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 arecollected 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 moreinformation 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 aroundJupiter.


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 empiricalcorrelations 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 collectedat 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, andhit 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 pulseserves 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 anExperiment 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 directlyexposed) 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 5milliseconds.

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 aprogram stored in the memory.

See Gruen et al.(1992a) [GRUENETAL1992A] for more information concerningthe 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 atone 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 oneangle.

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'sflight 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. Usingonly 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 speedmeasurement.


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 thedetectors 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 ExperimentData 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 toclassify 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 besatisfied 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 themodified 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 noisein 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 isavailable (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 onboard 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 last16 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 bestored 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 areoverwritten 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 ofabout 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 andtransmitted 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 cyclescover the full range of instrument memory.

All accumulator counters were read out and transmitted (or stored totape 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 transmissionrates.


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 aretoo 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 thethresholds 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 instrumentparameters 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 aresearched 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 whichreflect 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 consumes2.4 W.