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. (1992b) [GRUENETAL1992B] for
    more information concerning the instrument.
 
 
  Science Objectives Summary
  ==========================
    The objective of the Ulysses dust experiment is to investigate the
    physical and dynamical properties of small dust particles (10**-16
    to 10**-6g) as a function of ecliptic latitude and heliocentric
    distance, and the study of their interrelation with
    interplanetary/interstellar phenomena.  The parameters to be
    determined include the mass, speed, flight direction and electric
    charge of individual particles.  Specific objectives are:
 
    - To determine the impact rate, size frequency, and the distribution
      of flight directions and electric charges of interplanetary dust
      particles
    - To classify particle orbits into bound orbits around the Sun or
      hyperbolic orbits leaving or entering the solar system
    - To study the distributions of orbital elements (semi-major axis,
      eccentricity, inclination ) of particles in bound orbits
    - To determine as functions of heliocentric distance and ecliptic
      latitude the spatial density of the interplanetary large particle
      population which generally moves in bound orbits around the sun,
      and to determine the relative significance of comets and asteroids
      as sources for these zodiacal dust particles
    - To measure the flux and velocity of particles coming in hyperbolic
      orbits from the general direction of the Sun
    - To identify interstellar dust particles and perform direct
      measurements of the spatial density, heliocentric distribution,
      velocity and mass of interstellar grains traversing the solar
      system
    - To observe enhancements of cometary dust particles during the
      transit of the spacecraft through the plane of a comet's orbit
    - To investigate the spatial density of dust particles within the
      asteroid belt and determine the amount of dust produced by
      collisions in the asteroid belt
    - To investigate the influence of the Jovian gravitational field on
      the interplanetary dust population
    - To measure electric charges of dust particles and establish the
      relationship of these charges to the properties of the ambient
      plasma (plasma density, energy spectrum), the solar radiation
      spectrum and magnetic fields
 
 
  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 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 Q_i on the
    ion-collector, 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 if only
    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.
 
 
  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 and Gruen (1989) 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 risetimes 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.  The smallest impact charge Q_i detectable is
    about 10**14 coulomb which corresponds to a mass and speed dependent
    threshold that can be approximated by a power law (see Gruen et al.,
    1995a).
 
 
  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 aboard the instrument.  The thresholds
    are altered by telecommand from Earth.  The threshold levels of the
    detectors are included within the dataset.
 
 
  Onboard Processing
  ==================
    See Gruen et al, 1992b and 1995c [GRUENETAL1992B],
    [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.  The signal
    amplitudes and times of a single recorded event (dust impact or
    noise) are digitized and stored in an Experiment Data Frame (EDF) of
    16 bytes (i.e. 128 bits). Supplementary information like event time
    and instantaneous spin position are collected from the spacecraft
    and added in each EDF. Dead-time caused by the measurement cycle is
    5 ms.
 
    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, is shown:
 
 --------------------------------------------------------------------------
 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. The set points are 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.
 
    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 range, graphically depicted below, stores the last 8 events
    occurring within class 3. These events satisfy the most stringent
    constraints and are almost certainly true impacts.
 
    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 [BAGUHLETAL1993].
 
    The transmission of seven EDFs constitute an instrument read-out
    cycle (six A-range events and one of the subcommutated class 3
    events as well as all 24 accumulators) which is continuously
    repeated. The Ulysses mission is designed to provide continuous data
    coverage even when data transmission to Earth is only possible
    during one pass of approximately 8 hours per day. Continuous
    coverage is achieved by storing data from the instruments at a low
    rate into an on-board memory which is read out at a high rate
    together with real-time data transmission during a pass.  At a
    spacecraft data transmission rate of 1024 bps, one EDF is sent every
    16 seconds. Lower bit rates down to 128 bps during storage or real
    time transmission periods are possible.
 
 
  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.
 
    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 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 equipment platform of the
    spacecraft body and its axis is at an angle of 85 degrees with
    respect to the positive z axis,  where the z axis is the rotation
    axis of the spacecraft and the positive direction is where the axis
    points roughly towards Earth.  The Ulysses dust detector weighs 3.8
    kg and consumes 2.2 W.