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.