ISS Engineering Technical Write-up
ISS Engineering Technical Write-up
TL: Dr. Carolyn C. Porco
ISS General Description:
The Imaging Science Subsystem (ISS) consists of a wide angle camera, with angular resolution of 60 microradians per pixel, and a narrow angle camera, with angular resolution of 6.0 microradians per pixel. The sensors are 1024x1024 CCD arrays.
ISS Scientific Objectives:
- To map the 3-dimensional structure and motions within the Saturn/Titan atmospheres.
- To study the composition, distribution, and physical properties of clouds and aerosols.
- To investigate scattering, absorption, and solar heating within the S/T atmospheres.
- To search for evidence of lightning, aurorae, airglow, and planetary oscillations.
- To study the gravitational interactions between the rings and Saturn's satellites.
- To determine the rate and nature of energy and momemtum transfer within the rings.
- To determine ring thickness and sizes, composition, and physical nature of ring particles.
- To map the surfaces of the satellites (including Titan) to study their geological histories.
- To determine the nature and composition of the icy satellite surface materials.
- To determine the rotation states of the icy satellites.
ISS Sensing Instruments:
- Wide Angle Camera [WAC](20 cm f/3.5 refractor; 380-1100 nm; 18 filters; 3.5ox3.5o)
- Narrow Angle Camera [NAC](2 m f/10.5 reflector; 200-1100 nm; 24 filters; 0.35ox0.35o)
ISS Instrument Characteristics:
- Mass (current best estimate) = 57.83 kg
- Peak Operating Power (current best estimate) = 55.90 W
- Peak Data Rate (current best estimate) = 365.568 kilobits/sec
- Dimensions (approximate) = 95x40x33 cm (NAC); 55x35x33 cm (WAC)
The Cassini orbiter imaging experiments will encompass a wide variety of targets (Saturn, the rings, Titan, the icy satellites, and star fields) and a wide range of observing distances for various scientific purposes. The science objectives include studying the atmospheres of Saturn and Titan, the rings of Saturn and their interactions with the planet's satellites, and the surface characteristics of the satellites, including Titan. Because of these multiple objectives, the Imaging Science Subsystem (ISS) has two separate camera designs. The first is a narrow-angle camera (NAC) design that will obtain high-resolution images of the target of interest. The second is a wide-angle camera (WAC) design that provides a different scale of image resolution and more complete coverage spatially. The spacecraft will carry one NAC and one WAC. The NAC is also used to obtain optical navigation images for the mission with the WAC acting as a functionally redundant backup unit for this purpose.
The cameras are charge-coupled device (CCD) imagers. A CCD is essentially a large-scale integrated circuit (IC) that has a two-dimensional array of hundreds or thousands of charge-isolated wells, each representing a picture element or "pixel." Light falling on a well is absorbed by a photoconductive substrate, such as silicon, which releases a quantity of electrons proportional to the intensity of the light. The CCD detects and stores an accumulated electrical charge representing the light level on each well. These charges are subsequently read out for conversion to digital data. CCDs are much more sensitive to light of a wider spectrum than vidicon tube-type imagers, and they are less massive, require less energy, and interface more easily with digital circuitry.
The Cassini imagers differ primarily in the design of the optics. The NAC has a focal length of 2000 mm, and the WAC , which uses optics inherited from the Voyager mission, has a focal length of 200 mm. The cameras each have a focal plane shutter of the same type as used on both Voyager and Galileo, and they have a two-wheel filter-changing mechanism derived from the Hubble Space Telescope Wide Field/Planetary Camera (WF/PC) design. The CCD detector is cooled to suppress dark current (residual current in the CCD beyond that released by incident light), which is dependent upon temperature. It is also shielded from ionizing radiation.
The CCD detector design is a square array of 10242 pixels, each pixel 12 micrometers on a side. The IC chip will use three-phase, front-side-illuminated architecture, with a coating of lumogen phosphor to provide ultraviolet response. The detector is passively cooled by a radiator to approximately 10 degrees C below its nominal operating temperature (approximately minus 90 degrees C), and then it is controlled to the operating temperature by a proportional control heater. To minimize radiator size and heater power, the detector/radiator combination is thermally isolated from the rest of the camera head assembly (CHA).
The entire NAC is thermally isolated from the remote sensing pallet (RSP) on which it is mounted in order to minimize the effects of RSP thermal variations on NAC image quality. The WAC, being an inherited design with less stringent imaging requirements, is not thermally isolated.
The electronics for each camera are identical. All ISS command and telemetry functions will be handled by the electronics, including recipt of commands from the Command and Data Subsystem, expansion of commands, and collection and transmission of imaging data and telemetry to the CDS.
The ISS controls the amount of power it draws from the spacecraft during operations. To accomplish this, the profile of ISS command timing is structured to reduce the power the ISS requires for certain internal functions (e.g., shutter or filter wheel movement). When the filter is moving, the power from the optical heater (if present) in the active camera is turned off. When the movement is complete, the optical heater is turned on (if needed). In addition, simultaneous filter positioning within a single camera, either the WAC or NAC, is not permitted.
During the cruise phase of the mission, the cameras will periodically be turned on for maintenance, calibration, and monitoring of instrument health and performance. Other than these specified times, the ISS will be off and replacement heaters will be on. In addition, decontamination/radiation heater 1 will be on throughout most of the cruise.
Upon arrival at the Saturnian system, the cameras will be on most of the time. Spacecraft power limitations will be the controlling parameter determining whether the ISS will be turned off or put into a low-power state. During the Saturn tour, high-activity periods for Saturn and its rings will be clustered around periapsis (the point in the orbit closest to the planet); for the satellites, the high-activity periods will be when the spacecraft is closest to them. At these times, high-resolution images of all targets will be acquired through various camera filters, and the data will be stored in the spacecraft solid-state recorder (SSR). During lower activity periods (i.e., when the spacecraft is orbiting farther from the targets), long-term atmospheric and ring monitoring will take place, and ISS calibrations will be performed.
For additional information, see: