PDS_VERSION_ID = PDS3
LABEL_REVISION_NOTE = "Bill Harris, April 2002, minor
editing"
OBJECT = INSTRUMENT
INSTRUMENT_HOST_ID = "GO"
INSTRUMENT_ID = "SSD"
OBJECT = INSTRUMENT_INFORMATION
INSTRUMENT_NAME = "GALILEO ORBITER STAR SCANNER"
INSTRUMENT_TYPE = "STAR SCANNER"
INSTRUMENT_DESC = "
The Star Scanner is fully described by Fieseler, 2000, The Galileo
Star Scanner as an Instrument For Measuring High Energy Electrons
in the Jovian Environment, USC MS Thesis [FIESELER2000].
INTRODUCTION
The Galileo spacecraft carries aboard it a photomultiplier tube based
star scanner for the purpose of providing the spacecraft with an
inertial attitude reference. This device has been subjected to the
radiation environment within Jupiter's magnetosphere since 1995 and is
providing measurements of the omnidirectional flux of 1.5 to 30 MeV
electrons within about 12 Jupiter radii. The range of maximum
sensitivity is roughly 4 to 15 MeV. The star scanner is measuring
electrons in energy ranges similar to some channels of Galileo's
Energetic Particles Detector (EPD) but the star scanner operates
continuously thus providing a unique data set when EPD is not
operating. The star scanner is generally not sensitive to pitch angle
distribution. There is no data prior to the spacecraft reaching
Jupiter.
The star scanner provides a single channel of data for measuring
electrons termed 'background radiation count' (or sometimes 'raw
background') along with other data on the health of the instrument,
brightness of stars in the field of view and time.
Calibration:
Before Galileo's launch, an attempt was made to shield the star
scanner's Photomultiplier Tube (PMT) from the particle environment at
Jupiter. The attempt was not entirely successful resulting in this star
scanner data set. The conclusion that the star scanner is sensing
predominantly high energy electrons is based up multiple arguments.
1. Two analyses of the shielding around the star scanner PMT concluded
that the star scanner should be effectively shielded from electrons
below ~ 1 MeV and protons of several hundred MeV. The flux of such
protons is generally much less than the ~1 MeV electrons.
2. Theoretical arguments [RUSSELL2001B] based on the fact that the
star scanner measures longitudinal asymmetries at a given
jovicentric distance. It is argued that these asymmetries would
quickly smooth out unless drifting more or less with Io. This
suggests ~10 to 15 MeV electrons.
3. Qualitative comparison of star scanner data with Pioneer and
Voyager data. The star scanner measures a strong decrease in flux
at the Io L-shell similar to that seen in 5 MeV and 8 MeV electron
data from Pioneer. Although there are proton channels that show
this effect, they also show a noticeable flux decrease at the
Europa L-shell which is absent in both the Pioneer 5 MeV and
8 MeV electron data as well as star scanner data.
4. Quantitative comparison with Galileo Energetic Particles Detector
(EPD) data and Galileo Heavy Ion Counter (HIC) data. Extremely
strong correlations were found with the EPD DC3 channel measuring
> 11 MeV electrons. and with the EPD B1 channel measuring ~1.5
to 10.5 MeV electrons. A very strong correlation was also noted
with the EPD DC2 channel measuring > 2 MeV electrons.
5. Pre-flight testing of the unshielded PMT found that it was
sensitive to ~1 MeV electrons.
Using the above analyses, it was found that the range of energies the
star scanner is measuring could be bracketed between 1.5 and 30 MeV.
This, of course, does not imply that the star scanner would not react
to a 40 MeV electron or above. It is just that the flux of these
higher energy electrons drops off quickly with increasing energy in
the Jovian environment. The star scanner response was modeled using
the [DIVINE&GARRETT1983] model for flux in the Jovian environment which
predicted that the > 80% of the star scanner's response was caused by
electrons under 30 MeV.
The star scanner is a linear detector at the lower fluxes (<1000) but
becomes non-linear at higher fluxes due to saturation effects. A
derived channel called 'compensated counts' is included which corrects
for this non-linearity and other deterministic biases in the data.
Also using the above analyses, it is believed that the star scanner is
most sensitive to electrons in the range 4 to 15 MeV. An attempt was
made to convert star scanner counts to a flux of electrons by
correlating against the EPD DC3 channel and the Divine Model. In all
cases, a dependency on jovicentric distance was noted.
The equation:
Flux (#electrons cm*-2 sec*-1) = 1755 * CC * (RJ1.1208)
where RJ is radial distance from Jupiter, and CC is compensated counts
available in the star scanner text files. This conversion is
preliminary and thought to be correct only within a factor of five.
The corrected data from the star scanner (compensated counts) are
believed to be self-consistent; it is the conversion to flux that is
problematic. Additional calibration work is on-going and will be
updated here as work progresses.
Operational Considerations:
There are several situations which create loss of data. The most
likely of these is extended periods of no station coverage. All other
period where the data is missing or suspect are noted in the 'notes'
column of the star scanner text files. These include:
1. Brief periods (measured in minutes) near the point of closest
approach to a Galilean satellite in several encounters where the
star scanner shutter was closed to provide bright light protection.
The star scanner was not designed to return telemetry in this
condition.
2. Periods where the spacecraft antenna was turned sufficiently far
from the earth to cause loss of telemetry.
3. There is an operational mode termed 'OSAD' for One Star Attitude
Determination where only a single bright star is being
intentionally observed by the star scanner. This situation
increases the noise in the background radiation data noticeably
but does not otherwise harm the data.
4. Periods where Bright Body vectors were active. These are periods
during each rotation when the star scanner's is effectively shut
down to protect the photomultiplier tube from bright light sources.
These periods usually have no effect on the data set but, in a few
instances, can block a star from the field of view and thus reduce
the sampling of the data set.
It is noted that three of the four Io fly-bys appear to have 'spikes'
of increased flux right at Io closest approach. For orbits I25 and
I27 at least, it appears the star scanner was sensing bright light
reflected from Io rather than measuring a feature of the environment.
In other words, the bright body vectors were not effective due to
prior spacecraft anomalies. It cannot be ruled out that these spikes
don't partially or entirely reflect a feature of the electron
environment since a spike was apparently seen in the J0 orbit,
nevertheless, this spiked data must be used with extreme caution.
Detector:
There are two possible sources of the radiation signal within the star
scanner. Either direct electron stimulation of the photocathode of the
photomultiplier tube or light production by fluorescence and/or
Cerenkov radiation in the lenses that focus the light on the
photomultiplier. Pre-flight testing found that up to approximately 15%
of the signal was expected to come from the lenses. This can not be
verified in flight as there is no way to distinguish the two signals.
This does not cast doubt on the fact that the star scanner as a whole
is detecting electrons as described above but it does mean that no
meaningful geometric factors can be derived for the detector.
The PMT were specially modified by JPL starting with a 13 stage
tri-alkali off-the-shelf photo-multiplier tube supplied by EMR
photoelectric (model #549-01090). There are three lenses, but only
the crown glass Ohara SK18 is important as it is the least shielded
and closest to the photomultiplier tube. As this is the last lens in
the optical train, there is no focusing of any light generated within
this element. See [Fieseler, 2000] for more details.
Measured parameters and onboard processing:
The star scanner only provides the single measurement of radiation in
the unit of 'counts'. This actually is the average of 32 of the most
recent measurements that has been held in a special buffer. Each
measurement lasts for 3.2 milliseconds and are staggered by four
independent accumulators such that each measurement starts 0.8
millisecond after the previous. Thus the data that is downlinked is
actually an average taken over the previous 25.6 milliseconds. The
star scanner also provides the end time of this event, a code word
providing the health of the star scanner, the intensity of the most
recently seen star. Spacecraft twist information is also calculated
at the time the telemetry is prepared for downlink.
References
----------
DIVINE&GARRETT1983
Divine, N. and H. Garrett, Charged Particle Distributions in
Jupiter's Magnetosphere, J. Geophys. Res., 88, A9, 1983.
FIESELER2000
Fieseler, P., The Galileo Star Scanner as an Instrument for
Measuring Energetic Electrons in the Jovian Environment, MS
Thesis, University of Southern California, Los Angeles, 2000.
RUSSELLETAL2001B
Russell, C.T., P.D. Fieseler, D. Bindshadler, Z.J. Yu, S.P. Joy,
K.K. Khurana, and M.G. Kivelson, Large scale changes in the
highly energetic charged particles in the region of the Io torus,
Adv. Space Res., 28, 1495, 2001."
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REFERENCE_KEY_ID = "DIVINE&GARRETT1983"
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OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "FIESELER2000"
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REFERENCE_KEY_ID = "RUSSELLETAL2001B"
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