NASA Logo, National Aeronautics and Space Administration
NASA Space Science Data Coordinated Archive Header

GALILEO DUST DETECTOR SYSTEM V1.0 (PDS)

NSSDCA ID: PSSB-00444

Availability: Archived at NSSDC, accessible from elsewhere

Description

This description was generated automatically using input from the Planetary Data System.

Dataset Overview ---------------The data presented with this dataset includes 1) galddust.tab - data received from the dust detector, the spacecraft, and physical properties derived from the detector data [GRUNETAL1995B]. 2) galdcode.tab - value ranges corresponding to codes found in galddust.tab. 3) galdcalb.tab - laboratory calibration data used to relate instrument responses to physical properties of the impacting dust particles. 4) galdarea.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 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), DATE JULIAN (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 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 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]. 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 physcial properties determined from the data analysis. The calibration data are included as part of this dataset. Sampling Parameters ------------------The occurance 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 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 corresonding value of IT. VIT is then found in Table 5a of GRUNETAL1995C or galdcode.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 corresonding value of ET. VET is then found in Table 5a of GRUNETAL1995C or galdcode.tab. Note: If ET=0, then VET is invalid and set to -99.9. This differs from [GRUNETAL1995C]. 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<EA-IA or EA-IA<2, then there is no valid impact speed. If neither IT nor ET is valid, then there is no valid impact speed. Data Reduction - Impact Speed Error Factor -----------------------------------------The upper and lower estimates of impactor speed are obtained by multiplying and dividing, respectively, the mean particle speed by the velocity error factor, VEF. If only one speed is measured, and is from the electron detector, the minimum uncertainty is VEF=2. If only one speed is measured, and is from the ion detector, the minimum uncertainty is VEF=1.9. It is assumed that minimum error of 1.6 is achieved if both individual speeds agree to within a factor of 4. This error corresponds to the logarithmic mean of the minimum errors in the two cases when only a single speed is valid. Since these are all 1-sigma errors, it may happen that VIT or VET fall outside the error bar given for the mean impact speed, V. In order to avoid this, the error factor is 'stretched' to contain the values: If VIT > 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 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; 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 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; 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) (see B030696.ASC in the /document 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).

These data are available on-line from the Planetary Data System (PDS) at:

http://sbn.psi.edu/pds/dust/GO_D_GDDS_5_DUST_V1_0/

Alternate Names

  • GO-D-GDDS-5-DUST-V1.0

Discipline

  • Planetary Science: Small Bodies

Additional Information

Spacecraft

Experiments

Questions and comments about this data collection can be directed to: Dr. Edwin V. Bell, II

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Eberhard GruenData ProviderMax-Planck-Institut fur Kernphysikeberhard.gruen@mpi-hd.mpg.de
Dr. Mark V. SykesGeneral ContactUniversity of Arizonamsykes@as.arizona.edu
[USA.gov] NASA Logo - nasa.gov