Galileo undergoes calibration
Posted: April 24, 2001

An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL
This week sees the continuation of the set of instrument calibrations that began on Sunday. On Monday, the Near Infrared Mapping Spectrometer (NIMS) views a calibration plate mounted on the spacecraft. Since NIMS is sensitive to thermal emissions (heat), this Radiometric Calibration Target plate is warmed to a known temperature, and the instrument measures the signal it sees. By comparing the signal from this known source with those from observations of Jupiter's atmosphere or of the surfaces of the satellites, scientists can determine the correct temperatures of those features.

The Solid State Imaging camera (SSI), which imaged Saturn on Sunday, today views its largest satellite Titan. The observations are made through three filters that are sensitive to wavelengths of light characteristic of absorption by methane gas. Since both Saturn and Titan have prominent methane atmospheres, these bodies make excellent calibration targets.

Following these activities, Galileo will be turned approximately nine degrees so that the spin axis of the spacecraft will be pointed directly at the Sun. This allows the Sun to shine directly on another calibration plate on the spacecraft, the Photometric Calibration Target. When this plate is evenly illuminated, and not shadowed by any other parts of the spacecraft, nor varying in brightness as the spacecraft spins, both NIMS and SSI can view a flat field of known, uniform intensity. This allows the instruments to determine if their sensitivities vary across their respective fields of view, or have changed since the last calibration of this type in 1997.

This is the last time in the mission that we plan on calibrating SSI and NIMS in this way. These measurements will be stored on the spacecraft tape recorder. Late Monday night playback of the data will begin, and this will continue over the next month, completing prior to the next flyby of Callisto near the end of May.

After the calibrations are complete, the spacecraft is again turned to point the communications antenna towards Earth. Then a special engineering test of the SSI electronics will be done. During the last encounter with Ganymede in December of 2000, there were several times when an intermittent problem in the instrument electronics saturated the signal received from the imaging CCD sensor. This had the same effect as shining a bright light into the camera, washing out the pictures taken. Tests have shown that turning the instrument power off and on again clears up the problem. However, when the instrument is turned off, the software that governs the camera operations must be reloaded into its computer memory. This test exercises a new technique to reload that software more quickly and using fewer commands. This technique will make it easier to restore camera operations if the problem should recur.