INSTRUMENT: PHOTOPOLARIMETER SUBSYSTEM
SPACECRAFT: VOYAGER 2


Instrument Overview
===================
The Voyager Photopolarimeter Experiment (PPS) utilizes a general purpose filter photometer/polarimeter optimized for the encounterphase 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

Scientific Objectives
=====================
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, andrings.


Atmospheric Objectives----------------------
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 satellites were:
(1) to measure or set upper limits on the density of their atmospheres;
(2) to determine the texture and probable composition of their surfaces;
(3) to determine the bond albedo.

(4) to map the distribution of sodium vapor in the vicinity ofIo and in Jupiter's magnetosphere.


Planetary Ring Objectives-------------------------
Specific scientific objectives for the study of planetary rings were:
(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 distribution;
(2) to use observations of the extinction and scattering of starlight to give additional information on particle size andring 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 rotation;
(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(S-20) photocathode.

Individual photon events in the PMT are detected with pulsecounting electronics.

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 ofreducing 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 ofthe Sun.


Filters
=======
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 (Angstrom) (Angstrom)------------------------------------------------------------
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, Chromophore 5 2350 300 20 Si I, Rayleigh scattering 6 7500 300 8 K I, Aerosol scattering 7 7270 100 4 CH4 absorption band
*For a point source in counts accumulated during an 0.4 secondintegration per incident photon /cm^2/s/Angstrom.


Operational Modes
=================
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 eightfilters.

NOTE: Equipment failures and improved understanding of instrument usage considerations, however, caused substantial changes in this plan. See the section on operational considerations below forfurther details.

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 inEarth-based observations.

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 resolutionand the FOV set to 1 degree.


Measured Parameters
===================
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 beconverted 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 thedegree 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 aredetermined.

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 40bps, with a maximum of 1023 1/2 bps and a minimum of 0.6 bps.

NOTE: Data obtained during stellar occultation observations werenot subjected to compression.


Calibration
===========
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 comparisonpre-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 andUV to ~20 counts/Rayleigh in the infrared.

Further discussions of intensity and polarization measurements can be found in, for example, [WESTETAL1981], [WESTETAL1983], and[PRYOR&HORD1991].


Operational Considerations
==========================
The PPS aboard Voyager 1 suffered extreme sensitivity loss before and during Jupiter encounter. This was deemed to be irreparableand 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 PPSinternal 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 scatteredlight.

Measurement sequences could be modified by changing the PPSlook-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 properanalysis.