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.