

The Galileo Dust Detection System Data Collection Description
==============================================================
                                                           
    This data set contains information on the dust environment in             
    interplanetary space within the inner solar system and in the Jupiter     
    system, within and without the Jovian magnetosphere and around the        
    Galilean satellites. This information is collected with a dust impact     
    experiment, DDS, from which may be inferred direction of motion, mass,    
    velocity and charge. (See the instrument catalog file for further         
    information.)  The data presented in this dataset include instrumental    
    readouts, inferred metadata, calibration information and a calendar of    
    events. Specifically:                                                     
                                                                              
     1) galddust.tab - data received from the dust detector, the spacecraft,  
    and physical properties derived from the detector data for reliable dust  
    impacts (Gruen et al. 1995b, Krueger et al. 1999b, Krueger et al. 2006,   
    and Krueger et al. 2009).                                                 
                                                                              
     2) galdevnt.tab - data received from the dust detector, the spacecraft,  
    and physical properties derived from the detector data for reliable dust  
    impacts plus noise events.                                                
                                                                              
     3) galdcode.tab - value ranges corresponding to codes found in           
    galddust.tab.                                                             
                                                                              
     4) galdcalb.tab - laboratory calibration data used to relate instrument  
    responses to physical properties of the impacting dust particles.         
                                                                              
     5) galdarea.tab - the area of the dust detector exposed to particles as a
    function of their velocity direction relative to the detector axis.       
                                                                              
     6) galdstat.tab - time history of Galileo mission and dust detector      
    configuration, tests and other events.                                    
                                                                              
    The data received from the spacecraft are used for determining the        
    location and orientation of the spacecraft and instrument. Given are the  
    SPACECRAFT-SUN DISTANCE, ECLIPTIC LONGITUDE, ECLIPTIC LATITUDE,           
    SPACECRAFT-EARTH DISTANCE, ROTATION ANGLE, DETECTOR ECLIPTIC LONGITUDE,   
    and DETECTOR ECLIPTIC LATITUDE.  Full spacecraft ephemeris and            
    orbit/attitude data (SPICE) are available at the PDS Planetary Plasma     
    Interactions (PPI) Node, at http://ppi.pds.nasa.gov.                      
                                                                              
    Data received from the dust detector are given in an integer code format. 
    Some of the integer codes represent a range of values within which the    
    data could fall (e.g., ION AMPLITUDE CODE), some may represent a specific 
    value (e.g., ION COLLECTOR THRESHOLD), and others, a classification based 
    upon other integer codes (e.g., EVENT CLASS).  Information for            
    interpreting the codes and flag values given in the data files            
    galdevnt.tab and galddust.tab is given in the file galdinfo.asc in the    
    document directory.                                                       
                                                                              
     The instrument data consists of cataloging information, instrument       
    status, instrument readings at time of impact, and classification         
    information.  The cataloging information includes the SEQUENCE NUMBER     
    (impact number), JULIAN DATE (time of impact), and SECTOR (the pointing of
    the instrument at time of impact).  The instrument status data are the    
    threshold levels of the detectors and the CHANNELTRON VOLTAGE LEVEL.      
                                                                              
     The instrument readings include the amplitude codes of the detectors     
    aboard the instrument and the integer codes representing the charge level 
    rise times of the detectors, the difference in starting times of the ion  
    signal and the electron signal, electron and ion signal coincidence, and  
    ion and channeltron signal coincidence.                                   
                                                                              
     The classification information is used to assist in classifying an event 
    into probable impact and non-impact categories.  There are three variables
    used in classification:  EVENT DEFINITION which records which detectors   
    begin a measurement cycle; ION AMPLITUDE RANGE which is the classification
    of the ION AMPLITUDE CODE into 6 subranges (used with EVENT CLASS); and   
    EVENT CLASS which categorizes events into a range of probable impacts to  
    probable non-impacts.                                                     
                                                                              
     The PARTICLE SPEED and PARTICLE MASS and their corresponding error       
    factors are determined from the instrument and calibration data given in  
    galddust.tab and galdcalb.tab, respectively.                              
                                                                              
    In 2010 it was discovered that the julian dates provided in the files     
    galddust.tab and galdevnt.tab had been calculated incorrectly from the UT 
    times.  The julian dates were recalculated and the data set was           
    incremented from version 4.0 to version 4.1.                              
                                                                              
    Calibration Data                                                          
    ----------------                                                          
                                                                              
     ION RISE TIME, ELECTRON RISE TIME, ION CHARGE MASS RATIO, and ELECTRON   
    CHARGE MASS RATIO were measured for iron, glass, and carbon particles of  
    known mass and impacting at known speeds. Since the composition of        
    particles striking the Galileo spacecraft is unknown, logarithmic averages
    of the above values are used to infer the particle speed and mass from the
    instrumental measurements. See Goller (1988).                             
                                                                              
     The data were provided in a private communication to M. Sykes (Jun 29    
    03:04 MST 1995) by M. Baguhl. They are the results of these experiments   
    for impacts at an angle of 34 degrees from the detector axis.             
                                                                              
    Processing Level                                                          
    ----------------                                                          
                                                                              
     The data contain different levels of processing.  Some processing was    
    done at the time of the impact observation.  This processing categorized  
    the detector responses to transmit the data efficiently back to Earth.    
    Data received on Earth is given as an integer code. These integer codes   
    can, for example, represent ranges of values, or can be a classification  
    determined from other integer codes.  On Earth, these integer codes were  
    then fit to calibration curves to determine the speed and mass of the     
    impacting particle (Goller and Gruen 1989; Gruen et al., 1995c).          
                                                                              
     This data set contains the information from the spacecraft instrument as 
    received on Earth, information about the location and pointing direction  
    of the spacecraft, and the physical properties determined from the data   
    analysis.                                                                 
                                                                              
     The calibration data are included as part of this dataset.               
                                                                              
    Sampling Parameters                                                       
    -------------------                                                       
                                                                              
     The occurrence of an impact with the instrument begins a measurement     
    cycle.  The on-board detectors measure a charge accumulation versus time  
    in order to measure the rise time of the accumulation and any coincidences
    between detector readings.  The on-board computer converts these          
    measurements to integer codes to minimize the amount of data that is      
    transferred back to Earth.  After the conversion, the integer codes are   
    categorized to determine if an event is more likely to be an impact or    
    noise event.  The data are then stored until it is time to transmit to    
    Earth.                                                                    
                                                                              
    Data Reduction - Impact Speed                                             
    -----------------------------                                             
                                                                              
     Impact speed (V) is obtained from the rise-time measurements of the ion  
    and electron detectors (IT and ET, respectively) using procedures         
    described in part by Gruen et al. (1995c) and a private communication to  
    M. Sykes (Jul 22 03:43 MST 1995) from M. Baguhl. The calibration tables   
    used correspond to the mean values obtained for the three different       
    projectile materials with which the instruments were calibrated (Goller   
    and Gruen 1989; Gruen et al., 1995c). A rise-time measurement is started  
    when the respective signal exceeds its threshold and is stopped by a flag 
    pulse from the peak-detector. Impact calibration was performed in the     
    speed interval from about 2 km/s to 70 km/s, so impact speeds derived from
    rise-time measurements will be limited to this range.                     
                                                                              
     Dust accelerator tests as well as experience with flight data have shown 
    that (1) the shape of the ion signal is less susceptible to noise than the
    shape of the electron signal and (2) for true impacts, ELECTRON AMPLITUDE 
    CODE values (EA) are generally greater than the ION AMPLITUDE CODE values 
    (IA) by 2 to 6.  As a consequence, the electron rise-time is only used for
    impact speed determination if 2 =< EA-IA =< 6. Since both speed           
    measurements, if available, are independent, one obtains two (often       
    different) values VIT and VET, respectively. The impact speed is then     
    taken to be the geometric mean of VIT and VET.                            
                                                                              
     Determining VIT:                                                         
                                                                              
        If IA > 16 and IT > 12, then fix IT=14.                               
        Else, if IA > 16 and IT =< 12, then add 2 to the corresponding        
            value of IT.                                                      
        VIT is then found in Table 5a of Gruen et al. (1995c) or galdcode.tab.
                                                                              
        Note: If IT=0, then VIT is invalid. This differs from                 
              Gruen et al. (1995c).                                           
                                                                              
     Determining VET:                                                         
                                                                              
        If EA > 16 and ET > 12, then fix ET=14.                               
        Else, if EA > 16 and ET =< 12, then add 2 to the corresponding        
           value of ET.                                                       
        VET is then found in Table 5a of Gruen et al. (1995c) or galdcode.tab.
                                                                              
        Note: If ET=0, then VET is invalid. This differs from                 
              Gruen et al. (1995c).                                           
                                                                              
     If IA=49, or IA=18, or IA<3, then IT is not valid, and only VET is used  
    to determine impact speed.                                                
                                                                              
     If EA=49, or EA=31, or EA<5, then ET is not valid, and only VIT is used  
    to determine impact speed.                                                
                                                                              
     If IT is invalid and 6<EA-IA or EA-IA<2, then there is no valid impact   
    speed.                                                                    
                                                                              
     If neither IT nor ET is valid, then there is no valid impact speed.      
                                                                              
                                                                              
    Data Reduction - Impact Speed Error Factor                                
    ------------------------------------------                                
                                                                              
     The upper and lower estimates of impactor speed are obtained by          
    multiplying and dividing, respectively, the mean particle speed by the    
    velocity error factor, VEF. If only one speed is measured, and is from the
    electron detector, the minimum uncertainty is VEF=2. If only one speed is 
    measured, and is from the ion detector, the minimum uncertainty is        
    VEF=1.9. It is assumed that minimum error of 1.6 is achieved if both      
    individual speeds agree to within a factor of 4. This error corresponds to
    the logarithmic mean of the minimum errors in the two cases when only a   
    single speed is valid.                                                    
                                                                              
     Since these are all 1-sigma errors, it may happen that VIT or VET fall   
    outside the error bar given for the mean impact speed, V. In order to     
    avoid this, the error factor is 'stretched' to contain the values:        
                                                                              
     If VIT > 4*VET, then                                                     
                                                                              
         VEF=(VIT/VET-4.)/31.*(1.6*sqrt(35.)-1.6)+1.6                         
                                                                              
     If VET > 4*VIT, then                                                     
                                                                              
         VEF=(VET/VIT-4.)/31.*(1.6*sqrt(35.)-1.6)+1.6                         
                                                                              
     (private communication to M. Sykes from M. Baguhl, Mar  6 03:57 MST      
    1996).                                                                    
                                                                              
     If the ratio of both speeds exceeds 4, then the uncertainty can increase 
    to about 10 in the calibrated speed range. In any case, a speed value with
    an uncertainty factor VEF>6 should be ignored.                            
                                                                              
    Data Reduction - Impactor Mass                                            
    ------------------------------------------                                
                                                                              
     Once a particle's impact speed (V) has been determined, the charge to    
    mass ratio can be determined from calibration measurements (Figure 3,     
    Gruen et al. (1995c); galdcalb.tab). The charge to mass ratio for a given 
    impact speed (V) is determined by linear interpolation of the calibration 
    table (galdcalb.tab) on a double logarithmic scale, yielding a separate   
    value for the ion grid measurement (QIM) and and electron grid measurement
    (QEM).                                                                    
                                                                              
     From these values and the respective impact charges (QI and QE)          
    corresponding to IA and EA, respectively (Table 4, Gruen et al. (1995c);  
    galdcalb.tab), mass values (MQI=QI/QIM and MQE=QE/QEM) are determined     
    corresponding to the ion and electron grid measurements. When both MQI and
    MQE are valid, the impact particle mass, M, is the geometric mean of these
    two values, or the value corresponding to the valid measurement if the    
    other is invalid. If there is no valid impact speed, then there is no     
    valid impactor mass.                                                      
                                                                              
     Note: when V is invalid, M is invalid.                                   
                                                                              
     Note: when IA=0, QI is invalid and MQI is invalid.                       
                                                                              
     Note: when EA=0, QE is invalid and MQE is invalid.                       
                                                                              
    Data Reduction - Impactor Mass Error Factor                               
    ------------------------------------------                                
                                                                              
     The upper and lower estimate of impactor speed is obtained by multiplying
    and dividing, respectively, the mean particle speed by the mass error     
    factor, MEF. If the speed is well determined (VEF=1.6) then the mass value
    can be determined with an uncertainty factor MEF=6. Larger speed          
    uncertainties can result in mass uncertainty factors greater than 100.    
                                                                              
     The mass error is calculated from the speed error, keeping in mind that  
    mass detection threshold is proportional to speed to the 3.5th power. In  
    addition, there is an error factor of 2 from the amplitude                
    determination. Added together (logarithmically) these yield               
                                                                              
     MEF=10**(sqrt((3.5*log(VEF))**2+(log(2.))**2))                           
                                                                              
     (Private communication to M. Sykes from M. Baguhl, Mar  6 03:57 MST 1996.
    This differs from the exponent of 3.4 given in Gruen et al. (1995a))      
                                                                              
    Coordinate System                                                         
    -----------------                                                         
                                                                              
     The coordinates of the spacecraft are given in heliocentric ecliptic     
    latitude and longitude (equinox 1950.0), where the pointing direction of  
    the sensor is given in spacecraft centered ecliptic latitude and longitude
    (equinox 1950.0).                                                         
                                                                              
    On-off times and download times                                           
    -------------------------------                                           
                                                                              
    Galileo DDS was turned on all the time, even during Galileo probe release,
    Jupiter orbit insertion, a reprogramming event in December 1996 and during
    several spacecraft safings. However, during these events the high voltage 
    of the channeltron was reduced or switched off and the detection          
    thresholds were raised to reduce the instrument sensitivity for noise     
    events. During Galileo safings DDS continued to measure dust impacts but  
    because of the missing downlink and the limited instrument memory of DDS  
    the majority of the impact data were usually lost. Between 95-341 and     
    96-087 the high voltage was switched off entirely but still the instrument
    continued to measure dust impacts.                                        
                                                                              
    The downlink capability of the instrument limited the number              
    of transmitted events most of the time, in particular when the spacecraft 
    was in the inner jovian system. When no continuous downlink (Realtime     
    Science or RTS) was available, data were received as Memory Readouts      
    (MROs) which gave only snapshots of the dust activity during the previous 
    days or weeks.  The periods when these different types download methods   
    were used are listed in the status table (galdstat.tab).  These times give
    periods when DDS data were transmitted in the various modes.


Known issues or problems with the data
======================================

    Impact times                                                              
    ------------                                                              
                                                                              
       The impact times during the Cruise phase of the mission were recorded  
    with an accuracy of 1.1 hours. After June 25, 1990, inclusive, the        
    accuracy was 4.3 hours (this value has been set in order to bridge gaps in
    the data transmission as long as one month) (Gruen et al. 1995b; Krueger  
    et al. 1999b). There were also periods in which more frequent memory reads
    resulted in a time resolution of 2/3 seconds.                             
                                                                              
    After 1995, during periods of realtime science mode (RTS) and record mode,
    more accurate times were recorded.  The Time Error Value (TEV) listed in  
    the data files gives the time accuracy of any given impact or event.      
                                                                              
       Sector                                                                 
       ------                                                                 
                                                                              
    In a private communication to M. Sykes (Nov 17 02:25 MST 1998), H. Krueger
    stated that when the ROTATION ANGLE is invalid, SECTOR is also invalid. In
    the data that have been published in the literature electronically, prior 
    to 11/98, valid values of SEC are reported when ROTATION ANGLE is invalid.
    This has been corrected. See Baguhl (1993) for the relationship between   
    ROTATION ANGLE and SECTOR.  When SECTOR and ROTATION ANGLE are invalid,   
    detector ecliptic latitude and longitude are also invalid and represented 
    by a null value.                                                          
                                                                              
       In V1.0 of this data set, SECTOR was reported in degrees. In subsequent
    versions, Sector is reported as its original 8-bit word, and has a value  
    between 0 and 255 (when valid). Conversion to degrees may be accomplished 
    through scaling by 1.40625.                                               
                                                                              
       Ion Channeltron Coincidence (ICC)                                      
       ----------------------------------                                     
                                                                              
       The designation ICC is used following Gruen et al. (1995c) and Krueger 
    et al. (1999b), noting that in Gruen et al. (1995a and b) and Krueger et  
    al. (1999a) the designation is IIC.                                       
                                                                              
       Entrance Grid Amplitude Code (PA)                                      
       ---------------------------------                                      
                                                                              
       In the data that have been published in the literature and             
    electronically prior to 11/98, there are values of PA which exceed 47. In 
    a private communication to M. Sykes (Mar  6 03:57 MST 1996), Michael      
    Baguhl and Rainer Riemann stated:                                         
                                                                              
    'Values of PA greater 47 are caused by a bit flip (caused by a            
    timing bug in the sensor electronics) of the MSB. For values              
    greater 47, a value of 16 has to be subtracted.'                          
                                                                              
       This correction was made to all PDS DDS files prior to 11/98.          
                                                                              
       As a consequence of subsequent uncertainty about the origin of PA      
    values greater than 47, in a private communication to M. Sykes (Nov  6    
    04:07 MST 1998), H. Krueger requested that PA values greater than 47 be   
    corrected to '99'. This has been done in releases of the DDS data through 
    the PDS after 11/98.                                                      
                                                                              
       Channeltron Voltage Level (HV)                                         
       ------------------------------                                         
                                                                              
       The nominal high voltage HV=4 (1250V) could not be used before 1999    
    because of unexpected noise on the channeltron. It is assumed that the    
    nearby radioactive thermal generators (RTGs) are to blame, although other 
    causes cannot be excluded. During ground tests (without RTGs) no such     
    noise was observed. See Gruen et al. (1995b).  By 1999 the channeltron had
    significantly degraded and the instrument was not sensitive enough to     
    detect the RTG noise at HV=4.    In mid-1999 the channeltron was set to   
    HV=4 to compensate for the degradation, and later the channeltron voltage 
    was increased even further.  See the instrument status file, galdstat.tab,
    for   the times of voltage increases.                                     
                                                                              
       Spacecraft Earth distance                                              
       -------------------------                                              
                                                                              
       The value for the same impact event in galddust.tab and galdevnt.tab is
    different, but by less than 7500 km.                                      
                                                                              
       Impact speed                                                           
       ------------                                                           
                                                                              
       In a private communication to M. Sykes (Jul 22 03:43 MST 1995), M.     
    Baguhl stated that the reason for the exclusion of the values IA=49 and   
    EA=15 is empirical. These values are close to the switching points of the 
    amplifier ranges and therefore produce incorrect time measurements. The   
    adjustment of the times in amplifier range 2 was made in order to prevent 
    illegal time values.                                                      
                                                                              
       Calibration data                                                       
       ----------------                                                       
                                                                              
       Instrumental values were extrapolated for particle masses and speeds   
    outside the range of those tested, and are so marked. The accuracy of     
    these numbers is unknown. An explication of the experiments and data used 
    to generate the calibration may be found in Goller (1988).                
                                                                              
    Velocity and Mass Mismatches                                              
    ----------------------------                                              
                                                                              
    The calculated mass and velocity (and their error factors) of impact      
    numbers 340 and 418 do not match the values that would be derived from the
    data reduction procedures given in the data set description above.

PDS3 Source
===========

Version 1.0 of this bundle was migrated from version 4.1 of the PDS3 data set GO-D-GDDS-5-DUST-V4.1.
