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PPR - Photopolarimeter-Radiometer

MISSION OBJECTIVES

• Determine the vertical and horizontal distribution of cloud and haze particles in the Jovian atmosphere, including their size, shape, and refractive index.

• Determine the energy budget of Jupiter and the variation in amount and spatial distribution of reflected solar radiation and emitted thermal radiation for Jupiter and its satellites, including the thermal structure of the Jovian atmosphere.

• Measure and map the photometric, polarimetric, and radiometric properties of the Galilean satellites and other targets of opportunity.

SUMMARY

The polarimeter detects three spectral bands. Polarization is an important property of light (a fact known to the wearers of some types of sunglasses) and can reveal information about the nature of the object from which the light comes.

The photometer uses seven narrow spectral bands in the visible and near-infrared wavelengths. The bands in which to make these measurements have been carefully selected. For example, locations are covered where methane and ammonia strongly absorb light.

The PPR has seven radiometry bands. One of these uses no filters and observes all the radiation, both solar and thermal. Another band lets only solar radiation through. The difference between the solar- plus-thermal and the solar-only channels gives the total thermal radiation emitted. The PPR will also measure in five broadband channels that span the spectral range from 17 to 110 micrometers. The radiometer provides data on the temperatures of the Jovian satellites and Jupiter's atmosphere.

The design of the instrument is based on that of an instrument flown on the Pioneer Venus spacecraft. A 10-centimeter-aperture reflecting telescope collects light, directs it to a series of filters, and, from there, measurements are performed by the detectors of the PPR.

The PPR weighs 5.0 kilograms and consumes about 5 watts of power.

PPR DESCRIPTION

• The PPR is a Cassegrainian Dall-Kirkham telescope with a 10-cm aperture, a 50-cm focal length, and 2.5-mrad instantaneous field of view.

• A view of space is provided by reflection from a chopper mirror and a planar telescope mirror for radiometric measurements.

• The PPR main housing is mounted on the top of the electronics, which consists of a set of four stacked modules, which, in turn, are mounted onto the base plate. The main housing supports the scene-view and space-view telescopes and their ancillary optical elements.

DESIGN DETAILS

• The PPR makes polarimetry and photometry measurements by passing the flux collected by the scene-view and space-view telescopes through the optical elements located on the filter/retarder wheel, the Wollastom prism, and the chopper to two lenses that focus it on the two silicon photodiodes.

• For polarimetry and photometry measurements the chopper is stationary and blocks flux from the space-view telescope. For radiometry measurements, the chopper is operated so as to alternately direct the flux (at 30 Hz) from the scene-view and space-view telescopes through the field stop on to a lithium tantalate pyroelectric detector.

• Photopolarimetry is done at 410, 678, and 945 nm. Photometry is done at 619, 633, 648, 789, 829, 840, and 892 nm. Radiometry is done at < 4 u m (Solar), 17, 21, 27, 36, and > 42 u m.

INSTRUMENT PARAMETERS

• Instrument Mass: 5.2 Kg
• Power Consumption:
  • Instrument - 11.0 W
  • Heaters - 4.5 W
• Microprocessor: 1802 type
• ROM/RAM: 4 Kb/256 b
• Fields of View: 2.5 mrad circular
• Size: 44.8 x 39.1 x 32.7 cm
• Thermal Range:
  • Operating (GLL 3-210)
    • Electronics: -20 to 5 Deg. C
  • Non-operating (GLL3-210)
    • Electronics: -25 to 30 Deg. C
• Instrument Modes:
  • Off
  • POR
  • On
  • PP/PH
  • PH
  • RAD