This data set contains the RDR data for the Galileo Orbiter
        PPR instrument for the period corresponding to the
        observations of the SL-9 impacts with Jupiter in July 1994.

  Data Set Overview
  =================
    As described in the Project Galileo Experiment Data Record Software
    Interface Specification (cf. ASCII and word processor text versions
    in DOCUMENT subdirectory), the Galileo Data Management System
    generates Low Rate Science (LRS) EDR files for eight Galileo Orbiter
    instruments including the PPR.  EDR files are organized into blocks
    of data corresponding to a major frame, or one RIM cycle consisting
    of 91 minor frames.

    The PPR generates 18 bytes of instrument data for each 2/3 sec
    interval corresponding to one minor frame count.  For each of the
    18-byte PPR minor frame records, the first six bytes are housekeeping
    data that completely specify the instrument status, both commanded
    parameters and position within operational measurement mode cycles.
    The remaining twelve bytes are three sets of science data sample
    pairs and their associated identifying parameters.  Data files of the
    R_EDR archive are a reformatted version of the PPR EDR data and
    tabulate all of this data in an ASCII Table format.  The present
    Reduced DR (RDR) archive is also an ASCII Table format with the same
    number of records (rows) as the R_EDR files for the respective data
    sets, but with science data number values converted to reduced thermal
    radiometry brightness temperatures and polarimetry-photometry
    radiances and linear polarization degree and direction as appropriate.
    The detached PDS label for each file specifies the record format and
    describes in detail each parameter of the record.

    NOTE            =  The following is from the PPR instrument
                       paper as Published in Space Science
                       Reviews, Vol. 60, by Kluwer Academic
                       Publishers.  The complete document is
                       located at /DOCUMENT/PPR_INST/PPR.HTM.

    4.2. INFLIGHT CALIBRATION

    Several means of inflight calibration will be utilized by the PPR to
    update the preflight calibration of thephotopolarimetry channels and
    to provide the prime radiometric calibration of the radiometry
    channel. These include: (1) an internal calibration lamp within the
    PPR aft optics; (2) the third orientation position for each of the
    halfwave retarders which interchanges the roles of the two silicon
    detectors; (3) the radiometric calibration target (RCT-PPR) that is
    separately mounted on the spacecraft and can function either as a
    blackbody source or to provide a slightly polarized lamp output signal
    for the visible/near-infrared region; (4) a spacecraft supplied
    photometric calibration target (PCT) that provides a standard of
    spectral radiance that can be viewed by all instruments located on the
    spacecraft scan platform; and (5) viewing stars and spatially
    unresolved planets.

    4.2.1. Internal Cal Lamp
    The small tungsten filament lamp located within the aft optics of the
    PPR provides a means to track any changes with time of the silicon
    detector/amplifier channels. The spectral output of the lamp is
    modified with the use of a color glass filter (Schott BG-18). The
    stability of this long-life (50 000-hour) lamp is further enhanced by
    operating it only intermittently, at a derated power level, and with a
    controlled, slow turn-on characteristic. The lamp is energized
    intermittently only during cycle mode operation while the chopper is
    being driven to its rest position following radiometry mode sampling
    at the solar + thermal filter wheel position and if the cal lamp
    command bit is set to the ON state. (The estimated total on time is
    less than 200 hours during the 7-year pre-launch testing period plus
    the post-launch inflight period.)

    4.2.2. Internal Polarimetric Calibration
    As previously discussed, one key feature for achieving accurate
    polarimetry with the PPR is the ability to cross-calibrate the
    detectors by measuring simultaneously the orthogonal polarization
    components of the scene radiance. The third halfwave retarder position
    (with fast axis oriented at 45- to the plane of deviation of the
    Wollaston prism) effectively interchanges the scene polarization
    components incident on the two detectors. This permits maintaining the
    polarimetric accuracy even in the presence of slow relative changes of
    the two detector channels with time. temperature, radiation, etc.

    4.2.3. RCT-PPR Design
    The RCT-PPR will serve in a dual calibration role for the PPR. The
    primary role will be as a thermal calibration target which closely
    approximates a blackbody source when viewed along the RCT-PPR
    axisymmetric axis. Due to spacecraft space limitations, it was
    necessary to restrict the overall length of the target. To achieve the
    desired normal emittance (e > 0.998) the geometry of the interior
    portion of the target has a truncated conical form with a center
    cylindrical section. This provides on-axis performance approximately
    equivalent to a cone with half the apex angle and twice the overall
    length. The end of the central cylindrical portion is not viewed by
    the PPR since this area is within the central obscuration of the PPR
    telescope. The interior of the target is a smooth (specularly
    reflecting) black-painted surface to achieve a higher on-axis
    emittance than would be possible with a rough (diffuse) surface for
    the same geometry. The calculated on-axis emittance of the RCT-PPR is
    greater than 0.998 based on the reflectance versus incident angle for
    the interior surfaces.

    The RCT-PPR is designed and mounted such that it will be passively
    cooled at Jupiter to a temperature of 145 ~15 K. The wall thickness is
    chosen to assure worst case temperature gradients of less than 0.5 K.
    The temperature of the RCT-PPR is monitored by two platinum resistance
    thermometers (PRTs) that are calibrated by the manufacturer
    (resistance versus temperature) to an accuracy 0.2 K. These PRTs are
    read out directly by the PPR along with a low temperature coefficient
    resistor also mounted on the RCT-PPR to allow a first-order correction
    for spacecraft cabling resistance. The annular aperture of the target
    is designed to accommodate the 3-a worst case relative misalignments
    resulting from possible spacecraft environmental and mounting factors
    specified by the Galileo Project to assure that the PPR will view only
    the high emittance portion of the target during calibration. Through
    the use of the RCT-PPR and the preflight calibrations used to assess
    the influence of temperature changes of the PPR optical elements, it
    is expected that the overall radiometric calibration of the PPR
    thermal bands can be maintained within the desired ~1 K over the
    duration of the Galileo Mission.

    A small tungsten-filament lamp is mounted in one portion of the
    RCT-PPR interior surface. With the source commanded ON, flux from the
    source passes through an elliptically shaped, plane-parallel sapphire
    plate mounted such that the outer surface approximately conforms to
    the inner conical surface of the target. The flux transmitted to the
    PPR is partially polarized due to different S and P Fresnel
    reflectances of the inclined plate. Thus, this source will be useful
    in assessing possible photometric and polarimetric changes of the
    entire optical train of the PPR over the course of the mission.

    4.2.4. Spacecraft PCT
    The PCT is intended to serve as a standard of spectral radiance for
    the scan platform mounted instruments by reflecting sunlight from a
    diffusely reflecting surface with well-characterized reflectance
    properties. Since this target can be viewed by all scan platform
    mounted instruments, the PCT is expected to be particularly useful in
    the role of calibration intercomparison among instruments.

    4.2.5. Viewing Astronomical Objects
    Orienting the PPR to view such astronomical objects as stars or
    spatially unresolved planets will be used to provide both cross-check
    of the absolute photometric calibration of the PPR silicon photodiode
    channels and as an additional means to track any responsivity changes
    with time, temperature, radiation, etc. Sirius is a star which will
    provide an adequate signal-to-noise ratio for this purpose by
    aggregating a sufficient number of samples. Similarly, viewing
    unresolved (object subtending less than the PPR field of view) planets
    at phase angles accessible from Earth will allow intercomparisons to
    be made with ground-based photometric calibration.

    4.3. SIGNAL-TO-NOISE PERFORMANCE

    4.3.1. Photopolarimetry Channels
    Four separate band gains are used for the photopolarimetry channels,
    with the value applied (as described in the electronics section) being
    dependent on the filter/retarder wheel position as determined by the
    encoder. This will provide signal outputs of similar magnitude for the
    three polarimetry and seven photometry bands for typical scene
    spectral radiances. The channel and band gains were set to provide
    signal levels at Gain Step 8 of approximately 2000 DN for the three
    polarimetry bands (each with a separate band gain) and of
    approximately 1500 DN for the 648 nm photometry band (a single band
    gain is applied to all seven photometry bands). For setting these
    levels, the Jovian albedo values of Woodman et al. (1979) were used.

    The noise of the photopolarimetry channels is essentially independent
    of signal level, resulting primarily from the 100 megohm feedback
    resistors in the pre-amplifiers. As a result, the signal-to-noise
    ratio (SNR) varies only slowly with temperature over the PPR operating
    temperature range (since the Johnson noise varies as the square root
    of the absolute temperature). The measured SNR performance of the PPR
    photopolarimetry channels substantially exceeds the science-dictated,
    minimum SNR requirements of 1000 for the polarimetry bands and 200 for
    the photometry bands.

    4.3. 2. Radiometry Channel
    Achieving the SNR performance desired for the PPR science
    investigations utilizing the radiometry channel produces far greater
    stress on instrument design that is the case for the photopolarimetry
    channels. The inevitable Galileo mission mass and size constraints on
    science instruments required substantial compromise on performance
    characteristics. To optimize the SNR performance of the pyroelectric
    detector required thinning the LiTaO4 detector element to the maximum
    extent possible. For the PPR application, ion-beam milling was used to
    provide thicknesses in the 5 to 6 Ym range. In order to provide good
    optical absorption with low mass, the detector was coated with an
    evaporated gold-black coating. The wide range of absorbing
    characteristics found in the literature for gold blacks increased the
    risk with this approach, but on balance offered the best overall
    choice for the PPR requirements.

    Optimization curves for the noise components of the PPR pyroelectric
    detector is illustrated in Figure 11. Measured noise data for the PPR
    detector were near the levels predicted. However, relative spectral
    response measurements indicated levels substantially below
    specification. The lower than expected long wavelength responsivity,
    combined with the lower than specified filter transmittance for some
    of the filters (much lower for the 37 Ym band) led to the inability to
    meet the instrument SNR performance specifications for four of the
    seven radiometry channel bands. The measured versus specified SNR
    performance is indicated in Table VI. The solar plus thermal band
    measurement tabulated includes only the thermal component; the solar
    band is not included in the table, but comfortably exceeds the
    specifications. Three of the four out-of-spec bands have SNR
    performances about 60% of specification, while the 37 Ym band
    (D filter) is about one-third of the desired level.

    Fortunately the mission profile and the flexibility designed into
    instrument operation allows for observational 'work-arounds' to
    achieve nearly all of the anticipated science goals. The obvious
    approach of increasing the number of samples to improve the SNR
    (by the square root of the increase factor) is the principal
    observation strategy to achieve the radiometry science goals.

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