| 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.
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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