| DATA_SET_DESCRIPTION |
Version 1.1
===========
This data set, ULY-D-UDDS-5-DUST-V1.1, differs slightly from
the data set UL-D-UDDS-5-DUST-V1.0 created and reviewed at the
PDS/Small Bodies Dust Subnode. In addition to the change in
Data set ID, the following changes have been made:
1) ULYDDUST.TAB - the column 'TIME' has been added and the
missing data flag value for the column 'XMASS' has been
changed from 0 to 9.99e+10.
2) ULYDDUST.LBL - minor changes and updates to reflect changes
in ULYDDUST.TAB and to provide consistency.
Dataset Overview
================
The data presented with this data set include
1) ULYDDUST.TAB - data received from the dust detector, the
spacecraft, and physical properties derived from the
detector data [GRUNETAL1995A].
2) ULYDCODE.TAB - value ranges corresponding to codes found in
ULYDDUST.TAB.
3) ULYDCALB.TAB - laboratory calibration data used to relate
instrument responses to physical properties of the
impacting dust particles.
4) ULYDAREA.TAB - the area of the dust detector exposed to
particles as a function of their speed direction relative
to the detector axis.
The data received from the spacecraft are used for determining
the location and orientation of the spacecraft and instrument.
Given are the SPACECRAFT SOLAR DISTANCE, ECLIPTIC LONGITUDE,
ECLIPTIC LATITUDE, SPACECRAFT JUPITER 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 consist of cataloging information,
instrument status, instrument readings at time of impact, and
classification information. The cataloging information
includes the SEQUENCE NUMBER (impact number), TIME (time of
impact in the format yyyy-mm-ddThh:mm:ss.sssZ), JULIAN DATE
(Julian Date of the 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 are the amplitude codes of the the
detectors on-board the instrument, the rise time integer codes
of the charge level rise times of the detectors, the integer
code representing the difference in starting times of the ion
signal and the electron signal, and coincidences between the
electron and ion signals, and between the ion and channeltron
signals.
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 ULYDDUST.TAB and ULYDCALB.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 Ulysses
spacecraft is unknown, logarithmic averages of the above
values are used to infer the particle speed and mass from the
instrumental measurements. See [GOLLER1988].
Processing Level
----------------
The data contain different levels of processing. Some
processing is done at the time of the impact observation. This
processing categorizes 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 are then fit to calibration curves to
determine the speed and mass of the impacting particle. See
[GOLLER&GRUN1989 and GRUNETAL1995C].
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
meta-data determined from the data analysis.
The calibration data are included as part of this dataset.
Sampling Parameters
-------------------
The occurence 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 by GRUNETAL1995C and below
(see B072295.ASC in the /DOCUMENT/DUST directory of this
volume). The calibration tables used correspond to the mean
values obtained for the three different projectile materials
with which the instruments were calibrated [GOLLER&GRUN1989
and GRUNETAL1995C]. 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 5b of GRUNETAL1995C or
ULYDCODE.TAB.
Note: If IT=0, then VIT is invalid and set to -99.9. This
differs from [GRUNETAL1995C].
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 5b of GRUNETAL1995C or
ULYDCODE.TAB.
Note: If ET=0, then VET is invalid and set to -99.9. This
differs from [GRUNETAL1995C].
If IA=49, or IA>=60, or IA<3, then IT is not valid, and only
VET is used to determine impact speed.
If EA=15, or EA>=60, 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
(see B030696.ASC in the /DOCUMENT/DUST directory of this
volume).
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, GRUNETAL1995C; ULYDCALB.TAB). The
charge to mass ratio for a given impact speed (V) is
determined by linear interpolation of the calibration table
(ULYDCALB.TAB) on a double logarithmic scale, yielding a
separate value for the ion grid measurement (QIM) and and
electron grid measurement (QEM).
From these values and the respective impact charges (QI and
QE) corresponding to IA and EA, respectively (Table 4,
GRUNETAL1995C; ULYDCALB.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)
(see B030696.ASC in the /DOCUMENT/DUST directory of this
volume. This differs from the exponent of 3.4 given in
GRUNETAL1995A)
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).
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