PJS@GROUCH.JPL.NASA.GOV (Peter Scott) (07/21/89)
Extracted without permission from NASA _Voyager Bulletin_, Mission Status
Report No. 88, July 12:
WHAT DO THE HIEROGLYPHICS MEAN?
Images and some of the other data received at JPL from the two Voyager
spacecraft are sent to display devices at JPL from the computers in JPL's
Multimission Image Processing System. A subset of these data, primarily
images, is being broadcast on the NASA Select TV channel by the GE Satcom F2R
satellite. Currently there is a one-hour broadcast every Tuesday through
August 8. Broadcasts will probably be more frequent as Voyager 2 nears its
closest approach to Neptune on August 25.
The frames that appear on the monitors are black and white only and include
basic identification information for the image being displayed, as well as
information about the processing that has been applied to that image. Data from
the Planetary Radio Astronomy and Plasma Wave investigations can also be
displayed on video devices. The following is a brief summary of the information
shown for imaging, PRA, and PWS frames.
IMAGING FRAMES
Each Voyager spacecraft carries two imaging cameras: a 200-mm, f/3.5 wide-angle
camera using a refracting telescope and a 1500-mm f/8.5 narrow-angle
(telephoto) camera using a reflecting telescope. Each camera uses a one-inch
selenium-sulfur vidicon to convert an optical signal into electrical signals.
Each frame consists of 640,000 pixels, each of which is expressed as a level of
gray on a scale from 0 (black) to 255 (white). Color scenes are reconstructed
on Earth by electronically combining images taken through different filters.
The sensitivity of the filters ranges from 3460 (ultraviolet) to 6184 angstroms
(red-orange).
[The rest of this extract contains mostly information of interest to those
people receiving the images.]
The elements of the displayed imaging frames are described below:
[1-5 run left-to-right across the top line of the frame]
1. MIPL Multimission Image Processing System (a JPL facility).
2. Spacecraft identifier
3. System Source: RLTM Real Time. The data is displayed from telemetry as
soon as the data arrives at JPL; this data may be only 4 hours old (as long as
it took for the data to travel from the spacecraft to Earth); it may have been
taken earlier, recorded on the spacecraft's digital tape recorder, and played
back to Earth at an optimum time; or it may have been stored at the Data
Capture and Staging computer and read from there later. (See item 13, to learn
when the data was received at Earth.)
INTR Interactive. Image is displayed from a work station
where a scientist or analyst is interactively enhancing the data.
RPLA Replay, for NASA Select TV, of data received earlier.
4. Target Body (Neptune, Triton, Nereid, rings, etc).
5. FDS Count. Image identifiction in units of spacecraft clock time (the
spacecraft's clock is in the spacecraft's Flight Data Subsystem computers).
6. Frame identification in units of picture number, planet, spacecraft,
and days from encounter [displayed under #5].
7. Frame. The data are displayed at half resolution. As displayed, the
image area is 400 lines by 400 rows [sic] of picture elements. A full Voyager
image frame is 800 lines of 800 elements, but since the display devices
can display only 640x480 pixels, every other pixel of every other line is
displayed.
8. Reference Gray Scale [along left edge]
9. Histogram (frequency distribution) of number of bits at each of the
gray levels in the incoming image [top right].
10. Histogram of number of bits at each of the gray levels in the image
as displayed at left. [Center right]. (Note: the histograms are an aid
in evaluating the quality of the displayed image; e.g., how bright was the
image, how good was the exposure, etc).
11. [Text under #10] Describes processing that has been done to transform
the input image to the output image, e.g., magnification factor, dark current
subtraction, contrast enhancement.
12. [Text, bottom left to center] Identification information:
INSTR Instrument: ISSWA Imaging camera, Wide angle
ISSNA Imaging camera, Narrow angle
FILTER Clear, green, orange, blue, ultraviolet, methane (6910 or
5410 angstroms), or sodium.
EXP Exposure time (seconds)
IMODE Shutter mode:
NAONLY Narrow angle only
WAONLY Wide angle only
BOTALT Wide and narrow angle shuttered alternately
BOTSIM Both wide and narrow shuttered simultaneously
BODARK Neither wide nor narrow shuttered (used to calibrate
background noise (or "dark current"))
NOSHUT Neithr wide nor narrow read out
TMODE Spacecraft telemetry mode at which data is read out (e.g.
compressed, edited, slow scan, etc).
RANGE Distance from spacecraft to center of target body, in km.
INA Lighting angle (incidence angle of sunlight striking target)
EMA Viewing angle (emission angle of sunlight reflected from target)
PHA Phase angle (angle between incoming sunlight and emitted or
reflected light from target)
SCALE Distance across frame (at the target body)
13. [Bottom right] Time signal was received at Earth (day of year:hour:minute)
in Universal Time Coordinated (UTC).
14. [Right of #13] Calendar day (JPL local time), MIPS computer ID, display
device ID.
PLANETARY RADIO ASTRONOMY (PRA)
The planetary radio astronomy experiment uses two 10-meter whip antennas to
listen for radio emissions from the Sun, planets, and lightning in planetary
atmospheres over a range from 1.2kHz to 40.5MHz. The PRA high-rate receiver
gives high time resolution at two fixed frequencies that will be selected by
the PRA science team when the spacecraft is a few weeks away from the planet.
At Uranus the frequencies were 35.9424MHz and 1.230MHz.
Data from the PRA is sometimes displayed on the video devices. The following
discussion indicates information that differs from the Imaging frames:
7. Frame. A high-rate PRA frame contains 48 seconds of data formatted as 800
lines, each containing 800 8-bit samples. The total time per line is 0.06
second. Only 1/4 of the PRA data is displayed in the video convrsion of the
frame. The first 24 seconds of the displayed frame represents signals at the
first fixed frequency, at two antenna polarizations. The last 24 seconds show
only one polarization at the second fixed frequency. "Real" signals show as
horizontal white (or light) streaks running across the frame [an example is
pictured showing lightning-like electrical discharges at Uranus].
The pattern observed in each line represents the "loudness" as a function of
time, of an "audio" noise signal in a narrow band centered on the receiver
frequency. Real data increases the "loudness" of the signal, just like extra
static in a home radio. The amount and type of noise gives information about
what causes it.
9. Histogram of signal intensity as received (input).
10. Histogram of signal intensity values as displayed (output).
12. Only INSTRU and TMODE have any meaning for PRA and PWS frames.
PLASMA WAVES (PWS)
Plasma waves are low-frequency oscillations in the plasmas in interplanetary
space and in planetary magnetospheres. The plasma wave instrument detects and
measures plasma wave interactions in planetary magnetospheres and detects
interactions between a planetary magnetosphere and the solar wind. It can
detect particles in the ring plane and measure their impact rate on the
spacecraft. The PWS shares the two whip antennas with the PRA investigation to
provide the equivalent of a single 7-meter antenna. PWS covers the frequency
range from 10Hz to 56.2kHz.
The PWS high-rate receiver is a very sensitive audio amplifier. In the display
of a Fourier transform of the data, the horizontal axis is time (one 48-second
frame) and the vertical axis is frequency. Plasma wave scientists interpret
the signal patterns in terms of various mechanisms the interactions of charged
particles and elctromagnetic waves. Steady frequencies generally are associated
with interference signals from other subsystems on the spacecraft; for example,
the 2.4kHz hum of the spacecraft's power supply [shows up as a horizontal line
near the bottom of the sample frame].
Peter Scott (pjs@grouch.jpl.nasa.gov) Standard disclaimers.