ULYSSES 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. (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.

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, J. Kissel, D.    
        Linkert, D. Maas, J.A.M. McDonnell, G.E. Morfill, G. Schwehm, H.A.    
        Zook, and R. H. Giese, The Ulysses dust experiment, Astron Astrophys. 
        Suppl. Ser. 92, 411-423, 1992.

Gruen, E., M. Baguhl, N. Divine, H.     
        Fechtig, D.P. Hamilton, and 14 others, Two years of Ulysses dust data,
        Planet. Space Sci, 43, 971-999, 1995.

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.

