GALILEO DUST DETECTION SYSTEM (Data Set Description)

	 1995  Final data deliveries (through 1992) to PDS SBN by              
	       DDS Science Team (E. Gruen, MPI Heidelberg)                     
  01 Jan 1996  Creation of V1.0 (M. Sykes, SBN)                              
  06 Mar 1996  PDS SBN Peer Review (Tucson, Arizona)                           
	 1999  Final data updates and new data deliveries (through 1996)       
	       to PDS SBN by DDS Science Team (H. Krueger, MPI Heidelberg)     
     Jan 1999  Upgrades and corrections for V2.0 (M. Sykes, SBN)             
  09 Mar 1999  PDS SBN Peer Review (Heidelberg, Germany)        
                                                                               
  START_TIME                       1989-12-28T17:20                            
  STOP_TIME                        1997-12-20T03:10

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 and Krueger et al. 1999b).

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 Goller (1988).

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 and Gruen 1989; Gruen et al., 1995c).

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 by Gruen et al. (1995c) 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 and Gruen 1989; Gruen et al., 1995c). 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) or galdcode.tab.  
                                                                               
       Note: If IT=0, then VIT is invalid. This differs from                   
             Gruen et al. (1995c).

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) or galdcode.tab.  
                                                                               
       Note: If ET=0, then VET is invalid. This differs from                   
             Gruen et al. (1995c).

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

(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, Gruen et al. (1995c); 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, Gruen et al. (1995c); 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 Gruen et al. (1995a))

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) (Gruen et al. 1995b; Krueger et al. 1999b). 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 Gruen et al. (1995b) 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 Krueger et al. (1999b), 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 Gruen et al. (1995c) and Krueger et al. (1999b), noting that in Gruen et al. (1995a and b) and Krueger et al. (1999a) 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 Gruen et al. (1995b).

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 Goller (1988).