This data set contains the RDR data for the Galileo Orbiter PPR
       instrument for the period corresponding to the Venus encounter
       observations in February 1990.

  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 EDR (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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