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This data set contains the RDR data for the Galileo Orbiter
PPR instrument for the period corresponding to the initial
turn-on and checkout of the PPR in December 1989.
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 cycle consisting
of 91 minor frames.
The PPR generates 18 bytes of instrument data for each 2/3-sec
interval corresponding to one minor frame count. 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
R_EDR archive are a reformatted version of the PPR EDR data and
tabulate all of this data in an ASCII Table format. The present
Reduced DR (RDR) archive is also an ASCII Table format with the same
number of records (rows) as the R_EDR files for the respective data
sets, but with science data number values converted to reduced thermal
radiometry brightness temperatures and polarimetry-photometry
radiances and linear polarization degree and direction as appropriate.
The detached PDS label for each file specifies the record format and
describes in detail each parameter of the record.
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 45- 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 ~15 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 1 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 Ym 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 Ym 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 Ym 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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