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.