Instrument Information
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          
    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             
    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.                                                       
  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.                                                           
      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.                                         
  FILTER_TYPE                   = QUARTZ                                      
  MINIMUM_WAVELENGTH            = 0.38                                        
  CENTER_FILTER_WAVELENGTH      = 0.611                                       
  MAXIMUM_WAVELENGTH            = 0.82                                        
  FILTER_TYPE                   = INTERFERENCE                                
  MINIMUM_WAVELENGTH            = 0.38                                        
  CENTER_FILTER_WAVELENGTH      = 0.404                                       
  MAXIMUM_WAVELENGTH            = 0.43                                        
  FILTER_TYPE                   = INTERFERENCE                                
  MINIMUM_WAVELENGTH            = 0.53                                        
  CENTER_FILTER_WAVELENGTH      = 0.559                                       
  MAXIMUM_WAVELENGTH            = 0.59                                        
  FILTER_TYPE                   = INTERFERENCE                                
  MINIMUM_WAVELENGTH            = 0.64                                        
  CENTER_FILTER_WAVELENGTH      = 0.671                                       
  MAXIMUM_WAVELENGTH            = 0.70                                        
  FILTER_TYPE                   = INTERFERENCE                                
  MINIMUM_WAVELENGTH            = 0.729                                       
  CENTER_FILTER_WAVELENGTH      = 0.734                                       
  MAXIMUM_WAVELENGTH            = 0.739                                       
  FILTER_TYPE                   = INTERFERENCE                                
  MINIMUM_WAVELENGTH            = 0.747                                       
  CENTER_FILTER_WAVELENGTH      = 0.756                                       
  MAXIMUM_WAVELENGTH            = 0.765                                       
  FILTER_TYPE                   = INTERFERENCE                                
  MINIMUM_WAVELENGTH            = 0.779                                       
  CENTER_FILTER_WAVELENGTH      = 0.887                                       
  MAXIMUM_WAVELENGTH            = 0.895                                       
  FILTER_TYPE                   = INTERFERENCE                                
  MINIMUM_WAVELENGTH            = 0.96                                        
  CENTER_FILTER_WAVELENGTH      = 0.986                                       
  MAXIMUM_WAVELENGTH            = 1.0                                         
  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.