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Galileo Orbiter PPR Reformatted EDR V1.0 (PDS)

NSSDCA ID: PSPA-00552

Availability: Archived at NSSDC, accessible from elsewhere

Description

This description was generated automatically using input from the Planetary Data System.

Data Set Overview ================= The Photopolarimeter/Radiometer (PPR) subsystem is one of the four remote sensing instruments mounted on the Galileo Orbiter scan platform and is designed to measure the degree of linear polarization and intensity of reflected or scattered sunlight and the intensity of thermally emitted radiation from Jupiter and its major satellites. Primary science objectives and anticipated results of the PPR experiment are to: (1) determine the vertical and horizontal distribution of cloud and haze particles in the atmosphere of Jupiter; (2) determine the energy budget of Jupiter and the variation in the amount and spatial distribution of reflected solar radiation and emitted thermal radiation for Jupiter and its satellites, including the thermal structure of the atmosphere of Jupiter; and (3) measure and map the photometric, polarimetric, and thermal radiometric properties of the major satellites of Jupiter. These objectives of course contribute to the broader goals of the Galileo Mission science disciplines of atmospheres and satellites as planned and coordinated by the Atmospheres Working Group (AWG) and the Satellite Working Group (SWG).

Parameters ========== The PPR presents to the Galileo Orbiter Command and Data Subsystem (CDS), an 18-byte instrument data buffer for each 2/3-sec interval corresponding to one minor frame (or MOD91) count of the spacecraft clock (91 minor frames constitute one RIM count of the spacecraft clock). 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 PPR science data sample pairs and their associated identifying parameters and parity check bit.

Because of the differences in time required for specific steps of the instrument operation, the various operational modes of the PPR result in the generation of the 18-byte minor frame records at variable rates. Those rates range from just slightly slower than that at which the CDS performs the readout of those records every 2/3 second to a rate that is about three times slower. Accordingly, the PPR design uses two internal 18-byte buffers that are alternately filled, with one buffer being active, or in the state of being filled, and the other containing the previous 18 bytes of housekeeping and science data for the sequence. At the time of each CDS readout of PPR data, it is only the non-active buffer that is presented and placed into the CDS processing stream, and whenever that buffer has been previously transferred, the PPR sets a flag in the housekeeping data of that record to indicate that it is a 'repeat' record. For PPR observations that use the Low Rate Science format to record to the DMS tape, all PPR minor frame records are recorded and ultimately returned. With the PPR Burst-to-Tape record mode, however, the CDS checks the PPR housekeeping to identify and discard the repeat records, storing only the non-redundant records for playback and downlink telemetry.

Data ====

The PPR raw binary EDR data are reformatted into ASCII tables, i.e., R_EDR, with each record corresponding to the housekeeping and science data from each PPR minor frame record returned. Each record begins with the spacecraft clock RIM and MOD91 counts that correspond to the time that the respective buffer for that record was readout by the CDS. This is followed by housekeeping data and the three pairs of science data samples with their identifying parameters. For PPR observations that use the Burst-to-Tape record mode, the spacecraft scan platform pointing (viz., right ascension and declination angles) information is merged with the PPR science data because the Attitude and Articulation Control System (AACS) readouts that are included in the Low Rate Science record blocks and provide scan platform pointing would not otherwise be available with adequate frequency in the playback and downlink of the Burst-to-Tape mode. So the last two columns of the R_EDR records are scan platform right ascension and declination angles, which are set to zero when the PPR observation uses the Low Rate Science record mode. In all record formats other than PPR Burst-to-Tape, the AACS information is delivered directly to the SPICE System, which provides general target and observing geometry and is archived in PDS. For the Burst-to-Tape record mode, the scan platform right ascension and declination are obtained from the PPR science data and then delivered to the SPICE System.

Note also that in the PPR Burst-to-Tape record mode, the second byte of the PPR housekeeping is deleted from the data stored in the CDS memory buffer in order to reduce the total data volume slightly and to optimize the sizing of the buffer. The second byte of housekeeping was chosen because the bits therein correspond to the values for three parameters: photopolarimetry gain, radiometry gain, and number of samples, and all three of these parameters are set by command and do not change during instrument operation unless reset by sending a new command. For the present R_EDR data set, the values for these three parameters are set to default values of 0, 0, and 1, respectively, to reflect the omitted byte in the actual EDR data returned. See the data label file format for a detailed description of these parameters. When the EDR data are processed to generate reduced science data, the known values for these parameters are manually input based upon the Galileo Sequence of Events File (SEF), which indicates the timing of the PPR commands and the commanded parameter values.

Ancillary Data ==============

The housekeeping and science data in the full PPR minor frame records contain all of the commanded parameters and all variable parameter values that completely specify the instrument status, including the position within various measurement mode cycles. When observations are made using the PPR Burst-to-Tape record mode, the second byte of the PPR housekeeping is deleted before the data are stored in the CDS memory buffer. This byte contains the commanded values for the photopolarimetry gain, radiometry gain, and number of successive samples to be taken at each filter wheel position before stepping to the next position. These three parameters are set by command and do not change during instrument operation unless reset by a new command. The current data set uses default values of 0, 0, and 1, respectively, for these parameters for observations using the Burst-to-Tape mode. Actual values can be obtained from the Galileo Sequence of Events File (SEF), which is archived in the PDS by the Galileo Project and which indicates timing of the PPR commands and the commanded parameter values. When the EDR data are processed to generate reduced science data, the SEF or equivalent PPR-team-maintained command records are used to manually input the actual values for these three parameters. The target body for each observation is indicated in the observation ID and in the label file for each data file. Observed location on the target body and observing geometry (viz., target range and incidence, emission, and phase angles) for each measurement sample within an observation sequence must be obtained from the SPICE system for the spacecraft clock time corresponding to the measurement. Note that the spacecraft clock RIM and MOD91 counts for each record of the current R_EDR data set correspond to the time that the respective PPR memory buffer was readout by the CDS. The actual time at which the measurement samples were acquired is earlier than that time by varying amounts depending on instrument operation mode and the position of the data sample in the PPR buffer. When the EDR data are processed to generate reduced science data, the appropriate adjusted spacecraft clock time is determined and included as part of each measurement sample record in that data set. The SPICE data (kernels) and system tools are archived in PDS by the Galileo Project.

Coordinate System ================= For PPR observations that use the Burst-to-Tape record mode, the right ascension and declination angles of the scan platform instrument boresight direction are merged with the PPR housekeeping and science data and appear as the last two columns of each R_EDR data set record. All other record modes include the scan platform pointing in the AACS section of the Low Rate Science blocks, so it is necessary to use the SPICE system to obtain these angles for those observations. The SPICE system kernels contain Galileo spacecraft ephemeris, scan platform pointing, and instrument data necessary for providing target body coordinates and observing geometry as a function of time, using IAU standard coordinate systems as specified in the SPICE documentation.

Software ======== PPR R_EDR files are tables of ASCII format fields for each of the PPR housekeeping and science data elements. This simple conversion from the raw, binary EDR records involves no irreversible change or data reduction. As such, the R_EDR data set represents an appropriate primary archive of all of the raw PPR data delivered to the ground and there is no expectation of any need for reprocessing to perform this conversion in the future. Accordingly, the software used to read the raw, binary EDR files and create the R_EDR data is not archived with this data set.

Data Set Overview ================= The Photopolarimeter/Radiometer (PPR) subsystem is one of the four remote sensing instruments mounted on the Galileo Orbiter scan platform and is designed to measure the degree of linear polarization and intensity of reflected or scattered sunlight and the intensity of thermally emitted radiation from Jupiter and its major satellites. Primary science objectives and anticipated results of the PPR experiment are to: (1) determine the vertical and horizontal distribution of cloud and haze particles in the atmosphere of Jupiter; (2) determine the energy budget of Jupiter and the variation in the amount and spatial distribution of reflected solar radiation and emitted thermal radiation for Jupiter and its satellites, including the thermal structure of the atmosphere of Jupiter; and (3) measure and map the photometric, polarimetric, and thermal radiometric properties of the major satellites of Jupiter. These objectives of course contribute to the broader goals of the Galileo Mission science disciplines of atmospheres and satellites as planned and coordinated by the Atmospheres Working Group (AWG) and the Satellite Working Group (SWG).

Parameters ========== The PPR presents to the Galileo Orbiter Command and Data Subsystem (CDS), an 18-byte instrument data buffer for each 2/3-sec interval corresponding to one minor frame (or MOD91) count of the spacecraft clock (91 minor frames constitute one RIM count of the spacecraft clock). 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 PPR science data sample pairs and their associated identifying parameters and parity check bit.

Because of the differences in time required for specific steps of the instrument operation, the various operational modes of the PPR result in the generation of the 18-byte minor frame records at variable rates. Those rates range from just slightly slower than that at which the CDS performs the readout of those records every 2/3 second to a rate that is about three times slower. Accordingly, the PPR design uses two internal 18-byte buffers that are alternately filled, with one buffer being active, or in the state of being filled, and the other containing the previous 18 bytes of housekeeping and science data for the sequence. At the time of each CDS readout of PPR data, it is only the non-active buffer that is presented and placed into the CDS processing stream, and whenever that buffer has been previously transferred, the PPR sets a flag in the housekeeping data of that record to indicate that it is a 'repeat' record. For PPR observations that use the Low Rate Science format to record to the DMS tape, all PPR minor frame records are recorded and ultimately returned. With the PPR Burst-to-Tape record mode, however, the CDS checks the PPR housekeeping to identify and discard the repeat records, storing only the non-redundant records for playback and downlink telemetry. The PPR R_EDR data set is organized with the PPR minor frame record as the focus, so each record of the R_EDR ASCII table is a set of columns that displays all of the housekeeping and science data for that minor frame along with the spacecraft clock RIM and MOD91 counts that correspond to the time that the memory buffer containing those data was readout by the CDS. In contrast, the focus of the PPR RDR data set is on the science data unit, with a separate record generated for each of the three science data sample pairs of the minor frame record. Moreover, in order to appropriately represent the time at which a given science sample was acquired, we use the specific timing of the PPR operational modes to determine an 'adjusted' spacecraft clock RIM and MOD91 count. The ADJUSTED_RIM and ADJUSTED_MOD91 are the first two parameters of each RDR record and are followed in the record with the RIM and MOD91 values to provide an unambiguous identification of the R_EDR record from which the RDR data came. Note that the adjusted RIM and MOD91 naturally always represent earlier times than the RIM and MOD91 corresponding to the time of the readout of the PPR memory buffer. Following the adjusted and original spacecraft clock counts is the portion of the PPR housekeeping that describes the instrument status (i.e., not including parity information, valid command count, repeat memory buffer flag, and memory buffer ID contained in the R_EDR version archive), the PPR filter wheel position, the raw science data sample pair in DN, right ascension and declination (in degrees) of the scan platform instruments boresight direction, radiometry radiance in DN, radiometry brightness temperature in Kelvins, photometry or polarimetry radiance in DN, photometry or polarimetry radiance in absolute units of watts per square cm per steradian per nm, linear polarization degree in percent, polarization direction angle in degrees (i.e., the direction of the maximum E-field vector in the scene radiance relative to the PPR reference plane defined by the instrument baseplate), and a flag indicating whether the data sample pair was corrupted by a spacecraft boom passing through the PPR FOV.

Processing ========== The PPR makes three different types of measurements: photometry, polarimetry, and radiometry, with a range of positions on the rotating PPR filter wheel corresponding to each. Positions 0 - 17 are used for three polarimetry spectral bands, 18 - 24 for seven thermal radiometry bands, and 25 - 31 for seven photometry bands. At each of the thermal radiometry positions, a chopper mirror alternately directs the scene flux and the space-view reference flux through the filter and then from a mirror mounted on the back of the filter wheel to a pyroelectric detector. For radiometry measurements, the first of the pair of data samples is the digitized output from the detector while the second data sample is the reading from one of 10 thermistors monitoring the temperature of internal instrument elements or 2 platinum resistance thermometers and a reference resistance monitoring the temperature of the PPR radiometric calibration target (RCT). At the photometry and polarimetry positions, the scene flux is directed by a relay lens through a Wollaston prism that splits the input into two orthogonally-polarized output beams, which are then focused onto a pair of silicon photodiode detectors. Polarimetry measurements for a given spectral band are made at three filter wheel positions with three different orientations of half-wave retarders (mounted over the filter positions) in order to obtain the linear polarization degree and direction as well as intensity, whereas for photometry, the two orthogonal intensity components are simply added to get the intensity. The data reduction of the thermal radiometry entails the correction of the raw pyroelectric detector output for contributions to the measured flux by emission, albeit small, from internal elements such as mirrors and radiometric stops. These corrections use the element temperatures monitored by the thermistors along with calibration factors determined through ground thermal vacuum tests in which each element was in turn heated by several degrees to observe the sensitivity to that particular element. The corrected net thermal radiance is then converted to a brightness temperature in Kelvins.

Input data for the RDR data set are of course the R_EDR data, with three RDR records generated for each R_EDR record, corresponding to one RDR record for each of the three science data sample pairs in the R_EDR record, as described above. Each RDR data set record identifies the filter wheel position for that particular science data sample pair, which in turn indicates whether that record corresponds to radiometry, photometry, or polarimetry. Thus, meaningful reduced values for the respective type of PPR measurement appear in the appropriate columns and zero-fill is used in the other columns; e.g., when the filter wheel is at position 21, there are entries for the radiometry radiance in DN and the brightness temperature, but the polarimetry/photometry intensities, linear polarization degree, and polarization directions are set to zero. Since polarimetry requires a minimum of two, and preferably three successive retarder positions, the tabulated polarization quantities are based upon the current data sample position and the previous, most recent data for the other requisite position(s).

Data ====

The PPR reduced data are formatted as ASCII tables, one record for each PPR data sample pair corresponding to a single measurement. Each record begins with the adjusted spacecraft clock RIM and MOD91 counts that correspond to the time at which the sample was acquired and these are followed by the original RIM and MOD91 that provide the time tag identifying the R_EDR record that serves as the input for these RDR records. For PPR observations that use the Burst-to-Tape record mode, the spacecraft scan platform pointing (viz., right ascension and declination angles) information is merged with the PPR science data because the Attitude and Articulation Control System (AACS) readouts that are included in the Low Rate Science record blocks and provide scan platform pointing would not otherwise be available with adequate frequency in the playback and downlink of the Burst-to-Tape mode. So the two columns of the RDR records for the scan platform right ascension and declination angles, are set to zero when the PPR observation uses the Low Rate Science record mode. In all record formats other than PPR Burst-to-Tape, the AACS information is delivered directly to the SPICE System, which provides general target and observing geometry and is archived in PDS. For the Burst-to-Tape record mode, the scan platform right ascension and declination are obtained from the PPR science data and then delivered to the SPICE System.

Note also that in the PPR Burst-to-Tape record mode, the second byte of the PPR housekeeping is deleted from the data stored in the CDS memory buffer in order to reduce the total data volume slightly and to optimize the sizing of the buffer. The second byte of housekeeping was chosen because the bits therein correspond to the values for three parameters: photopolarimetry gain, radiometry gain, and number of samples, and all three of these parameters are set by command and do not change during instrument operation unless reset by sending a new command. See the data label file format for a detailed description of these parameters. In the processing to generate the present reduced science data, the known values for these parameters are manually input based upon the Galileo Sequence of Events File (SEF), which indicates the timing of the PPR commands and the commanded parameter values.

Ancillary Data ============== The housekeeping and science data in the full PPR minor frame records contain all of the commanded parameters and all variable parameter values that completely specify the instrument status, including the position within various measurement mode cycles. When observations are made using the PPR Burst-to-Tape record mode, the second byte of the PPR housekeeping is deleted before the data are stored in the CDS memory buffer. This byte contains the commanded values for the photopolarimetry gain, radiometry gain, and number of successive samples to be taken at each filter wheel position before stepping to the next position. These three parameters are set by command and do not change during instrument operation unless reset by a new command. Actual values can be obtained from the Galileo Sequence of Events File (SEF), which is archived in the PDS by the Galileo Project and which indicates timing of the PPR commands and the commanded parameter values. In processing to generate the present reduced science data, the SEF or equivalent PPR-team-maintained command records are used to manually input the actual values for these three parameters. The target body for each observation is indicated in the observation ID and in the label file for each data file. Observed location on the target body and observing geometry (viz., target range and incidence, emission, and phase angles) for each measurement sample within an observation sequence must be obtained from the SPICE system for the spacecraft clock time corresponding to the measurement. Note that the spacecraft clock RIM and MOD91 counts for each record of the R_EDR data set correspond to the time that the respective PPR memory buffer was readout by the CDS. The actual time at which the measurement samples were acquired is earlier than that time by varying amounts depending on instrument operation mode and the position of the data sample in the PPR buffer. In the processing to generate the present reduced science data, the appropriate adjusted spacecraft clock time is determined and represented as adjusted RIM and MOD91 counts, which are the first two entries in each RDR data set record. The SPICE data (kernels) and system tools are archived in PDS by the Galileo Project. The present reduced science data set includes geometric information (observed location on target body, observing geometry, range, etc.) as determined by the PPR team using the SPICE system. Subdirectory GEOMETRY on this volume contains a *.GEO file with that information for each *.TAB reduced science data file.

Calibration coefficients to convert PPR photometry and polarimetry measurements of intensity from DN values at a particular instrument gain level to absolute units are based upon ground calibration measurements. Thermal radiometry measurement conversion to brightness temperature is a substantially more complex reduction process based upon various calibration tests performed before launch, but was always expected to require significant iteration based upon in-flight observations of the PPR radiometric calibration target (RCT), whose temperature is monitored and readout in the PPR science data stream. The ancillary data in CALINFO.TXT provides the photometry and polarimetry calibration coefficients and tabulates the results of the in-flight RCT observations.

Coordinate System ================= For PPR observations that use the Burst-to-Tape record mode, the right ascension and declination angles of the scan platform instrument boresight direction are merged with the PPR housekeeping and science data and appear as two columns of each RDR data set record. All other record modes include the scan platform pointing in the AACS section of the Low Rate Science blocks, so it is necessary to use the SPICE system to obtain these angles for those observations. The SPICE system kernels contain Galileo spacecraft ephemeris, scan platform pointing, and instrument data necessary for providing target body coordinates and observing geometry as a function of time, using IAU standard coordinate systems as specified in the SPICE documentation. As indicated above, geometry information generated by the PPR science team using the SPICE system is provided in files contained in the GEOMETRY subdirectory of this volume. The reduced PPR polarimetry observations provide the radiance, or intensity, the linear polarization degree, and the direction of the polarization of the light scattered-reflected from the viewed target. Polarization direction is the plane in which the maximum electric field vector of the incident flux lies and in the present RDR data is referenced to the PPR instrument baseplate. In the case of a photon scattered or reflected just once, i.e., single scattering, the polarization direction must lie precisely either in the scattering plane or perpendicular to it, where the scattering plane is the plane defined by the vector between the Sun and the observed point and the vector from the spacecraft to the observed point. Even in the more general multiple scattering case, the polarization direction is rarely more than a few degrees away from being either parallel or perpendicular to the scattering plane unless the linear polarization is close to zero, where the direction of course becomes undefined. As a consequence, polarimetry data will typically show polarization direction angles clustered, either about a single angle or two angles separated by about 90 degrees. Because of this approximate discrete two-valued nature rather than continuous over the full range of angles, linear polarization is often presented as a signed value, with negative values corresponding to the situation where the direction lies in or very near the scattering plane.

These data are available on-line from the Planetary Data System (PDS) at: http://pds-atmospheres.nmsu.edu/PDS/data/gopr_5001/

Alternate Names

  • GO-J-PPR-2-REDR-V1.0
  • GO-J-PPR-3-RDR-V1.0
  • Galileo Orbiter PPR Reduced Data Record (RDR) V1.0 (PDS)

Discipline

  • Planetary Science: Atmospheres

Additional Information

Spacecraft

Experiments

Questions and comments about this data collection can be directed to: Dr. Edwin V. Bell, II

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. James E. HansenData ProviderNASA Goddard Institute for Space Studiesjhansen@giss.nasa.gov
Dr. Larry D. TravisGeneral ContactNASA Goddard Institute for Space Studiesltravis@giss.nasa.gov
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