PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = "2004-04-24 A.C.Raugh Minor editing to conform to current standards" OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "GO" INSTRUMENT_ID = "SSI" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "SOLID STATE IMAGING SYSTEM" INSTRUMENT_TYPE = "FRAMING CAMERA" INSTRUMENT_DESC = " Instrument Overview =================== The SSI is a single-camera system based on an 800-line-by-800- element solid-state silicon image sensor array called a charge- coupled device (CCD). The camera head, composed of a radiation- shielded, radiatively cooled CCD, and supporting electronics, is coupled to a 1500 mm optical system. The optics subassembly, inherited from the Voyager project and recoated to complement the CCD's spectral characteristics, consists of an all- spherical, catadioptric Cassegrain telescope operating at a fixed relative aperture of f/8.5. Scientific Objectives ===================== The primary scientific objectives of the imaging experiment are to investigate the chemical composition and physical state of the Jovian satellites and the structure and dynamics of the Jovian atmosphere. Instrument Calibration ====================== In-flight/radiometric calibration is implemented by imaging a flat, solar-illuminated calibration target carried aboard the orbiter. Stars or other celestial objects may also be imaged in support of SSI in-flight calibration. To establish CCD dark- current correction values, it is possible to read out SSI frames without prior shuttering. Operational Considerations ========================== Several practical constraints limit the degree to which the science objectives can be met. The capacity of the telemetry link between the spacecraft and Earth limits the acceptable data rate from the camera, thereby placing constraints on the format size, encoding level, and frame rate. These constraints are alleviated somewhat by including 9 x 10**8 bits of tape recorder storage for onboard buffering and by the capability for data compression in a ratio of about 2.5:1. SSI pointing is accomplished by using an articulating scan platform attached to the orbiter. This platform, while extremely stable, does have some residual motions associated with it. Smear considerations then influence requirements regarding camera sensitivity, short shutter times, and filter transmission and passband width. The harsh Jovian radiation environment necessitates extensive shielding, particularly of the SSI sensor. Rapid image readout onto the tape recorder is used to minimize the buildup of radiation-induced noise. Power constraints limit the rate of filter stepping permitted. Mass limitations partially define the telescope aperture and the amount of radiation shielding that can be used. Detectors ========= SSI --- DETECTOR_TYPE = SI CCD DETECTOR_ASPECT_RATIO = 1.0 MINIMUM_WAVELENGTH = 0.404 MAXIMUM_WAVELENGTH = 0.986 NOMINAL_OPERATING_TEMPERATURE = 163 The image sensor is a virtual-phase, buried-channel, frontside-illuminated, 800-line-by-800-column charge-coupled device developed by Texas Instruments, Inc. The CCD employs a polysilicon gate structure with 15.2 micrometer center-to- center spacing between photoelements. During image readout, all 800 lines are simultaneously shifted one line in the column (also called parallel) direction, causing the first image line to be shifted into the line transport (also called serial) register. One of the most important parameters of an imaging sensor is the thermally generated dark current. For any CCD there are basically three sources of dark current (aside from local dark-current blemishes, e.g., dark-current spikes): the surface component due to the silicon/silicon dioxide interface states, the depletion region component, and the diffusion component from the undepleted bulk of the silicon. Of these sources, the contribution from the surface states has been shown to be the dominant contributor to the dark current. For the virtual phase CCD, however, the surface component can be significantly lower than that measured for other CCD technologies. This is because, if the gate bias is held at a sufficiently negative gate potential during integration and readout, holes from the channel-stop regions will flow over the surface of the imager area, suppressing surface state generation in the clocked-phase regions as well. For such a gate potential, channel inversion occurs. The dark current measured under these conditions for the 800 X 800 VP imager is 0.4 nA/cm**2 (at 25 deg C), which is an order of magnitude better than other buried-channel CCD technologies. At the SSI CCD temperature of -110 deg C, the typical 0.4 nA/cm**2 level of dark current produces charge at a rate of about 10**-5 electrons/pixel/s. With a noninverted channel, dark current is typically around 10**-3 electrons/pixel/s for the SSI at -110 deg C. The signal-generation rate of dark spikes is not affected by channel inversion and ranges from about 0.01 to 10 electrons/pixel/s at -110 deg C. Electronics =========== To maintain a wide dynamic range for this slow-scan camera system, it is necessary, in addition to using a low-noise signal chain, to suppress thermally induced CCD dark current. To reduce the normal dark current to an acceptable level for the longest SSI frame readout interval, 60-2/3 s, CCD cooling to at least -70 deg C is required. To keep the dark current small at localized sites of dark-current blemishes, an operating temperature of -110 deg C has been selected. This cooling is implemented through use of a closed-loop, heater-modulated, radiatively cooled temperature-control system. The temperature controller maintains CCD temperature to within 0.5 deg C of the design value over the full range of view factors 'seen' by the thermal-control radiators as SSI pointing is articulated. The SSI has four operating modes for Phase 1 and five for Phase 2. These modes are characterized by frame repetition rates of 2- 1/3 s with 2 x 2 pixel summation, 8-2/3 s, 30-1/3 s, and 60-2/3 s and an additional 15-1/6 s for Phase 2. Each frame sequence is composed of a prepare and a readout cycle. During the prepare cycle the shutter is reset, the filter wheel is stepped if commanded, the sensor is read out to reduce dark current, and the shutter is activated to expose the image. The image readout cycle follows, and the data are read out either into the onboard tape recorder for later transmission to Earth or put directly on the downlink for real-time transmission. The video analog-to-digital converter (ADC) converts the analog video data to eight bits. The SSI has four gain states commandable on an individual frame basis by SSI control parameter words. The lowest gain state is scaled to provide full-scale data for the full well of the CCD during summation mode readout. The highest gain state is scaled to provide full- scale data for a CCD signal of 10,000 electrons. SSI image parameter control (including commandable selection of spectral filters, exposure duration, gain state, and image readout rate/mode), timing signal generation, pixel shifting and analog-to-digital conversion, internal sequencing, and engineering and status data acquisition are performed under programmed microcomputer (~muC) control. The SSI ~muC is composed of an RCA 1802 microprocessor (~muP), a bus adapter to interface with the spacecraft command and data subsystem (CDS), 3 kwords of read-only memory (ROM), 3 kwords of random access memory (RAM), two 256-word scratchpad memories, two input ports, and three output ports. To enhance image data return over the available spacecraft-to- Earth telecommunication channel, the SSI includes a block adaptive rate controlled (BARC) data compressor and added an additional Integer Cosine Transform data compressor for the Phase 2 mission. By using the BARC data compressor, 8-bit pixel data are compressed to an average of 3.24 bits/pixel. Because of the error sensitivity of compressed imaging data, the SSI includes a Reed-Solomon coder that is active whenever the SSI is outputting compressed data. Use of Reed- Solomon coding provides virtually error-free data at a telemetry rate for which an uncoded data link results in a bit-error-rate of one in fifty. Filters ======= CLEAR ----- FILTER_TYPE = QUARTZ MINIMUM_WAVELENGTH = 0.38 CENTER_FILTER_WAVELENGTH = 0.611 MAXIMUM_WAVELENGTH = 0.82 VIOLET ------ FILTER_TYPE = INTERFERENCE MINIMUM_WAVELENGTH = 0.38 CENTER_FILTER_WAVELENGTH = 0.404 MAXIMUM_WAVELENGTH = 0.43 GREEN ----- FILTER_TYPE = INTERFERENCE MINIMUM_WAVELENGTH = 0.53 CENTER_FILTER_WAVELENGTH = 0.559 MAXIMUM_WAVELENGTH = 0.59 RED --- FILTER_TYPE = INTERFERENCE MINIMUM_WAVELENGTH = 0.64 CENTER_FILTER_WAVELENGTH = 0.671 MAXIMUM_WAVELENGTH = 0.70 ETHANE7270 ---------- FILTER_TYPE = INTERFERENCE MINIMUM_WAVELENGTH = 0.729 CENTER_FILTER_WAVELENGTH = 0.734 MAXIMUM_WAVELENGTH = 0.739 CONTINUUM --------- FILTER_TYPE = INTERFERENCE MINIMUM_WAVELENGTH = 0.747 CENTER_FILTER_WAVELENGTH = 0.756 MAXIMUM_WAVELENGTH = 0.765 METHANE8890 ----------- FILTER_TYPE = INTERFERENCE MINIMUM_WAVELENGTH = 0.779 CENTER_FILTER_WAVELENGTH = 0.887 MAXIMUM_WAVELENGTH = 0.895 INFRARED -------- FILTER_TYPE = INTERFERENCE MINIMUM_WAVELENGTH = 0.96 CENTER_FILTER_WAVELENGTH = 0.986 MAXIMUM_WAVELENGTH = 1.0 Optics ====== TELESCOPE_ID = SSI TELESCOPE_FOCAL_LENGTH = 1.5 TELESCOPE_DIAMETER = 0.176 TELESCOPE_F_NUMBER = 8.5 TELESCOPE_TRANSMITTANCE = 0.50 TELESCOPE_T_NUMBER = 10.8 The optics subassembly, inherited from the Voyager project and recoated to complement the CCD's spectral characteristics, consists of an all-spherical, catadioptric Cassegrain telescope with a 1500 mm focal-length lens operating at a fixed relative aperture of f/8.5. Based on the CCD density of 65.6 elements per mm, the angular resolution is 10. 16 ~murad per pixel. Transmittance is about 50% over the range of 400 to 1 nm. Mounting Offset =============== The SSI is mounted on a two-axis scan platform, coaligned with three other instruments: the Near IR Mapping Spectrometer, UV Spectrometer, and Photopolarimeter Radiometer. Field of View ============= FOV_SHAPE_NAME = SQUARE HORIZONTAL_PIXEL_FOV = 5.7E-04 VERTICAL_PIXEL_FOV = 5.7E-04 HORIZONTAL_FOV = 0.458 VERTICAL_FOV = 0.458 FOVS = 1 Operation Modes =============== INSTRUMENT_MODE_ID = NORMAL DATA_PATH_TYPE = BOTH INSTRUMENT_POWER_CONSUMPTION = 23 INSTRUMENT_MODE_DESC = The SSI has four operating modes for the Phase 1 cruise mission and five operating modes for the Phase2 Orbital mission. The Phase 1 operating modes are characterized by frame repetition rates of 2-1/3 s with 2 x 2 pixel summation, 8-2/3 s, 30-1/3 s, and 60-2/3 s. The Phase 2 includes an addition operating mode of 15-1/6 s with 2x2 pixel summation. Normal mode refers to frame rates of once per 8.666 sec or slower. Normal mode data can either be recorded or channelled directly for real-time transmission. In the normal modes the data can, if necessary, be compressed by a factor of about 2.5 in either an information preserving fashion (lines may be truncated), or in a non- information preserving (lines are complete but pixel values may lose accuracy). Normal mode is distinct from 'summation mode'. INSTRUMENT_MODE_ID = SUMMATION DATA_PATH_TYPE = RECORD INSTRUMENT_POWER_CONSUMPTION = 23 INSTRUMENT_MODE_DESC = Summation mode was designed to minimize the effect of radiation- induced noise in the vicinity of Io. The frame time in summation mode is 2.333 s or 15.1667 s (Phase 2 only), and in order to match the read-out rate of the camera to the on-board tape recorder, it was necessary to reduce the image format by the same factor. The SSI team chose an option in which adjacent pixels in the image are summed (one 'summed' pixel equals four mutually adjacent pixels; the resulting image is then in a 400 x 400 pixel format) during the read-out of the chip. The summation mode data must be recorded." END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "BELTONETAL1984" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "BELTONETAL1992" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END