INSTRUMENT: PHOTOPOLARIMETER SUBSYSTEM
SPACECRAFT: VOYAGER 2
The Voyager Photopolarimeter Experiment (PPS) utilizes a general
purpose filter photometer/polarimeter optimized for the encounter
phase of the mission.
Instrument Id : PPS
Instrument Host Id : VG2
Pi Pds User Id : ALLANE
Instrument Name : PHOTOPOLARIMETER SUBSYSTEM
Instrument Type : PHOTOPOLARIMETER
Build Date : NULL
Instrument Mass : 2.55
Instrument Length : NULL
Instrument Width : NULL
Instrument Height : NULL
Instrument Serial Number : NULL
Instrument Manufacturer Name : NULL
From [LILLIEETAL1977], pp. 159-160.
The primary scientific objectives of the photopolarimeter
investigation on the Voyager mission were divided into three
categories---studies of atmospheres, satellite surfaces, and
Specific objectives associated with the atmospheres were:
(1) to use intensity and polarization measurements as a
function of viewing angle and wavelength to determine the
macrostructure (vertical distribution of atmospheric aerosols)
and microstructure (particle size, shape, and probable
composition) of atmospheres;
(2) to use polarization measurements at large phase angles to
constrain the particle shapes and compositions within clouds;
(3) to search for dark side auroral emissions.
Satellite Surface Objectives
Specific scientific objectives associated with the surfaces of
(1) to measure or set upper limits on the density of their
(2) to determine the texture and probable composition of their
(3) to determine the bond albedo.
(4) to map the distribution of sodium vapor in the vicinity of
Io and in Jupiter's magnetosphere.
Planetary Ring Objectives
Specific scientific objectives for the study of planetary rings
(1) to use intensity and polarization measurements of scattered
light as a function of wavelength and viewing angle to provide
information on the size, shape, and probable composition of the
ring particles, as well as their density and radial
(2) to use observations of the extinction and scattering of
starlight to give additional information on particle size and
ring optical depth.
Optics and Detectors
From [LILLIEETAL1977], pp. 160-161.
The instrument consists of the following components:
(1) a 6-inch f/1.4 Dahl-Kirkham type Cassegrain telescope;
(2) a four-position aperture wheel providing circular fields of
view (FOVs) with diameters of 1/16, 1/4, 1, and 3.5 degrees;
(3) an eight-position analyzer wheel with open, dark and
calibration positions, plus with five polacoat analyzers with
transmitting axes oriented at 0, 60, 120, 45, and 135 degrees
(4) an eight-position filter wheel holding thin interference
filters (see below);
(5) an EMR 510-E-06 photomultiplier tube (PMT) with a tri-alkali
Individual photon events in the PMT are detected with pulse
The wide dynamical range required by the mission (4 to 6.e11
photons /cm^2/s/Angstrom/ster) could be accommodated by FOV
changes and an electronic gain change in the PMT capable of
reducing the instrumental sensitivity by a factor of ~50.
A shadow caster prevents direct sunlight from entering the
aperture for phase angles < 160 degrees. A solar sensor turns
off the high voltage if the PPS is pointed within 20 degrees of
The effective wavelength of each filter, its nominal band width,
typical instrumental sensitivities, and the particular atomic and
molecular species to which it is sensitive are listed below (From
[LILLIEETAL1977], Table I, p. 164).
Position Effective Half-power Nominal Spectral
number wavelength bandwidth sensitivity* features
0 5900 100 30 Sodium D
1 4900 100 50 H beta
2 3900 100 45 He I, Ca II
3 3100 300 40 OH Emission
4 2630 300 25 O3, Mg II,
5 2350 300 20 Si I, Rayleigh
6 7500 300 8 K I, Aerosol
7 7270 100 4 CH4 absorption
*For a point source in counts accumulated during an 0.4 second
integration per incident photon /cm^2/s/Angstrom.
From [LILLIEETAL1977], pp. 161-162.
The planned normal operational (encounter) mode of the PPS was to
step through a programmed sequence of 40 filter/analyzer wheel
combinations once every 24 sec. Each measurement was to consist
of a 400 millisecond integration period followed by a 200 ms
period during which the next filter or analyzer would be stepped
into place. A full measurement set would thus consist of
readings in the open, 0 degrees, 60 degrees, 120 degrees, and
dark positions of the analyzer wheel for each of the eight
NOTE: Equipment failures and improved understanding of instrument
usage considerations, however, caused substantial changes in this
plan. See the section on operational considerations below for
For stellar occultation and satellite eclipse measurements, the
experiment was operated with filters and analyzers fixed in
position and a 10-ms integration period. This provided rapid
measurements in order to resolve spikes in the light curves due
to turbulence in the occulting planet's atmosphere, as seen in
For ring observations, stellar occultations were observed using
filter #4 (2650 Angstroms) and polarizer #7 (45 degrees). A
10-ms integration time was used to obtain maximum time resolution
and the FOV set to 1 degree.
From [LILLIEETAL1977], pp. 164-165.
PPS raw values represent the number of photons events in the PMT
counted by the detector during the given integration time. Based
on FOV, filter, and gain settings, this count could then be
converted to an intensity.
Four Stokes' parameters, I, Q, U, and V, completely specify the
state of polarization of a quasi-monochromatic wave. The
advantages of measuring and using Stokes' parameters are:
(1) They all have the same dimension of intensity;
(2) They are additive;
(3) From the Stokes' parameters it is possible to generate the
degree and plane of polarization and ellipticity.
Since the value of V is generally small and its measurement
requires a much more complex instrument, only I, Q, and U are
Total intensity I can be measured either with no polarizer in the
optical train, or by summing polarizers with transmission axes at
0, 60 and 120 degrees:
I = 2[I(0) + I(60) + I(120)] / 3 .
The Stokes' parameter Q is given by
Q = 2[2I(0) - I(60) - I(120)] / 3 .
Similarly, U can be determined from
U = 2[I(60) - I(120)] / sqrt(3) .
The PPS data readout consists of a 30-bit digital word, of which
20 bits provided the data count accumulated during the
integration period, and 10 bits indicated instrument status. In
order to reduce the telemetry rate, data count bits were log
compressed in the spacecraft FDS to 14 bits (a 10 bit mantissa
and 4 bit exponent). Log compression was removed from the FDS
for the PPS occultation modes. The nominal data rate was thus 40
bps, with a maximum of 1023 1/2 bps and a minimum of 0.6 bps.
NOTE: Data obtained during stellar occultation observations were
not subjected to compression.
From [LILLIEETAL1977], p. 161.
In-flight calibration was accomplished by observing a set of
standard stars of known brightness and polarization, the sunlight
scattered by an on-board calibration target (unpolarized light),
and the light from stars and the planets reflected into the PPS
from a mirror tilted to the Brewster angle (yielding 100%
polarized light). An internal Cerenkov radiation source mounted
on the analyzer wheel provided a short term measure of the
instrument's stability but was not used because comparison
pre-flight calibration data was lost.
The instrument is capable of measuring the polarization of
reflected light from the planets and their satellites with a
precision of +/- 0.5%, and their relative brightness with an
accuracy of +/- 0.5 to 1%. Absolute calibration is known to +/-
3% in the visible and infrared, and to +/- 10% in the UV. For
measurements of low surface brightnesses the instrument's
sensitivity ranged from ~140 counts/Rayleigh in the visible and
UV to ~20 counts/Rayleigh in the infrared.
Further discussions of intensity and polarization measurements
can be found in, for example, [WESTETAL1981], [WESTETAL1983], and
The PPS aboard Voyager 1 suffered extreme sensitivity loss before
and during Jupiter encounter. This was deemed to be irreparable
and the instrument was turned off before Saturn encounter.
Voyager 1 data were never analyzed or archived.
The PPS instrument on board Voyager 2 suffered two hardware
failures that affected the ability to access wheel positions. A
spacecraft decoder failure affected the analyzer and a PPS
internal chip failure affected the available filter positions.
At Jupiter, filter positions 0, 2, 4, and 6 were used.
Afterwards, only three positions, 2, 4, and 6, were used. Four
of the eight analyzer wheel positions were available. Of these,
135 and 45 degree orientations at wheel positions 6 and 7 were
used to acquire polarization information. Before closest
approach at Jupiter, data taken are unreliable due to scattered
Measurement sequences could be modified by changing the PPS
look-up-table (LUT) in the spacecraft's Flight Data System (FDS).
This controlled the filter and analyzer wheel positioning. The
changes were predominantly a result of instrument electronic
failures and PMT usage issues more fully understood as the
mission progressed. Knowledge of which FDS load was in effect
during each data observation is therefore necessary for proper
Lillie, C.F., C.W. Hord, K. Pang, D.L. Coffeen, and J.E. Hansen, TheVoyager mission photopolarimeter experiment, Space Sci. Rev., 21, 159-181,1977.
Pryor, W.R., and C.W. Hord, A study of photopolarimeter system UVabsorption data on Jupiter, Saturn, Uranus, and Neptune: implications forauroral haze formation, Icarus, 91, 161-172, 1991.
West, R.A., C.W. Hord, K.E. Simmons, D.L. Coffeen, M. Sato, and A.L. Lane,Near-ultraviolet scattering properties of Jupiter, J. Geophys. Res., 86,8783-8792, 1981.
West, R.A., M. Sato, H. Hart, A.L. Lane, C.W. Hord, K.E. Simmons, L.W.Esposito, D.L. Coffeen, and R.B. Pomphrey, Photometry and polarimetry ofSaturn at 2640 and 7500 Angstroms, J. Geophys. Res., 88, 8679-8697, 1983.
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