Multimedia Mania/abacus-multimedia-mania.iso/dp/0117/01174.txt

Text File | 1993-07-27 | 6KB | 107 lines

$Unique_ID{bob01174}
$Pretitle{}
$Title{Pioneer
Appendix: The Imaging Photopolarimeter}
$Subtitle{}
$Author{Fimmel, Richard O.;Allen, James Van;Burgess, Eric}
$Affiliation{Ames Research Center;University Of Iowa;Science Writer}
$Subject{data
spacecraft
instrument
ipp
saturn
imaging
mode
rate
jupiter
scan}
$Date{1980}
$Log{}
Title:       Pioneer
Book:        Pioneer: First To Jupiter, Saturn, And Beyond
Author:      Fimmel, Richard O.;Allen, James Van;Burgess, Eric
Affiliation: Ames Research Center;University Of Iowa;Science Writer
Date:        1980

Appendix: The Imaging Photopolarimeter

     The entire complex of scientific instruments carried onboard each Pioneer
spacecraft was needed to provide the first close-up investigations of Jupiter
and Saturn and their environments.  The imaging photopolarimeter (IPP)
returned the data from which the colored images of the cloud covers of Jupiter
and Saturn and of the magnificent rings of Saturn were constructed to reveal
details never before seen.  This appendix provides further technical details
of this instrument.

     The IPP consisted basically of a positionable optics-detector assembly
and an electronic equipment housing, supported on a central mounting frame.
Special optical materials were selected to retain their transparency even
after being subjected to radiation effects from trapped energetic protons and
electrons in the radiation belts of Jupiter and Saturn.

     The optical system consisted of a 2.54-cm (1-in.) diameter Maksutov-type
telescope, a calcite Wollaston prism polarization analyzer, multilayer filters
to separate red and blue components of the reflected light from Jupiter and
Saturn, relay optics, and two dual-channel multiplier detectors, each designed
to sense two polarization components in one of two colors (a total of four
channels).  The field of view of the instrument could be varied by use of
three apertures on a carrier that also carried polarization processing
elements (depolarizer and half-wave retardation plate) and an internal
calibration lamp.

     Analog signals from the detectors were digitized, buffered in the
spacecraft's data storage unit, and transmitted together with instrument
status information in either of two telemetry formats.

     After a command into the data-taking mode had been received, the
electronic logic processor automatically provided all internal commands
required to sequence a complete measurement operation with the IPP and then
return the instrument to standby.  Additional commands were available to
adjust power supply voltages, thereby controlling the gain of the instrument,
to alter sampling rates, to inhibit functions, to change the direction in
which the telescope stepped, and so on.

     The field of view of 0.028 square was employed in the imaging mode of the
IPP.  This field of view was moved slightly in steps every roll of the
spacecraft, unless inhibited by command.  Step direction could be selected by
command.  A six-bit telemetry format provided 64 shades of gray for imaging.
In that mode only two of the four detector channels were used, and the light
was depolarized before detection.  Sampling took place on the dark sky, and
the resultant output was used to compensate for zero-level shifts and
background caused by the radiation belt environment.  Detector output was
measured, alternating colors, each 0.0150 of spacecraft roll, or 0.03 if the
low sampling rate had been commanded.  The spacecraft buffer stored imaging
data collected over 140 of each roll or 280 in the low sampling rate.

     Scan lines, analogous to the horizontal lines on a television screen,
were produced by the instrument looking in a fixed direction with respect to
the spacecraft as the spaceecraft spun on its axis.  The start of each scan
was controlled by a series of "spoke" commands that related the start to the
spin position.  An alternate mode, the "start data at threshold" mode, allowed
the scan to be started automatically by the telescope receiving light from the
limb of the planet.

     The equivalent of television vertical scanning was achieved either by
mechanically stepping the instrument's telescope 0.5 mrad with respect to the
spin axis between each rotation of the spacecraft or, during closest approach
to a planet, by holding the telescope fixed and letting the relative motion of
the spacecraft and the planet produce the scanning steps.  However, if the
latter technique were used, during the close approach sequence, the scan lines
could overlap or have gaps between them, depending on whether the relative
motion of spacecraft and planet was too slow or too fast, respectively.

     In the imaging mode, the data were converted to 64 levels of intensities
and stored in a 6144-bit buffer onboard the spacecraft.  The instrument
overwrote the buffer as it started each "vertical" scan with each rotation of
the spacecraft.  The memory read-in time was approximately 0.5 sec and the
rotation period of Pioneer 10 was approximately 12.5 sec, which meant that
approximately 12 sec were available for reading out the data from the memory.
To read out the 6144 bits in the 12 sec available required a data rate of 512
bits/sec.  The IPP instrument received 50% service rate on the spacecraft's
telemetry downlink.  Thus a 1024-bit/sec telemetry downlink to Earth was the
minimum data rate at which all the IPP data taken could be returned to Earth.
During the encounter with Saturn, when the data rate had to be reduced because
of interference from the Sun, the area of coverage, but not the resolution, of
some of the IPP images had to be restricted so that the data could be returned
at the low bit rates permissible.