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In aerial photography, comparing different colors can highlight ground features that cannot be seen otherwise. Similarly, the Galileo Orbiter has an instrument that views an assortment of spectral bands. The Photopolarimeter-Radiometer (PPR) is in many respects three instruments combined into one: a photometer, a polarimeter, and a radiometer. Combining three major functions into one instrument makes a flexible and powerful experiment, but it required some compromises and a great deal of clever design.
The PPR will determine the amounts of radiation at Jupiter and its moons, provide atmospheric temperature profiles in the topmost (smog-like) layer and in the stratosphere just below, and help us understand cloud and haze properties and structures.
At Jupiter, the Probe will measure the atmospheric conditions in its path. PPR data for each area will then be compared with remotely sensed data for the entire planet.
Like Galileo's Ultraviolet Spectrometer and Near Infrared Mapping Spectrometer, the PPR is aligned with the imaging system. That way, data from all three instruments can be correlated for the object being viewed and can be used to investigate major elements of the Jovian atmosphere.
A detailed look at the tasks performed by each of the PPR's functions starts with the Photopolarimeter, whose design comes almost directly from the cloud photopolarimeter on the Pioneer Venus Orbiter.
Cloud particles can play a dominant role in determining the polarization of reflected sunlight. (Polarization is the suppression of the vibration of light waves in a certain direction.) For instance, water droplets in the Earth's atmosphere produce rainbows with a very strong signature in the polarization as a function of the phase angle (the angle between the Sun, the water droplet/scatterer, and the observer).
Scattering due to the molecules of an atmosphere (Rayleigh scattering) produces a very distinctive polarization signature and is most prominent at shorter wavelengths. (It is this wavelength dependence that makes the sky appear blue on Earth.) An examination of the relative contribution of Rayleigh scattering provides an estimate of how much gas is above the cloud tops. This "optical barometer" technique helps to see variations in the height of the cloud tops associated with major Jovian cloud features. The photometer investigates at seven narrow bands in the visible and near infrared wavelengths. These channels correspond to the positions of several absorption bands that are due to atmospheric methane and ammonia. The behavior of the intensities of these bands across Jupiter will help deduce the vertical structure of clouds and haze.
The radiometer's infrared wavelengths overlap some of those used by the Probe's radiometer. Because they correspond to regions (mainly hydrogen) with different atmospheric absorption, the radiometer can see to different depths and measure each region's brightness temperature.
The PPR has two additional radiometry bands. One of these uses no filter and, thus, observes all radiation from the visible through thermal and solar infrared wavelengths; the other lets only solar radiation come through . The difference between the solar-plus-thermal and the solar-only channels gives the total thermal radiation emitted.
For viewing the moons of Jupiter, this radiometer is the only source of data on direct "temperatures" of the surface. It is expected to be able to make such measurements for many regions, including the interesting "hot spots" near the volcanoes of Io.
A 10-centimeter (4-inch) diameter telescope collects the radiation for all three functions of the PPR. Light from the telescope is focused through one point, giving all three functions exactly the same field of view. The light then strikes a filter/retarder wheel, which can step through 32 positions.
A single polarimetry observation requires three wheel positions, each with a filter and a half-wave retarder. After passing through these, the light enters a prism, separating it into beams of vertically and horizontally polarized components. These beams are directed to a pair of photodiode detectors. (These detectors measure the light by converting it into electricity.) Thus, the polarization of the incoming light is determined by rotating the beam itself using the retarder.
Photometry, which simply measures the intensity of the incoming light, requires only one position on the wheel with an appropriate filter. In principle, this filtered beam could be immediately directed to a detector to measure its intensity. The PPR, instead, passes the beam through a prism and to the silicon detectors used for polarimetry, thus avoiding the added complexity of an additional optical path and detector. The photometry intensity is then just the sum of the intensities measured by the two detectors.
Another advantage of this design is that some information on the polarization of the observed light is available if the polarization direction is known or inferred from other measurements.
For radiometry (measuring thermal infrared radiation), a separate telescope allows radiation to come in from space (corresponding to a 3-K (-454 deg F) blackbody) to provide a reference signal. This beam intersects the incoming light's path just ahead of the filter wheel. Filters are used to select the desired wavelength; then mirrors send the beam to the side. There, it strikes a conical mirror that focuses it onto a pyroelectric detector.
For radiometric calibration, there is a target on Galileo's sunshade, which can be viewed and its temperature monitored by means of platinum resistance thermometers connected to the PPR's electronics. Similarly, the photometer's response can be checked with an internal calibration lamp.
Trade-offs were crucial to the PPR's design. An actively cooled radiometry detector would have been more sensitive, but incompatible with the photometry and polarimetry requirements. Also, of course, one instrument splitting time among three functions has less time for any one of them. However, there is a tremendous advantage in having the functions and wavelengths sampled with exactly the same field of view. Despite all its capabilities, the PPR has a mass of only 5.0 kilograms and is less than half a meter on its longest axis. It uses a peak power of 11 watts and an average of 6 watts.
The PPR was designed and built at the Santa Barbara Research Center (SBRC) in California. The SBRC has supplied sensors for Landsats and weather satellites and has built radiometers for many deep-space missions.
J. E. Hansen, Principal Investigator for the PPR, is at the Goddard Institute for Space Studies in New York City, as are investigators M. D. Allison, A. D. Del Genio, A. A. Lacis, W. B. Rossow, and L. D. Travis. Other investigators include G. S. Orton and T. Martin at JPL, P. H. Stone at the Massachusetts Institute of Technology, Y. L. Yung at the California Institute of Technology, and D. Morrison at the University of Hawaii.
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