PDS_VERSION_ID = PDS3
LABEL_REVISION_NOTE = "
01 Jan 1996 Creation of V1.0 by M. Sykes (SBN)
Dec 1998 Final data updates and new data deliveries (through 1995)
to PDS SBN by DDS Science Team (H. Krueger, MPI Heidelberg)
Upgrades and corrections for V2.0 by M. Sykes (SBN)
Aug 2003 Updated reference style to PDS standard format by S.Joy (PPI)
"
OBJECT = INSTRUMENT
INSTRUMENT_HOST_ID = GO
INSTRUMENT_ID = GDDS
OBJECT = INSTRUMENT_INFORMATION
INSTRUMENT_NAME = "GALILEO DUST DETECTION SYSTEM"
INSTRUMENT_TYPE = "DUST IMPACT DETECTOR"
INSTRUMENT_DESC = "
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) 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.
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:
--------------------------------------------------------------------------
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).
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). 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.
"
END_OBJECT = INSTRUMENT_INFORMATION
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "GOLLER&GRUEN1989"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "GRUENETAL1992A"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "BAGUHLETAL1993"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "GRUENETAL1995C"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "KRUEGERETAL1999B"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
END_OBJECT = INSTRUMENT
END