Description
    ===========
      The PPR Radiometric Calibration Target (RCT) is mounted on the
      spacecraft so that it is viewed by the PPR when the Galileo Orbiter
      scan platform is pointed near the zero cone angle direction.
      Intended primarily for calibration of the PPR thermal radiometry
      bands, the RCT has a truncated conical interior with a center
      cylindrical section.  The interior of the target is a smooth,
      specularly reflecting, black-painted surface that with the truncated
      conical geometry achieves a thermal emittance of 0.998 when viewed
      on axis by the PPR.  Note that the end of the central cylindrical
      portion of the target is not viewed by the PPR since that area is
      within the central obscuration from the secondary mirror of the PPR
      telescope.
 
      The RCT is designed and mounted such that simply through passive
      thermal equilibrium, it will have a temperature in the range, 125 K
      to 140 K, during the Jupiter Phase of the Galileo Mission.  Actual
      temperature of the RCT is monitored by two platinum resistance
      thermometers (PRTs) that are calibrated to an accuracy of 0.2 K.
      These PRTs are connected to the PPR so that their readouts can be
      included in the PPR science data stream.
 
      Although the RCT is designed primarily to provide a target that
      closely approximates a blackbody source at its monitored temperature
      in order to perform inflight calibration of the PPR thermal
      radiometry bands, it includes a small tungsten-filament lamp mounted
      in the RCT interior so that it can be used as a radiometric and
      polarimetric reference standard for the shortwave PPR bands as
      well.  When this lamp is commanded on for non-radiometry band
      calibration, its output passes through two elliptically shaped,
      plane-parallel sapphire plates mounted such that the outer surface
      of the stack approximately conforms to the inner conical surface of
      the RCT.  This configuration provides shortwave band calibration
      while having negligible effect on the thermal emittance of the RCT.