| DATA_SET_DESCRIPTION |
This data set contains the R_EDR data for the Galileo
Orbiter PPR instrument for the period corresponding
to the Earth-2 encounter observations in
November-December 1992.
Data Set Overview
=================
As described in the Project Galileo Experiment Data Record Software
Interface Specification (cf. ASCII and word processor text versions
in DOCUMENT subdirectory), the Galileo Data Management System
generates Low Rate Science (LRS) EDR files for eight Galileo Orbiter
instruments including the PPR. EDR files are organized into blocks
of data corresponding to a major frame, or one RIM (major frame count)
cycle consisting of 91 minor frames. The EDR generation program
extracts the AACS, PPR science, and science related engineering
channels from each LRS minor frame using the spacecraft clock (SCLK)
count to control the building of EDR blocks of data. With normal,
uninterrupted LRS data, the SCLK count increments through minor frame
(MOD91 count) numbers 0 through 90 and then the next minor frame
starts a new major frame with the RIM count incremented by one. If
LRS minor frames are missing, appropriate locations within the EDR
blocks corresponding to these missing data are filled with binary
zeros. The MOD91 count and the spacecraft event time (SCET) that
corresponds to the first actual (non-filler) minor frame in a block is
placed in the header record of that block, thus allowing for missing
data at the beginning of the block as well as for the initial block of
an EDR file which may begin at a MOD91 count other than 0.
The PPR EDR data for a major frame consist of a pair of blocks, the
first being 2252 bytes in length and the second, 1924 bytes. At the
beginning of the first block is a header of 68 bytes and this is
followed by 24 bytes of spacecraft and scan platform attitude data
for each of the 91 minor frames of that major frame. The second
block of the pair begins with a header of 68 bytes, followed by a
subheader of 216 bytes containing science related engineering data
and then 18 bytes of PPR housekeeping and science data for each of
the 91 minor frames. (Since this total of 1922 bytes is not an
integral number of 4-byte words, zero fill for byte numbers 1923 and
1924 is used to complete the second block.) For each of the 18-byte
PPR minor frame records, the first six bytes are housekeeping data
that completely specify the instrument status, both commanded
parameters and position within operational measurement mode cycles.
The remaining twelve bytes are three sets of science data sample
pairs and their associated identifying parameters. Data files of the
present archive are a reformatted version of the PPR EDR data (hence,
R_EDR). These files are organized in an ASCII Table format with one
record of 51 parameters, or columns, corresponding to each PPR
science data sample pair (three per minor frame). In addition to the
data number (DN) values for the sample pair itself, all parameters
from the PPR housekeeping and supplementary EDR data (including
appropriately adjusted time and scan platform pointing information)
that identify and characterize that sample pair are included among
the 51 parameters. Each data file has an attached PDS label that
specifies the record format and describes each parameter of the
record.
Russell, E.E., F.G. Brown, R.A. Chandos, W.C. Fincher,
L.F. Kubel, A.A. Lacis, and L.T. Travis,
Galileo Photopolarimeter/Radiometer Experiment,
Space Sci. Rev. 60 p. 531-563, 1992.
Hunten, D.M., L. Colin and J.E. Hansen,
Atmospheric Science on the Galileo Mission,
Space Sci. Rev., 44, 191-240, 1986
Johnson, T.V., C.M. Yeates and R. Young,
Space Science Reviews Volume on Galileo Mission Overview,
Space Sci. Rev., 60, 3-21, 1992
NOTE = The following is from the PPR instrument paper as Published
in Space Science Reviews, Vol. 60, by Kluwer Academic
Publishers. The complete document is located at
/DOCUMENT/PPR_INST/PPR.HTM.
4.2. INFLIGHT CALIBRATION
Several means of inflight calibration will be utilized by the PPR to
update the preflight calibration of thephotopolarimetry channels and
to provide the prime radiometric calibration of the radiometry
channel. These include: (1) an internal calibration lamp within the
PPR aft optics; (2) the third orientation position for each of the
halfwave retarders which interchanges the roles of the two silicon
detectors; (3) the radiometric calibration target (RCT-PPR) that is
separately mounted on the spacecraft and can function either as a
blackbody source or to provide a slightly polarized lamp output signal
for the visible/near-infrared region; (4) a spacecraft supplied
photometric calibration target (PCT) that provides a standard of
spectral radiance that can be viewed by all instruments located on the
spacecraft scan platform; and (5) viewing stars and spatially
unresolved planets.
4.2.1. Internal Cal Lamp
The small tungsten filament lamp located within the aft optics of the
PPR provides a means to track any changes with time of the silicon
detector/amplifier channels. The spectral output of the lamp is
modified with the use of a color glass filter (Schott BG-18). The
stability of this long-life (50 000-hour) lamp is further enhanced by
operating it only intermittently, at a derated power level, and with a
controlled, slow turn-on characteristic. The lamp is energized
intermittently only during cycle mode operation while the chopper is
being driven to its rest position following radiometry mode sampling
at the solar + thermal filter wheel position and if the cal lamp
command bit is set to the ON state. (The estimated total on time is
less than 200 hours during the 7-year pre-launch testing period plus
the post-launch inflight period.)
4.2.2. Internal Polarimetric Calibration
As previously discussed, one key feature for achieving accurate
polarimetry with the PPR is the ability to cross-calibrate the
detectors by measuring simultaneously the orthogonal polarization
components of the scene radiance. The third halfwave retarder position
(with fast axis oriented at 45u to the plane of deviation of the
Wollaston prism) effectively interchanges the scene polarization
components incident on the two detectors. This permits maintaining the
polarimetric accuracy even in the presence of slow relative changes of
the two detector channels with time. temperature, radiation, etc.
4.2.3. RCT-PPR Design
The RCT-PPR will serve in a dual calibration role for the PPR. The
primary role will be as a thermal calibration target which closely
approximates a blackbody source when viewed along the RCT-PPR
axisymmetric axis. Due to spacecraft space limitations, it was
necessary to restrict the overall length of the target. To achieve the
desired normal emittance (e > 0.998) the geometry of the interior
portion of the target has a truncated conical form with a center
cylindrical section. This provides on-axis performance approximately
equivalent to a cone with half the apex angle and twice the overall
length. The end of the central cylindrical portion is not viewed by
the PPR since this area is within the central obscuration of the PPR
telescope. The interior of the target is a smooth
(specularly reflecting) black-painted surface to achieve a higher
on-axis emittance than would be possible with a rough (diffuse)
surface for the same geometry. The calculated on-axis emittance of the
RCT-PPR is greater than 0.998 based on the reflectance versus incident
angle for the interior surfaces.
The RCT-PPR is designed and mounted such that it will be passively
cooled at Jupiter to a temperature of 145 y15 K. The wall thickness is
chosen to assure worst case temperature gradients of less than 0.5 K.
The temperature of the RCT-PPR is monitored by two platinum resistance
thermometers (PRTs) that are calibrated by the manufacturer
(resistance versus temperature) to an accuracy 0.2 K. These PRTs are
read out directly by the PPR along with a low temperature coefficient
resistor also mounted on the RCT-PPR to allow a first-order correction
for spacecraft cabling resistance. The annular aperture of the target
is designed to accommodate the 3-a worst case relative misalignments
resulting from possible spacecraft environmental and mounting factors
specified by the Galileo Project to assure that the PPR will view only
the high emittance portion of the target during calibration. Through
the use of the RCT-PPR and the preflight calibrations used to assess
the influence of temperature changes of the PPR optical elements, it
is expected that the overall radiometric calibration of the PPR
thermal bands can be maintained within the desired y1 K over the
duration of the Galileo Mission.
A small tungsten-filament lamp is mounted in one portion of the
RCT-PPR interior surface. With the source commanded ON, flux from the
source passes through an elliptically shaped, plane-parallel sapphire
plate mounted such that the outer surface approximately conforms to
the inner conical surface of the target. The flux transmitted to the
PPR is partially polarized due to different S and P Fresnel
reflectances of the inclined plate. Thus, this source will be useful
in assessing possible photometric and polarimetric changes of the
entire optical train of the PPR over the course of the mission.
4.2.4. Spacecraft PCT
The PCT is intended to serve as a standard of spectral radiance for
the scan platform mounted instruments by reflecting sunlight from a
diffusely reflecting surface with well-characterized reflectance
properties. Since this target can be viewed by all scan platform
mounted instruments, the PCT is expected to be particularly useful in
the role of calibration intercomparison among instruments.
4.2.5. Viewing Astronomical Objects
Orienting the PPR to view such astronomical objects as stars or
spatially unresolved planets will be used to provide both cross-check
of the absolute photometric calibration of the PPR silicon photodiode
channels and as an additional means to track any responsivity changes
with time, temperature, radiation, etc. Sirius is a star which will
provide an adequate signal-to-noise ratio for this purpose by
aggregating a sufficient number of samples. Similarly, viewing
unresolved (object subtending less than the PPR field of view) planets
at phase angles accessible from Earth will allow intercomparisons to
be made with ground-based photometric calibration.
4.3. SIGNAL-TO-NOISE PERFORMANCE
4.3.1. Photopolarimetry Channels
Four separate band gains are used for the photopolarimetry channels,
with the value applied (as described in the electronics section) being
dependent on the filter/retarder wheel position as determined by the
encoder. This will provide signal outputs of similar magnitude for the
three polarimetry and seven photometry bands for typical scene
spectral radiances. The channel and band gains were set to provide
signal levels at Gain Step 8 of approximately 2000 DN for the three
polarimetry bands (each with a separate band gain) and of
approximately 1500 DN for the 648 nm photometry band (a single band
gain is applied to all seven photometry bands). For setting these
levels, the Jovian albedo values of Woodman et al. (1979) were used.
The noise of the photopolarimetry channels is essentially independent
of signal level, resulting primarily from the 100 megohm feedback
resistors in the pre-amplifiers. As a result, the signal-to-noise
ratio (SNR) varies only slowly with temperature over the PPR operating
temperature range (since the Johnson noise varies as the square root
of the absolute temperature). The measured SNR performance of the PPR
photopolarimetry channels substantially exceeds the science-dictated,
minimum SNR requirements of 1000 for the polarimetry bands and 200 for
the photometry bands.
4.3. 2. Radiometry Channel
Achieving the SNR performance desired for the PPR science
investigations utilizing the radiometry channel produces far greater
stress on instrument design that is the case for the photopolarimetry
channels. The inevitable Galileo mission mass and size constraints on
science instruments required substantial compromise on performance
characteristics. To optimize the SNR performance of the pyroelectric
detector required thinning the LiTaO4 detector element to the maximum
extent possible. For the PPR application, ion-beam milling was used to
provide thicknesses in the 5 to 6 3m range. In order to provide good
optical absorption with low mass, the detector was coated with an
evaporated gold-black coating. The wide range of absorbing
characteristics found in the literature for gold blacks increased the
risk with this approach, but on balance offered the best overall
choice for the PPR requirements.
Optimization curves for the noise components of the PPR pyroelectric
detector is illustrated in Figure 11. Measured noise data for the PPR
detector were near the levels predicted. However, relative spectral
response measurements indicated levels substantially below
specification. The lower than expected long wavelength responsivity,
combined with the lower than specified filter transmittance for some
of the filters (much lower for the 37 3m band) led to the inability to
meet the instrument SNR performance specifications for four of the
seven radiometry channel bands. The measured versus specified SNR
performance is indicated in Table VI. The solar plus thermal band
measurement tabulated includes only the thermal component; the solar
band is not included in the table, but comfortably exceeds the
specifications. Three of the four out-of-spec bands have SNR
performances about 60% of specification, while the 37 3m band
(D filter) is about one-third of the desired level.
Fortunately the mission profile and the flexibility designed into
instrument operation allows for observational 'work-arounds' to
achieve nearly all of the anticipated science goals. The obvious
approach of increasing the number of samples to improve the SNR (by
the square root of the increase factor) is the principal observation
strategy to achieve the radiometry science goals.
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