| DATA_SET_DESCRIPTION |
Dataset Overview
================
This data set contains information on the dust environment in
interplanetary space within the inner solar system and in the
Jupiter system, within and without the Jovian magnetosphere and
around the Galilean satellites. This information is collected with a
dust impact experiment, DDS, from which may be inferred direction of
motion, mass, velocity and charge (see GALDINST.CAT). The data
presented in this dataset include instrumental readouts, inferred
metadata, calibration information and a calendar of events.
Specifically:
1) galddust.tab - data received from the dust detector, the
spacecraft, and physical properties derived from the detector data
for reliable dust impacts (Gruen et al. 1995b [GRUENETAL1995B] and
Krueger et al. 1999b [KRUEGERETAL1999B]).
2) galdevnt.tab - data received from the dust detector, the
spacecraft, and physical properties derived from the detector data
for reliable dust impacts plus noise events.
3) galdcode.tab - value ranges corresponding to codes found in
galddust.tab.
4) galdcalb.tab - laboratory calibration data used to relate
instrument responses to physical properties of the impacting dust
particles.
5) galdarea.tab - the area of the dust detector exposed to particles
as a function of their velocity direction relative to the detector
axis.
6) galdstat.tab - time history of Galileo mission and dust detector
configuration, tests and other events.
The data received from the spacecraft are used for determining the
location and orientation of the spacecraft and instrument. Given are
the SPACECRAFT-SUN DISTANCE, ECLIPTIC LONGITUDE, ECLIPTIC LATITUDE,
SPACECRAFT-EARTH DISTANCE, ROTATION ANGLE, DETECTOR ECLIPTIC
LONGITUDE, and DETECTOR ECLIPTIC LATITUDE.
Data received from the dust detector are given in an integer code
format. Some of the integer codes represent a range of values
within which the data could fall (e.g., ION AMPLITUDE CODE), some
may represent a specific value (e.g., ION COLLECTOR THRESHOLD), and
others, a classification based upon other integer codes (e.g., EVENT
CLASS).
The instrument data consists of cataloging information, instrument
status, instrument readings at time of impact, and classification
information. The cataloging information includes the SEQUENCE
NUMBER (impact number), JULIAN DATE (time of impact), and SECTOR
(the pointing of the instrument at time of impact). The instrument
status data are the threshold levels of the detectors and the
CHANNELTRON VOLTAGE LEVEL.
The instrument readings include the amplitude codes of the detectors
aboard the instrument and the integer codes representing the charge
level rise times of the detectors, the difference in starting times
of the ion signal and the electron signal, electron and ion signal
coincidence, and ion and channeltron signal coincidence.
The classification information is used to assist in classifying an
event into probable impact and non-impact categories. There are
three variables used in classification: EVENT DEFINITION which
records which detectors begin a measurement cycle; ION AMPLITUDE
RANGE which is the classification of the ION AMPLITUDE CODE into 6
subranges (used with EVENT CLASS); and EVENT CLASS which categorizes
events into a range of probable impacts to probable non-impacts.
The PARTICLE SPEED and PARTICLE MASS and their corresponding error
factors are determined from the instrument and calibration data
given in galddust.tab and galdcalb.tab, respectively.
Calibration Data
================
ION RISE TIME, ELECTRON RISE TIME, ION CHARGE MASS RATIO, and
ELECTRON CHARGE MASS RATIO were measured for iron, glass, and carbon
particles of known mass and impacting at known speeds. Since the
composition of particles striking the Galileo spacecraft is unknown,
logarithmic averages of the above values are used to infer the
particle speed and mass from the instrumental measurements. See
[GOLLER1988].
The data were provided in a private communication to M. Sykes (Jun
29 03:04 MST 1995) by M. Baguhl. They are the results of these
experiments for impacts at an angle of 34 degrees from the detector
axis.
Processing Level
================
The data contain different levels of processing. Some processing
was done at the time of the impact observation. This processing
categorized the detector responses to transmit the data efficiently
back to Earth. Data received on Earth is given as an integer code.
These integer codes can, for example, represent ranges of values, or
can be a classification determined from other integer codes. On
Earth, these integer codes were then fit to calibration curves to
determine the speed and mass of the impacting particle
([GOLLER&GRUEN1989]; [GRUENETAL1995C]).
This data set contains the information from the spacecraft
instrument as received on Earth, information about the location and
pointing direction of the spacecraft, and the physical properties
determined from the data analysis.
The calibration data are included as part of this dataset.
Sampling Parameters
===================
The occurrence of an impact with the instrument begins a measurement
cycle. The on-board detectors measure a charge accumulation versus
time in order to measure the rise time of the accumulation and any
coincidences between detector readings. The on-board computer
converts these measurements to integer codes to minimize the amount
of data that is transferred back to Earth. After the conversion,
the integer codes are categorized to determine if an event is more
likely to be an impact or noise event. The data are then stored
until it is time to transmit to Earth.
Data Reduction - Impact Speed
=============================
Impact speed (V) is obtained from the rise-time measurements of the
ion and electron detectors (IT and ET, respectively) using
procedures described in part in [GRUENETAL1995C] and a private
communication to M. Sykes (Jul 22 03:43 MST 1995) from M. Baguhl.
The calibration tables used correspond to the mean values obtained
for the three different projectile materials with which the
instruments were calibrated ([GOLLER&GRUEN1989]; [GRUENETAL1995C]).
A rise-time measurement is started when the respective signal
exceeds its threshold and is stopped by a flag pulse from the peak-
detector. Impact calibration was performed in the speed interval
from about 2 km/s to 70 km/s, so impact speeds derived from rise-
time measurements will be limited to this range.
Dust accelerator tests as well as experience with flight data have
shown that (1) the shape of the ion signal is less susceptible to
noise than the shape of the electron signal and (2) for true
impacts, ELECTRON AMPLITUDE CODE values (EA) are generally greater
than the ION AMPLITUDE CODE values (IA) by 2 to 6. As a
consequence, the electron rise-time is only used for impact speed
determination if 2 =< EA-IA =< 6. Since both speed measurements, if
available, are independent, one obtains two (often different) values
VIT and VET, respectively. The impact speed is then taken to be the
geometric mean of VIT and VET.
Determining VIT:
If IA > 16 and IT > 12, then fix IT=14. Else, if IA > 16 and IT
=< 12, then add 2 to the corresponding value of IT.
VIT is then found in Table 5a of Gruen et al.(1995c)
[GRUENETAL1995C] or galdcode.tab.
Note: If IT=0, then VIT is invalid. This differs from
Gruen et al. (1995c) [GRUENETAL1995C].
Determining VET:
If EA > 16 and ET > 12, then fix ET=14.
Else, if EA > 16 and ET =< 12, then add 2 to the corresponding
value of ET.
VET is then found in Table 5a of Gruen et al.(1995c)
[GRUENETAL1995C] or galdcode.tab.
Note: If ET=0, then VET is invalid. This differs from
Gruen et al. (1995c) [GRUENETAL1995C].
If IA=49, or IA=18, or IA<3, then IT is not valid, and only VET is
used to determine impact speed.
If EA=49, or EA=31, or EA<5, then ET is not valid, and only VIT is
used to determine impact speed.
If IT is invalid and 6 4*VET, then
VEF=(VIT/VET-4.)/31.*(1.6*sqrt(35.)-1.6)+1.6
If VET > 4*VIT, then
VEF=(VET/VIT-4.)/31.*(1.6*sqrt(35.)-1.6)+1.6
(private communication to M. Sykes from M. Baguhl, Mar 6 03:57 MST
1996).
If the ratio of both speeds exceeds 4, then the uncertainty can
increase to about 10 in the calibrated speed range. In any case, a
speed value with an uncertainty factor VEF>6 should be ignored.
Data Reduction - Impactor Mass
==============================
Once a particle's impact speed (V) has been determined, the charge
to mass ratio can be determined from calibration measurements
(Figure 3, [GRUENETAL1995C]); galdcalb.tab). The charge to mass
ratio for a given impact speed (V) is determined by linear
interpolation of the calibration table (galdcalb.tab) on a double
logarithmic scale, yielding a separate value for the ion grid
measurement (QIM) and electron grid measurement (QEM).
From these values and the respective impact charges (QI and QE)
corresponding to IA and EA, respectively (Table 4, Gruen et al.
(1995c) [GRUENETAL1995C]; galdcalb.tab), mass values (MQI=QI/QIM and
MQE=QE/QEM) are determined corresponding to the ion and electron
grid measurements. When both MQI and MQE are valid, the impact
particle mass, M, is the geometric mean of these two values, or the
value corresponding to the valid measurement if the other is
invalid. If there is no valid impact speed, then there is no valid
impactor mass.
Note: when V is invalid, M is invalid.
Note: when IA=0, QI is invalid and MQI is invalid.
Note: when EA=0, QE is invalid and MQE is invalid.
Data Reduction - Impactor Mass Error Factor
===========================================
The upper and lower estimate of impactor speed is obtained by
multiplying and dividing, respectively, the mean particle speed by
the mass error factor, MEF. If the speed is well determined
(VEF=1.6) then the mass value can be determined with an uncertainty
factor MEF=6. Larger speed uncertainties can result in mass
uncertainty factors greater than 100.
The mass error is calculated from the speed error, keeping in mind
that mass detection threshold is proportional to speed to the 3.5th
power. In addition, there is an error factor of 2 from the amplitude
determination. Added together (logarithmically) these yield
MEF=10**(sqrt((3.5*log(VEF))**2+(log(2.))**2))
(Private communication to M. Sykes from M. Baguhl, Mar 6 03:57 MST
1996. This differs from the exponent of 3.4 given in
[GRUENETAL1995A])
Coordinate System
=================
The coordinates of the spacecraft are given in heliocentric ecliptic
latitude and longitude (equinox 1950.0), where the pointing
direction of the sensor is given in spacecraft centered ecliptic
latitude and longitude (equinox 1950.0).
|
| CONFIDENCE_LEVEL_NOTE |
Impact times
============
The impact times during the Cruise phase of the mission were
recorded with an accuracy of 1.1 hours. After June 25, 1990,
inclusive, the accuracy was 4.3 hours (this value has been set in
order to bridge gaps in the data transmission as long as one month)
([GRUENETAL1995B]; [KRUEGERETAL1999B]). There were also periods in
which more frequent memory reads resulted in a time resolution of
2/3 seconds.
Time Error Value (TEV)
======================
Prior to 1993, data were not released with individual TEV values.
Time resolutions given by [GRUENETAL1995B] and applied
retrospectively to data prior to 1993 yields the following
distribution of TEV across those impact and noise events:
galddust galdevnt
IMPACT SEQUENCE NUMBER | TEV EVENT SEQUENCE NUMBER | TEV
------------------------------ -----------------------------
001-099 | 66 0001-0607 | 0
100-138 | 259 0608-1099 | 66
139-147 | 0 1100-1218 | 259
148-344 | 259 1219-1851 | 0
345-352 | 0 1852-3244 | 259
353-359 | 259 3245-4627 | 0
4628-5446 | 259
These values were confirmed in a private communication to M. Sykes
(Dec 9 05:06 MST 1998) by H. Krueger.
Based on information in [KRUEGERETAL1999B], TEV values were changed
for the following events to the values below:
IMPACT SEQUENCE NUMBER | TEV EVENT SEQUENCE NUMBER | TEV
------------------------------ -----------------------------
2762-2767 | 70 8991-8996 | 70
2768-2837 | 2 8997-9069 | 2
2848-2851 | 33 9080-9083 | 33
2852-2869 | 2 9084-9101 | 2
NOTE: In the current (1998) PDS release, the Galileo DDS Status File
(galdstat.tab) extends only through the end of 1995. Thus, Time
Error Factors for 1996 and 1997 impact and event data cannot be
checked against the sampling mode of the DDS until the release of
the updated status file.
Sector
======
In V1.0 of this data set, SECTOR was reported in degrees. In V2.0
Sector is reported as its original 8-bit word, and has a value
between 0 and 255 (when valid). Conversion to degrees may be
accomplished through scaling by 1.40625.
Ion Channeltron Coincidence (ICC)
=================================
The designation ICC is used following [GRUENETAL1995B] and
[KRUEGERETAL1999B], noting that in [GRUENETAL1995A] and
[GRUENETAL1995C], and [KRUEGERETAL1999A], the designation is IIC.
Entrance Grid Amplitude Code (PA)
=================================
In the data that have been published in the literature and
electronically prior to 11/98, there are values of PA which exceed
47. In a private communication to M. Sykes (Mar 6 03:57 MST 1996),
Michael Baguhl and Rainer Riemann stated:
'Values of PA greater 47 are caused by a bit flip (caused by a
timing bug in the sensor electronics) of the MSB. For values
greater 47, a value of 16 has to be subtracted.'
This correction was made to all PDS DDS files prior to 11/98.
As a consequence of subsequent uncertainty about the origin of PA
values greater than 47, in a private communication to M. Sykes (Nov
6 04:07 MST 1998), H. Krueger requested that PA values greater than
47 be corrected to '99'. This has been done in releases of the DDS
data through the PDS after 11/98.
Channeltron Voltage Level (HV)
==============================
The nominal high voltage HV=4 (1250V) could not be used because of
unexpected noise on the channeltron. It is assumed that the nearby
radioactive thermal generators (RTGs) are to blame, although other
causes cannot be excluded. During ground tests (without RTGs) no
such noise was observed. See [GRUENETAL1995B].
Spacecraft Earth distance
=========================
The value for the same impact event in galddust.tab and galdevnt.tab
is different, but less than 7500 km.
Impact speed
============
In a private communication to M. Sykes (Jul 22 03:43 MST 1995), M.
Baguhl stated that the reason for the exclusion of the values IA=49
and EA=15 is empirical. These values are close to the switching
points of the amplifier ranges and therefore produce incorrect time
measurements. The adjustment of the times in amplifier range 2 was
made in order to prevent illegal time values.
Calibration data
================
Instrumental values were extrapolated for particle masses and speeds
outside the range of those tested, and are so marked. The accuracy
of these numbers is unknown. An explication of the experiments and
data used to generate the calibration may be found in [GOLLER1988].
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