The primary scientific objectives of the photopolarimeter investigation on the Voyager mission were divided into three categories:
(1) the study of atmospheric properties,
(2) the study of surface properties of satellites where little or no atmosphere exists
(3) the study of planetary ring systems.
Additionally, the search for dark side auroral emissions provided both magnetic field and charged particle information, as well as atmospheric data.
Specific scientific objectives associated with the atmospheres of Jupiter, Saturn, Uranus, Neptune, Titan, and Triton were:
(1) to use intensity and polarization measurements as a function of viewing angle, wavelength, and latitude to determine the vertical distribution of cloud particles (atmospheric aerosols) down to an optical depth of unity, as well as information on particle size, shape, and composition
(2) to use observations of the extinction and scattering of starlight to determine atmospheric optical depth as a function of altitude.
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 surface texture and probable composition
(3) to determine the bond albedo.
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 and ring optical depth.
The Voyager Photopolarimeter Experiment (PPS) utilized a general purpose filter photometer/polarimeter optimized for the encounter phase of the mission. The instrument consisted of a 6-inch f/1.4 Dahl-Kirkham type Cassegrain telescope with a four-position aperture wheel providing Gaussian fields of view (FOV) with full widths at half-maximum (FWHM) of 0.12, 0.25, 0.8, and 3.5 degrees. Behind this were mounted an eight position analyzer wheel with five positions for polacoat analyzers, the transmitting axes of which were oriented at 0, 60, 120, 45, and 135 degrees, and three positions for open, dark, and calibration, an eight position filter wheel with thin film interference filters, and an EMR 510-E-06 photomultiplier tube (PMT) with a tri-alkali (S-20) photocathode. Individual photon events in the PMT were detected with pulse counting electronics (LILLIE_ETAL_1977).
The wide dynamical range required by the mission (4 to 6.e11 photons /cm^2/s/Angstrom/ster) could thus be accommodated by FOV changes and gain change in the PMT capable of reducing the instrumental sensitivity by a factor of approximately 50.
The PPS massed a total of 2.55 kg, including 0.78 kg of tungsten and aluminum shielding to protect critical components from radiation trapped in the Jovian magnetosphere. A shadow caster prevented direct sunlight from entering the aperture of the instrument for phase angles less than 160 degrees. A solar sensor was provided to turn off the high voltage if the instrument was pointed within 20 degrees of the Sun.
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.
Position Effective Half-power Nominal Spectral number wavelength bandwidth sensitivity* features (Angstroms) (Angstroms) 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 2650 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
* For a point source in counts accumulated during an 0.4 second integration per incident photon /cm^2/s/Angstrom.
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 filters. Equipment failures and improved understanding of instrument usage considerations, however, caused substantial changes in this plan. See the section on operational considerations for further 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 in Earth-based observations. Unlike other observational modes, data obtained during this mode were not subjected to compression.
For ring observations, stellar occultations (delta Scorpio at Saturn, beta Perseus at Uranus, sigma Sagittarius at Saturn, Uranus and Neptune) were observed using filter #4 (2650 Angstroms) and polarizer #7 (45 degrees). A 10 ms integration time was used to get maximum time resolution and the FOV set to 0.8 degrees.
All the above measurement sequences could be modified during flight by changing the PPS look-up-tables (LUT) in the spacecraft's Flight Data System (FDS). These controlled the filter and analyzer wheel positioning and changed according to mission phase and FDS count. The variations 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 analysis.
Inflight calibration of the experiment was accomplished by observing a set of standard stars of known brightness and polarization, the sunlight scattered by an on-board calibration target, and the light from stars and the planets reflected into the PPS from a mirror tilted to the Brewster angle. 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 was 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 1.0 %. Absolute calibration was 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 about 140 counts/Rayleigh in the visible and UV to about 20 counts/Rayleigh in the infrared (LILLIE_ETAL_1977).
Distance from the target body was determined from other experiments on board. Further discussions of intensity and polarization measurements can be found in, for example, WEST_ETAL_1981, WEST_ETAL_1983, PRYOR_AND_HORD_1991.
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 nor 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 utilised. 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 light.
Measurement sequences were 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 variations were predominantly a result of sequencing considerations, i.e. where slews ended and images were shuttered, as well as the above-mentioned 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 analysis.
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."
West, R. A., C. W. Hord, K. E. Simmons, D. L. Coffeen, M. Sato, and A. L. Lane, 1981. Near-ultraviolet scattering properties of Jupiter. J. Geophys. Res. 86, 8783-8792.
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, 1983. Photometry and polarimetry of Saturn at 2640 and 7500 Angstroms. J. Geophys. Res. 88, 8679-8697.
Pryor, W. R., and C. W. Hord, 1991. A study of photopolarimeter system UV absorption data on Jupiter, Saturn, Uranus, and Neptune: implications for auroral haze formation. Icarus 91, 161-172.