GALILEO DUST DETECTION SYSTEM
=============================

    Instrument Overview                                                       
    ===================                                                       
    The instrument consists of a 0.1 mm thick gold foil of hemispherical      
    shape with three grids at the entrance (entrance grid, charge grid,       
    and shield), as well as an ion collector and channeltron detector.        
    The maximum sensitive area (for particles moving parallel to the          
    sensor axis) is 0.1 m**2.  Upon impact the particle produces a            
    plasma, whose charge carriers are separated by an electric field          
    between the target and the ion collector.  Negative charges (mainly       
    electrons) are collected at the target; the positive charges are          
    collected partly by the ion collector and partly by a channeltron.        
    The channeltron is used as it is insensitive to electric and              
    vibrational noise.  See Gruen et al.(1992a) [GRUENETAL1992A] for more     
    information concerning the instrument.                                    
                                                                              
                                                                              
    Science Objectives Summary                                                
    ==========================                                                
    The objective of the Galileo dust experiment is to investigate the        
    physical and dynamical properties of small dust particles (10**-16 to     
    10**-6g) in the Jovian environment.  The parameters to be determined      
    include the mass, speed, flight direction and electric charge of          
    individual particles.  Specific objectives are:                           
    - To investigate the interaction of the Galilean satellites with          
    their dust environment in order to study the relationship between         
    dust influx on satellites and their surface properties, and to            
    perform direct measurements of ejecta particles from the satellites;      
    - To study the interaction between dust particles and magnetospheric      
    plasma, high-energy electrons and protons, and magnetic fields, to        
    determine the relationship between dust concentrations and                
    attenuation of the radiation belts, and to investigate the effects of     
    the Jovian magnetic field on the trajectories of charged dust             
    particles;                                                                
    - To investigate the influence of the Jovian gravitational field on       
    the interplanetary dust population and to search for rings around         
    Jupiter.                                                                  
                                                                              
                                                                              
    Instrument Measurements                                                   
    =======================                                                   
    Positively or negatively charged particles entering the sensor are        
    first detected via the charge which they induce in the charge grid        
    while flying between the entrance and shield grids.  The grids            
    adjacent to the charge pick-up grid are kept at the same potential in     
    order to minimize the susceptibility of the charge measurement to         
    mechanical noise.  All dust particles - charged or uncharged - are        
    detected by the ionization they produce during the impact on the          
    hemispherical impact sensor.  After separation by an electric field,      
    the ions and electrons of the plasma are accumulated by charge            
    sensitive amplifiers (CSA), thus delivering two coincident pulses of      
    opposite polarity.  The rise times of the pulses, which are               
    independent of the particle mass, decrease with increasing particle       
    speed.  From both the pulse heights and rise times, the mass and          
    impact speed of the dust particles are derived by using empirical         
    correlations between these four quantities.                               
                                                                              
                                                                              
   Detector Description                                                       
   ====================                                                       
    The sensor consists of a grid system for the measurement of the           
    particle charge, an electrically grounded target (hemisphere) and a       
    negatively biased ion collector.  A charged dust particle entering        
    the sensor will induce a charge in the charge grid, which is              
    connected to a charge sensitive amplifier.  The output voltage of         
    this amplifier rises until the particle passes this grid, and falls       
    off to zero when it reaches the shield grid.  The peak value (Q_p) is     
    stored for a maximum of 600 microseconds and is only processed if an      
    impact is detected by the impact ionization detector within this          
    time.  A dust particle hitting the hemispherical target produces          
    electrons and ions, which are separated by the electric field between     
    the hemisphere and ion collector into negative charges (electrons and     
    negative ions) and positive ions. The negative charges are collected      
    at the hemisphere and measured by a charge sensitive amplifier (Q_e).     
    Positive ions are collected and measured at the negatively biased ion     
    collector with a charge sensitive amplifier (Q_i).  Some of the ions      
    penetrate the ion collector (which is partly transparent - total          
    transmission approximately 40 percent), are further accelerated, and      
    hit the entrance cone of an electron multiplier (channeltron).            
    Secondary electrons are produced, amplified, and measured by a charge     
    sensitive amplifier (Q_c).  Other quantities measured are the rise        
    times of both the positive and negative charge pulses.  The               
    measurement of the time delay between electron pulse and ion pulse        
    serves as a means for distinguishing impact events from noise.            
    Impact events have time delays of 2-50 microseconds, while mechanical     
    noise has a time delay of milliseconds. These signal amplitudes and       
    times of a single recorded event are digitized and stored in an           
    Experiment Data Frame (EDF).                                              
                                                                              
    A measurement cycle is initiated if either the negative charge Q_e on     
    the hemispherical target, or the positive charge on the ion-collector     
    Q_i, or the positive charge Q_c on the channeltron exceeds a              
    threshold. Since the hemisphere has a large area which is directly        
    exposed to interplanetary plasma and high-energy radiation, this may      
    cause some interference for the Q_e measurement. To avoid this            
    interference during high activity times, it is possible to switch by      
    command to a mode in which a measurement cycle is initiated only when     
    the charge on the ion collector Q_i (small area and not directly          
    exposed) or channeltron signal Q_c exceeds the threshold.                 
                                                                              
    If more than one event occurs within the transmission time of one         
    EDF, then these events are counted by several amplitude-dependent         
    counters. The dead-time caused by the measurement cycles is 5             
    milliseconds.                                                             
                                                                              
    The signals from the sensor are conditioned and analysed.  The            
    microprocessor coordinates the experiment measurement cycle, collects     
    the buffered measurement data and processes the data according to a       
    program stored in the memory.                                             
                                                                              
    See Gruen et al.(1992a) [GRUENETAL1992A] for more information concerning  
    the operation of the detector.                                            
                                                                              
                                                                              
    Calibration Description                                                   
    =======================                                                   
    Impact tests with iron, carbon, and silicate particles were performed     
    at the Heidelberg dust accelerator facility.  The particles were in       
    the speed range from 1 to 70 km/s and in the mass range from 1.0E-15      
    to 1.0E-10 grams.  In addition to the projectile material variation,      
    calibrations for iron particles with varying impact angles were done.     
    See [GOLLER&GRUEN1989] for more information.                              
                                                                              
    To obtain calibrations without information about the impact angle and     
    the composition of an impacting micrometeoroid, a set of curves (one      
    for each measurement channel) was calculated, which were averaged         
    over three different materials (iron, carbon, and silicate) and over      
    the range of relevant impact angles (20 to 53 degrees).  The              
    measurements were done at different angles with iron particles and at     
    one fixed angle (20 degrees) with carbon and silicate projectiles.        
    Difficulties in accelerating glass and carbon projectiles and the low     
    acceleration rate made it impossible to do tests at more than one         
    angle.                                                                    
                                                                              
    A computer simulation of the detector exposed to an isotropic             
    particle flux leads to the result that 50 percent of the particles        
    hit the detector under an angle of 32 degrees or lower, relative to       
    the sensor axis.  Its effective viewing cone covers a solid angle of      
    1.4 sr.  As the target is curved (hemispherical) the impact angle,        
    measured relative to the target normal at the point of impact, is         
    generally different from the angle of incidence (relative to the          
    sensor axis).  The direction of travel of the impacting particle can      
    not be determined.  From the computer simulation the most probable        
    impact angle is 28 degrees, the average angle is 36 degrees.  This        
    information, used with the pointing of the instrument, can be used to     
    obtain a rough estimate of the particle trajectory.  The particle's       
    flight path inside the detector was determined to be 20 +/- 5 cm.         
                                                                              
    There are three possibilities for the determination of a particle's       
    speed (the rise times and the ratio Q_c/Q_i).  Using all three            
    measurements and comparing them with the calibration curves, the          
    speed can be determined with an accuracy of a factor of 1.6. Using        
    only one the accuracy is given by a factor of 2.                          
                                                                              
    With a known particle speed the mass can be determined from the           
    charge yields Q_i/m and Q_e/m.  If the speed is known within a factor     
    of 1.6 and both yields are used for mass measurements the value can       
    be measured with an uncertainty of a factor of 6.  The main part of       
    this error is caused by the limited accuracy of the speed                 
    measurement.                                                              
                                                                              
                                                                              
    Instrument Modes                                                          
    ================                                                          
    Different instrument modes exist to alter the instrument's                
    susceptibility to noise.  These modes are changed by adjusting the        
    thresholds of the detectors on board the instrument.  The thresholds      
    are altered by telecommand from Earth.  The threshold levels of the       
    detectors are included within the dataset.                                
                                                                              
                                                                              
    Onboard Processing                                                        
    ==================                                                        
    See [GRUENETAL1995C].                                                     
                                                                              
    First, the instrument microprocessor, which controls the experiment       
    measurement cycle, collects the buffered data and processes the data      
    according to its onboard program. This takes about 5 ms (10 ms for        
    Galileo after reprogramming in June 1990).  The information on a          
    single event (dust impact or noise) is contained in an Experiment         
    Data Frame (EDF) of 16 bytes (i.e. 128 bits).                             
                                                                              
    The instruments are designed to reliably operate under noisy              
    conditions thereby allowing the reliable extraction of true dust          
    impacts from noise events. True impacts can be detected at rates of       
    as low as one per month. This is achieved by raising the threshold        
    levels of all impact signals individually by telecommand which allows     
    instrument sensitivity to be adapted to the actual noise environment      
    on board the spacecraft. Coincidences between the signals are             
    established which, along with the signal amplitudes, are used to          
    classify each event.                                                      
                                                                              
    Each measured event (noise or impact) is classified according to the      
    strength of its ion signal (IA) into one of six amplitude ranges          
    (AR=1 to 6). Each amplitude range correspond roughly to one decade in     
    electronic charge, Q_I. In addition, each event is categorized into       
    one of four event classes (described by the class number CLN). The        
    event classification scheme, which defines criteria that must be          
    satisfied for each class, as it stood before July 14, 1994, is shown.     
                                                                              
    The abbreviations below refer to EVENT CLASS (CLN), ION AMPLITUDE         
    (IA), CHANNELTRON AMPLITUDE (CA), ELECTRON AMPLITUDE (EA), ELECTRON RISE  
    TIME (ET), ION RISE TIME (IT), ION CHANNELTRON COINCIDENCE (ICC), ELECTRON
    ION COINCIDENCE (EIC), and noise counters of the electron collector (EN), 
    ion collector (IN), and the channeltron (CN).                             
                                                                              
  -----------------------------------------------------------------------     
  Parameters:| CLN=0  |  CLN=1  |       CLN=2        |       CLN=3            
  -----------------------------------------------------------------------     
     IA      | IA > 0 |  IA > 0 |       IA > 0       |       IA > SP16        
  -----------|   or   |    or   |----------------------------------------     
     EA      | EA > 0 |  EA > 0 |       EA > 0       |       EA > SP14        
  -----------|   or   |--------------------------------------------------     
     CA      | CA > 0 |  CA > 0 |       CA > 0       |       CA > SP15        
  -----------------------------------------------------------------------     
     ET      |        |         | SP03 <= ET <= SP04 | SP03 <= ET <= SP04     
  -----------------------------------------------------------------------     
     IT      |        |         | SP01 <= IT <= SP02 | SP01 <= IT <= SP02     
  -----------------------------------------------------------------------     
     EIC     |        |         |       EIC = 0      |      EIC = 0           
  -----------------------------------------------------------------------     
     ICC     |        |         |       ICC = 1      |      ICC = 1           
  -----------------------------------------------------------------------     
  Noise      |        |         |                    |                        
  counter of:|        |         |                    |                        
     EN      |        |         |                    |      EN <= SP11        
     IN      |        |         |                    |      IN <= SP09        
     CN      |        |         |                    |      CN <= SP10        
  -----------------------------------------------------------------------     
                                                                              
    Within each class these conditions are connected by logical 'and'         
    except where noted. Class 0 (CLN = 0) includes all events that are        
    not categorized in a higher class (typically noise and unusual impact     
    events - e.g. impacts onto the sensor's internal structure other than     
    the impact target).  In classes 1 through 3, the criteria become          
    increasingly restricted so that CLN = 3 generally represents true         
    dust impact events only. Some of the set point values (SP01 to SP15),     
    which can be set by ground command, are used in the classification        
    scheme. Prior to July 14, 1994, the set points were as follows:           
                                                                              
                        SP01 =  1                                             
                        SP02 = 15                                             
                        SP03 =  1                                             
                        SP04 = 15                                             
                        SP09 =  2                                             
                        SP10 =  8                                             
                        SP11 =  8                                             
                        SP14 =  0                                             
                        SP15 =  0                                             
                        SP16 =  0                                             
                                                                              
    The on board classification can be adapted to the in-flight noise         
    environment by changing the thresholds and classification parameters      
    (set points) or by adjusting the onboard classification program           
    through telecommands. Detailed information on noise is mandatory in       
    order to evaluate the reliability of impact detection for the various     
    event categories, to minimize the effect on dead-time and to optimize     
    memory utilization. Such a modification of the on board                   
    classification scheme was done on July 14, 1994 after a detailed          
    analysis of data from Ulysses [BAGUHLETAL1993] identified a number of     
    'small' impacts in the three lowest categories. Baguhl et al. deduced     
    a modified event classification scheme which allowed for a better         
    discrimination between noise events and real dust impacts:                
                                                                              
  -----------------------------------------------------------------------     
  Parameters:  |CLN=0 |      CLN=1    |         CLN=2       |  CLN=3          
  -----------------------------------------------------------------------     
     IA        | IA>0 |  IA>0  | IA>0 |   IA>0     | IA>0   |  IA>0           
  -------------|  or  |--------|------|------------|--------|-----------      
     EA        | EA>0 |  EA>0  |      |   EA>0     |        |  EA>0           
  -------------|  or  |--------|------|------------|--------|-----------      
     CA        | CA>0 |        | CA>0 |            | CA>0   |  CA>0           
  -------------|------|--------|------|------------|--------|-----------      
     ET        |      |        |      |            |        | 1<=ET<=15       
  -------------|------|--------|------|------------|--------|-----------      
     IT        |      |        |      |            |        | 1<=IT<=15       
  -------------|------|--------|------|------------|--------|-----------      
     EIC       |      | EIC=1  |      |   EIC=0    |        |  EIC=0          
  -------------|------|--------|------|------------|--------|-----------      
     ICC       |      |        |      |            | ICC=1  |  ICC=1          
  -------------|------|--------|------|------------|--------|-----------      
               |      | EIT=0  |      |            |        |                 
     EIT       |      |   or   |      | 3<=EIT<=15 |        | 3<=EIT<=15      
               |      | EIT=15 |      |            |        |                 
  -------------|------|--------|------|------------|--------|-----------      
  Noise counter|      |        |      |            |        |                 
  of:          |      |        |      |            |        |                 
     EN        |      |        |      | EN<=8      |        |  EN<=8          
     IN        |      |        |      | IN<=14     |        |  IN<=2          
     CN        |      |        |      |            | CN<=14 |  CN<=2          
  -----------------------------------------------------------------------     
                                                                              
    The definition of class 3 remained unchanged with respect to the old      
    scheme. Classes 1 and 2 were divided into two subclasses. With the        
    modified scheme, noise events are usually restricted to Class 0.          
    However, Class 0 may still contain good dust impacts, especially in       
    the higher amplitude ranges. Although noise events are normally           
    restricted to Class 0, Classes 1 and 2 are also contaminated by noise     
    in extreme radiation environments [KRUEGERETAL199B].                      
                                                                              
    The above four classes, together with six amplitude ranges,               
    constitute twenty-four separate categories. Each of these categories      
    has its own 8-bit accumulator:                                            
                                                                              
                          |         |    Class number (CLN)                   
                          |Amplitude|                                         
                     IA   |  Range  |  0      1      2      3                 
                    -------------------------------------------               
                     0- 7 | AR = 1  | AC01 | AC11 | AC21 | AC31               
                     8-15 | AR = 2  | AC02 | AC12 | AC22 | AC32               
                    16-23 | AR = 3  | AC03 | AC13 | AC23 | AC33               
                    24-32 | AR = 4  | AC04 | AC14 | AC24 | AC34               
                    48-55 | AR = 5  | AC05 | AC15 | AC25 | AC35               
                    56-63 | AR = 6  | AC06 | AC16 | AC26 | AC36               
                                                                              
    As long as the respective accumulator does not overflow, each event       
    is counted even if the complete information is not received on            
    ground. Generally, the event rate is so low (even in the low              
    amplitude and low class ranges) that the true increment can be            
    reliably determined. All categories and corresponding accumulators -      
    excluding AC01, AC11 and AC02 - contain primarily impact events. Even     
    in these latter categories, true impacts can be identified and            
    separated from noise events if the complete data set for an event is      
    available (Baguhl et al., 1993)[BAGUHLETAL1993].                          
                                                                              
    The on board data processing supports the application of a priority       
    scheme for the data transmission. Data from events with different         
    categories are stored in different ranges of the on board memory. The     
    organization of the memory is particularly important because of its       
    severely limited transmission rate. Data must be safely stored on         
    board for long periods of time.                                           
                                                                              
    The memory is divided into separate ranges in which various data is       
    given priority. The A-range of instrument memory stores the six most      
    recent EDFs - one for each amplitude range regardless of class. The       
    E/E2 range, graphically depicted below, stores the last 8 (the last       
    16 after reprogramming in June 1990) events occurring within class 3.     
    These events satisfy the most stringent constraints and are almost        
    certainly true impacts. Additional memory ranges F, G, and H were         
    added to the Galileo memory scheme during reprogramming. The last 8       
    EDFs in each of these ranges are also stored. Thus, 46 EDFs can be        
    stored in DDS memory.                                                     
                                                                              
              |         | Class number (CLN)                                  
              |Amplitude|                                                     
         IA   |  Range  | 0   1   2    3                                      
    -------------------------------------------                               
         0- 7 | AR = 1  | H | G | G | E/E2                                    
         8-15 | AR = 2  | F | F | F | E/E2                                    
        16-23 | AR = 3  | F | F | F | E/E2                                    
        24-32 | AR = 4  | F | F | F | E/E2                                    
        48-55 | AR = 5  | F | F | F | E/E2                                    
        56-63 | AR = 6  | F | F | F | E/E2                                    
                                                                              
                                                                              
    Data Readout Modes                                                        
    ==================                                                        
    During most of the interplanetary cruise (i.e. before December 7,         
    1995) DDS data was received as instrument memory readouts (MROs).         
    MROs return event data which have accumulated in the instrument           
    memory over time. The contents of all 46 instrument data frames of        
    DDS is transmitted to Earth during an MRO. If too many events in a        
    given range occur between two MROs, the oldest EDFs in that range are     
    overwritten in the instrument memory and lost.                            
                                                                              
    In April 1996 the spacecraft computer on board Galileo was                
    reprogrammed (Phase 2 software) which provided a new mode for high-       
    rate dust data transmission to the Earth, the so-called realtime          
    science mode (RTS). In RTS mode, DDS data were read-out wither every      
    7 or every 21 minutes, depending on the spacecraft data transmission      
    rate, and were usually directly transmitted to Earth with a rate of       
    about 1 or 3 bits per second.                                             
                                                                              
    For short periods around satellite closest approaches, DDS data were      
    collected with a higher rate at about one minute intervals, recorded      
    on the tape recorder and transmitted to Earth several days to a few       
    weeks later. This was known as 'record mode'. Sometimes RTS data for      
    short time intervals were also stored on the tape recorder and            
    transmitted later, but this does not change the labeling.                 
                                                                              
    In both RTS and record mode only seven instrument data frames were        
    read out and transmitted to Earth, rather than the complete               
    instrument memory. This read out would consist of the six A-range         
    events and one of the E, F, G, and H range events. The E, F, G, and H     
    ranges were cyclically permuted so that 40 successive read-out cycles     
    cover the full range of instrument memory.                                
                                                                              
    All accumulator counters were read out and transmitted (or stored to      
    tape and transmitted) during each MRO, RTS and record mode read out.      
    Because of the low data transmission rates required for this              
    instrument, event rates were unaffected by spacecraft transmission        
    rates.                                                                    
                                                                              
                                                                              
   Data processing on the ground                                              
   =============================                                              
    After receiving the partially processed data from the spacecraft, the     
    following data processing steps are performed on the ground:              
                                                                              
         (1) instrument health check                                          
         (2) generation of accumulator histories                              
         (3) extraction of discrete events                                    
         (4) reduction of impact data                                         
         (5) generation of data products                                      
                                                                              
    The instrument health check involves inspection of instrument house       
    keeping data such as temperatures, voltages, currents and a check of      
    the test pulse data. If, for example, the temperature readings are        
    too high, the heater power level can be set accordingly.                  
                                                                              
    Once per day (during encounter times more frequently) all 24              
    accumulators are checked and history plots covering appropriate time      
    intervals for impact and noise events are produced. If excessive          
    noise is detected then appropriate measures, such as changing the         
    thresholds or channeltron high voltage by telecommand, can be taken.      
    Occasionally, tests of different instrument modes are performed in        
    order to probe the actual noise environment; the instrument               
    parameters can then be adjusted accordingly.                              
                                                                              
    The extraction of discrete event data, includes the removal of            
    redundant information, which can occur because of the design of the       
    instrument's memory, and a completeness check during which all events     
    that have caused an increment of one of the 24 accumulators are           
    searched for. Data of these events are put in time order.                 
                                                                              
    The preparation of data products is the final routine step of dust        
    data processing. A number of separate files are produced which            
    reflect various stages of data processing.                                
                                                                              
                                                                              
   Instrument Mounting                                                        
   ===================                                                        
   The instrument is located on the spinning section of the spacecraft        
   underneath the magnetometer boom. The sensor axis is offset by an          
   angle of 60 degrees from the positive z axis (Krueger et al. 1999b         
   [KRUEGERETAL1999B]). The z axis is the rotation axis of the                
   spacecraft.  The positive direction is antiparallel to the spacecraft      
   antenna. During most of the initial 3 years of the mission the antenna     
   pointed towards the Sun. Since 1993, the antenna usually points            
   towards Earth.  The Galileo dust detector weighs 4.2 kg and consumes       
   2.4 W.

References
==========

Baguhl, M., E. Gruen, E., D. Linkert, G.
        Linkert, and N. Siddique, Identification of 'small' dust impacts in   
        the Ulysses dust detector data, Planetary and Space Science 41,       
        1085-1098, 1993.

Goller, J.R., and Gruen E., Calibration 
        of the Galileo/Ulysses dust detectors with different projectile       
        materials and at varying impact angle, Planet. Space Sci. 37,         
        1197-1206, 1989.

Gruen, E., H. Fechtig, M.S. Hanner, J.  
        Kissel, B.A. Lindblad, D. Linkert, D. Maas, G.E. Morfill, and H.A.    
        Zook, The Galileo dust detector, Space Sci. Rev., 60, 317-340, 1992.

Gruen, E., M. Baguhl, D.P. Hamilton, J. 
        Kissel, D. Linkert, and 2 others, Reduction of Galileo and Ulysses    
        dust data, Planet. Space Sci, 43, 941-951, 1995.

Krueger, H., E. Gruen, D. Hamilton, M.  
        Baguhl, S. Dermott, and 16 others, Three years of Galileo dust data:  
        II. 1993 to 1995, Planetary and Space Science 47, 85-106, 1999.

