This data set contains the R_EDR datafor the Galileo Orbiter PPR instrument for the period correspondingto the Earth-2 encounter observations in November-December 1992.As described in the Project Galileo Experiment Data Record SoftwareInterface Specification (cf. ASCII and word processor text versionsin DOCUMENT subdirectory), the Galileo Data Management Systemgenerates Low Rate Science (LRS) EDR files for eight Galileo Orbiterinstruments including the PPR. EDR files are organized into blocksof data corresponding to a major frame, or one RIM (major frame count)cycle consisting of 91 minor frames. The EDR generation program extractsthe AACS, PPR science, and science related engineering channels from eachLRS minor frame using the spacecraft clock (SCLK) count to control thebuilding of EDR blocks of data. With normal, uninterrupted LRS data,the SCLK count increments through minor frame (MOD91 count) numbers 0through 90 and then the next minor frame starts a new major framewith the RIM count incremented by one. If LRS minor frames aremissing, appropriate locations within the EDR blocks corresponding tothese missing data are filled with binary zeros. The MOD91 count andthe spacecraft event time (SCET) that corresponds to the first actual(non-filler) minor frame in a block is placed in the header record ofthat block, thus allowing for missing data at the beginning of theblock as well as for the initial block of an EDR file which may beginat a MOD91 count other than 0.The PPR EDR data for a major frame consist of a pair of blocks, thefirst being 2252 bytes in length and the second, 1924 bytes. At thebeginning of the first block is a header of 68 bytes and this isfollowed by 24 bytes of spacecraft and scan platform attitude datafor each of the 91 minor frames of that major frame. The secondblock of the pair begins with a header of 68 bytes, followed by asubheader of 216 bytes containing science related engineering dataand then 18 bytes of PPR housekeeping and science data for each ofthe 91 minor frames. (Since this total of 1922 bytes is not anintegral number of 4-byte words, zero fill for byte numbers 1923 and1924 is used to complete the second block.) For each of the 18-bytePPR minor frame records, the first six bytes are housekeeping datathat completely specify the instrument status, both commandedparameters and position within operational measurement mode cycles.The remaining twelve bytes are three sets of science data samplepairs and their associated identifying parameters. Data files of thepresent archive are a reformatted version of the PPR EDR data (hence,R_EDR). These files are organized in an ASCII Table format with onerecord of 51 parameters, or columns, corresponding to each PPRscience data sample pair (three per minor frame). In addition to thedata number (DN) values for the sample pair itself, all parametersfrom the PPR housekeeping and supplementary EDR data (includingappropriately adjusted time and scan platform pointing information)that identify and characterize that sample pair are included amongthe 51 parameters. Each data file has an attached PDS label thatspecifies the record format and describes each parameter of therecord. Russell, E.E., F.G. Brown, R.A. Chandos, W.C. Fincher, L.F. Kubel, A.A. Lacis, and L.T. Travis, Galileo Photopolarimeter/Radiometer Experiment, Space Sci. Rev. 60 p. 531-563, 1992. Hunten, D.M., L. Colin and J.E. Hansen, Atmospheric Science on the Galileo Mission, Space Sci. Rev., 44, 191-240, 1986 Johnson, T.V., C.M. Yeates and R. Young, Space Science Reviews Volume on Galileo Mission Overview, Space Sci. Rev., 60, 3-21, 1992 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 CALIBRATIONSeveral means of inflight calibration will be utilized by the PPR to updatethe preflight calibration of thephotopolarimetry channels and to provide theprime radiometric calibration of the radiometry channel. These include: (1) aninternal calibration lamp within the PPR aft optics; (2) the third orientationposition for each of the halfwave retarders which interchanges the roles ofthe two silicon detectors; (3) the radiometric calibration target (RCT-PPR)that is separately mounted on the spacecraft and can function either as ablackbody source or to provide a slightly polarized lamp output signal for thevisible/near-infrared region; (4) a spacecraft supplied photometriccalibration target (PCT) that provides a standard of spectral radiance thatcan be viewed by all instruments located on the spacecraft scan platform; and(5) viewing stars and spatially unresolved planets.4.2.1. Internal Cal LampThe small tungsten filament lamp located within the aft optics of the PPRprovides a means to track any changes with time of the silicondetector/amplifier channels. The spectral output of the lamp is modified withthe use of a color glass filter (Schott BG-18). The stability of thislong-life (50 000-hour) lamp is further enhanced by operating it onlyintermittently, at a derated power level, and with a controlled, slow turn-oncharacteristic. The lamp is energized intermittently only during cycle modeoperation while the chopper is being driven to its rest position followingradiometry mode sampling at the solar + thermal filter wheel position and ifthe cal lamp command bit is set to the ON state. (The estimated total on timeis less than 200 hours during the 7-year pre-launch testing period plus thepost-launch inflight period.)4.2.2. Internal Polarimetric CalibrationAs previously discussed, one key feature for achieving accurate polarimetrywith the PPR is the ability to cross-calibrate the detectors by measuringsimultaneously the orthogonal polarization components of the scene radiance.The third halfwave retarder position (with fast axis oriented at 45u to theplane of deviation of the Wollaston prism) effectively interchanges the scenepolarization components incident on the two detectors. This permitsmaintaining the polarimetric accuracy even in the presence of slow relativechanges of the two detector channels with time. temperature, radiation, etc.4.2.3. RCT-PPR DesignThe RCT-PPR will serve in a dual calibration role for the PPR. The primaryrole will be as a thermal calibration target which closely approximates ablackbody source when viewed along the RCT-PPR axisymmetric axis. Due tospacecraft space limitations, it was necessary to restrict the overall lengthof the target. To achieve the desired normal emittance (e > 0.998) thegeometry of the interior portion of the target has a truncated conical formwith a center cylindrical section. This provides on-axis performanceapproximately equivalent to a cone with half the apex angle and twice theoverall length. The end of the central cylindrical portion is not viewed bythe PPR since this area is within the central obscuration of the PPRtelescope. The interior of the target is a smooth (specularly reflecting)black-painted surface to achieve a higher on-axis emittance than would bepossible with a rough (diffuse) surface for the same geometry. The calculatedon-axis emittance of the RCT-PPR is greater than 0.998 based on thereflectance versus incident angle for the interior surfaces.The RCT-PPR is designed and mounted such that it will be passively cooled atJupiter to a temperature of 145 y15 K. The wall thickness is chosen toassure worst case temperature gradients of less than 0.5 K. The temperature ofthe RCT-PPR is monitored by two platinum resistance thermometers (PRTs) thatare calibrated by the manufacturer (resistance versus temperature) to anaccuracy 0.2 K. These PRTs are read out directly by the PPR along with a lowtemperature coefficient resistor also mounted on the RCT-PPR to allow afirst-order correction for spacecraft cabling resistance. The annular apertureof the target is designed to accommodate the 3-a worst case relativemisalignments resulting from possible spacecraft environmental and mountingfactors specified by the Galileo Project to assure that the PPR will view onlythe high emittance portion of the target during calibration. Through the useof the RCT-PPR and the preflight calibrations used to assess the influence oftemperature changes of the PPR optical elements, it is expected that theoverall radiometric calibration of the PPR thermal bands can be maintainedwithin the desired y1 K over the duration of the Galileo Mission.A small tungsten-filament lamp is mounted in one portion of the RCT-PPRinterior surface. With the source commanded ON, flux from the source passesthrough an elliptically shaped, plane-parallel sapphire plate mounted suchthat the outer surface approximately conforms to the inner conical surface ofthe target. The flux transmitted to the PPR is partially polarized due todifferent S and P Fresnel reflectances of the inclined plate. Thus, thissource will be useful in assessing possible photometric and polarimetricchanges of the entire optical train of the PPR over the course of the mission.4.2.4. Spacecraft PCTThe PCT is intended to serve as a standard of spectral radiance for the scanplatform mounted instruments by reflecting sunlight from a diffuselyreflecting surface with well-characterized reflectance properties. Since thistarget can be viewed by all scan platform mounted instruments, the PCT isexpected to be particularly useful in the role of calibration intercomparisonamong instruments.4.2.5. Viewing Astronomical ObjectsOrienting the PPR to view such astronomical objects as stars or spatiallyunresolved planets will be used to provide both cross-check of the absolutephotometric calibration of the PPR silicon photodiode channels and as anadditional 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 ofview) planets at phase angles accessible from Earth will allow intercomparisons to be made with ground-based photometric calibration.4.3. SIGNAL-TO-NOISE PERFORMANCE4.3.1. Photopolarimetry ChannelsFour separate band gains are used for the photopolarimetry channels, with thevalue applied (as described in the electronics section) being dependent on thefilter/retarder wheel position as determined by the encoder. This will providesignal outputs of similar magnitude for the three polarimetry and sevenphotometry bands for typical scene spectral radiances. The channel and bandgains were set to provide signal levels at Gain Step 8 of approximately 2000DN for the three polarimetry bands (each with a separate band gain) and ofapproximately 1500 DN for the 648 nm photometry band (a single band gain isapplied to all seven photometry bands). For setting these levels, the Jovianalbedo values of Woodman et al. (1979) were used.The noise of the photopolarimetry channels is essentially independent ofsignal level, resulting primarily from the 100 megohm feedback resistors inthe pre-amplifiers. As a result, the signal-to-noise ratio (SNR) varies onlyslowly with temperature over the PPR operating temperature range (since theJohnson noise varies as the square root of the absolute temperature). Themeasured SNR performance of the PPR photopolarimetry channels substantiallyexceeds the science-dictated, minimum SNR requirements of 1000 for thepolarimetry bands and 200 for the photometry bands.4.3. 2. Radiometry ChannelAchieving the SNR performance desired for the PPR science investigationsutilizing the radiometry channel produces far greater stress on instrumentdesign that is the case for the photopolarimetry channels. The inevitableGalileo mission mass and size constraints on science instruments requiredsubstantial compromise on performance characteristics. To optimize the SNRperformance of the pyroelectric detector required thinning the LiTaO4 detectorelement to the maximum extent possible. For the PPR application, ion-beammilling was used to provide thicknesses in the 5 to 6 3m range. In order toprovide good optical absorption with low mass, the detector was coated with anevaporated gold-black coating. The wide range of absorbing characteristicsfound 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 detectoris illustrated in Figure 11. Measured noise data for the PPR detector werenear the levels predicted. However, relative spectral response measurementsindicated levels substantially below specification. The lower than expectedlong wavelength responsivity, combined with the lower than specified filtertransmittance for some of the filters (much lower for the 37 3m band) ledto the inability to meet the instrument SNR performance specifications forfour of the seven radiometry channel bands. The measured versus specified SNRperformance is indicated in Table VI. The solar plus thermal band measurementtabulated includes only the thermal component; the solar band is not includedin the table, but comfortably exceeds the specifications. Three of the fourout-of-spec bands have SNR performances about 60% of specification, while the37 3m band (D filter) is about one-third of the desired level.Fortunately the mission profile and the flexibility designed into instrumentoperation allows for observational 'work-arounds' to achieve nearly all of theanticipated science goals. The obvious approach of increasing the number ofsamples to improve the SNR (by the square root of the increase factor) is theprincipal observation strategy to achieve the radiometry science goals.

Since this data set consists of thebasic EDR level data, the appropriate confidence indicators are theDATA_PRESENT_FLAG parameter taken from the header record of the EDRdata blocks and the internal PPR housekeeping and science data parityindicators.