon in our solar system with a dense atmosphere.
A critical component in Cassini’s Radio Science Subsystem is a traveling-wave tube (TWT) that was designed at the NASA Lewis Research Center and built by Hughes Electronic Dynamics Division (ref. 1). This TWT will amplify downlink microwave signals at a frequency of 32 GHz for the Deep Space Network and will be involved in a number of experiments. These include occultation experiments in which the microwave signal will be beamed through rings and atmospheres toward Earth. Researchers will analyze the received signals to determine the sizes and distributions of the particles in the rings and the structure and composition of the atmospheres. The Radio Science Subsystem also will also be used to more accurately determine the mass and size of Saturn and its moons, to investigate the solar corona, and to search for gravity waves from outside the solar system.
During tests of the TWT in 1995, the Jet Propulsion Laboratory (JPL) discovered an unexpected anomalous signal at a frequency of 35 GHz. Curiously, this signal only occurred at power levels below that at which the TWT was to be operated. Mission planners were very concerned that a small change in operating conditions might cause the spurious signal to occur at higher power levels and interfere with detection of the desired signal at 32 GHz. Because the Jet Propulsion Laboratory was unable to determine the cause of the anomalous signal experimentally, an in-house effort was initiated at Lewis to try to determine the cause of the anomalous signal with Lewis-developed computer analysis techniques.
Lewis’ extensive computer analysis determined that the anomalous signal is an intermodulation product of the 32-GHz signal with a 67-GHz oscillation (ref. 2). The oscillation is induced by coupling to a second harmonic beam current, which is significantly high only at low power levels. The analysis indicated that possible changes in operating conditions would not cause the anomaly to be a problem. The analysis convinced the Jet Propulsion Laboratory to not remove the Radio Science Subsystem from the Cassini spacecraft, and the experiments will proceed as originally planned.
Lewis contacts: Dr. Jeffrey D. Wilson, (216) 433-3513,
Jeffrey.D.Wilson@grc.nasa.gov
Authors: Dr. Jeffrey D. Wilson
Headquarters program office: OSS
Programs/Projects: Cassini Mission to Saturn
Last updated May 25, 1999, by Nancy.L.Obryan@nasa.gov
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