The emphasis of the Juno mission is on the interior, atmosphere, and
magnetosphere of Jupiter. The spacecraft has been highly optimized
for the operation of its seven science instruments, leading to a
solar-powered, sun-pointing, spinning design. Such a design presents
challenges for visible imaging. But it was understood that visible
imaging is an important component of public engagement for any
mission, so a visible camera, JunoCam, was included primarily for
education and public outreach (EPO), funded from the mission's EPO
budget and given a fairly constrained allocation of spacecraft mass
However, Juno's polar orbit offers a unique vantage point compared to
other missions that have orbited or flown by Jupiter. The orbit
allows observation of the poles at low emission angles and much closer
approaches to Jupiter near perijove -- about 5000 km above the
cloudtops, compared to the closest previous approach of about 71500 km
late in the Galileo mission.
After a series of engineering trade studies, the minimum requirements
defined for JunoCam were to image the entire pole of Jupiter when the
spacecraft is ~1 hr from perijove with 50 km/pixel spatial scale in
three colors. This leads to a field of view requirement of about 60
degrees. To minimize cost, the camera electronics design was based on
that developed earlier for the Mars Science Laboratory mission; the
flexibility inherent in that design allowed the addition of multiband
'pushframe' imaging and time-delayed integration (TDI) to achieve
adequate signal-to-noise ratio (SNR) despite spacecraft spin. A
fourth filter in a narrow methane absorption band allows the
instrument to address atmospheric science objectives related to the
vertical structure of Jupiter's cloudtops.
The radiation environment for Juno, while avoiding the worst areas of
the jovian radiation belts, is still many times harsher than that of
the MSL mission. This led to the addition of substantial extra
shielding mass to both the optics and electronics and some revision of
the selected parts. While JunoCam is only qualified to survive for
the first three months of the mission (through orbit 8), we expect the
degradation of the instrument to be graceful.
For further information about the JunoCam instrument, please see
Detailed instrument description
The JunoCam instrument consists of two subsystems: the camera head
(CH), which uses a build-to-print copy of the camera head electronics
developed for the Mars Science Laboratory (MSL) mission (MALINETAL2005)
with slightly modified logic and Juno-specific optics and housings, and
the Juno Digital Electronics Assembly (JDEA), which contains an image
buffer, power conversion circuitry, and the interface to the spacecraft.
The camera head is mounted on the spacecraft's upper deck, while the JDEA
is mounted to the side of the main avionics vault. The two subsystems
communicate via a spacecraft-provided wiring harness.
The CH electronics are designed around the Kodak KAI-2020
Charge-Coupled Device (CCD) image sensor. This sensor has 1640 X 1214
7.4-micron pixels (1600 X 1200 photoactive), and uses interline
transfer to implement electronic shuttering. The sensor incorporates
microlenses to improve its quantum efficiency, which peaks at about
55%. The 'fast-dump' capability of the sensor is used to clear
residual charge prior to integration and also allows vertical
subframing of the final image.
The output signal from the CCD is AC-coupled and then amplified. The
amplified signal is digitized to 12 bits at a maximum rate of 5
Mpixels/s. For each pixel, both reset and video levels are digitized
and then subtracted in the digital domain to perform correlated double
sampling (CDS), resulting in a typical 11 bits of dynamic range.
The CH electronics are laid out as a single rigid-flex printed circuit
board (PCB) with three rigid sections. The sections are sandwiched
between housing sections that provide mechanical support and radiation
shielding, and the flexible interconnects are enclosed in metal
covers. For JunoCam, additional radiation shielding was required and
was incorporated into the housings, which are made of titanium. An
additional copper-tungsten enclosure surrounds the image sensor. The
total mass of the CH is about 2.6 kg.
A color filter array (CFA) with four spectral bands is
bonded to the CCD. The four bands are red (600-800 nm), green
(500-600 nm), blue (420-520 nm), and methane absorption (880-900 nm).
The JunoCam filters were fabricated by Barr Associates.
The JunoCam optics is a 14-element all-refractive lens with a nominal
focal length of 11 mm and a field of view of about 58 degrees
(horizontal.) T/number varies somewhat across the field and with
wavelength, but the nominal on-axis T/number is 3.2. The first five
front elements are made of radiation-hard glasses to provide shielding
for the remaining elements, and the optics are additionally shielded by
a thick titanium housing. An alignment cube is mounted to the optics
to facilitate precision mounting on the spacectaft.
The JunoCam lens was fabricated by Rockwell-Collins Optronics.
The JDEA provides regulated power to the camera head, implements a
minimal command sequencing capability to manage camera head pushframe
operation, receives the raw digital image data from the camera head,
applies 12-to-8-bit non-linear companding, and stores the image data
in a 128 MB internal DRAM buffer. The CH command/data interface is a
three-signal LVDS synchronous serial link transmitting commands from
the JDEA to the CH at 2 Mbit/s and a four-signal synchronous 3-bit
parallel interface from the CH to the JDEA at a rate of 30 Mbit/sec.
The JDEA also contains a command/data interface with the spacecraft,
receiving higher-level imaging commands and returning image data. The
command interface is a bidirectional asynchronous RS-422 interface
running at 57.6Kbaud; the data interface is a unidirectional three-
signal RS-422 synchronous interface running at 20 Mbits/sec.
The JDEA electronics are laid out as a single rectangular PCB,
sandwiched between housing sections that provide mechanical support
and radiation shielding. The JDEA housings are aluminum, since
considerable radiation shielding is provided by the avionics vault.
The JDEA mass is about 1 kg.
There is no software resident in the instrument. All additional
processing is performed by software provided by JunoCam and running in
the spacecraft computer. This software has significant commonality
with that previously developed by MSSS for the Mars Odyssey and MRO
missions. It is written in ANSI C and uses the VxWorks multitasking
facility so that processing can occur when the spacecraft computer is
The software receives commands to acquire images from the spacecraft's
command sequence engine. Each image command contains parameters such
as exposure time, number of TDI stages, number of frames, interframe
time, summing, and compression. Optionally, each image can be
commanded relative to the spin phase (based on information provided by
the spacecraft's attitude control system) so that only frames which
are pointed at the planet need be acquired. The software instructs
the JDEA to begin imaging at the appropriate time and then delays
until the entire multi-frame image is acquired. It then reads out the
JDEA DRAM. The raw image data are stored in spacecraft DRAM and then
read out, processed, and formatted for downlinking. Processing
consists of frame editing, optional summing, optional median filtering
to remove radiation-induced pixel transients, and optional lossy
transform-based or lossless predictive image compression.
Like previous MSSS cameras (e.g., Mars Reconnaissance Orbiter Mars Color
Imager (MARCI)(BELLETAL2009, MALINETAL2001)) JunoCam is a 'pushframe'
imager. Its detector has multiple filter strips, each with a different
bandpass, bonded directly to its photoactive surface. Each strip extends
the entire width of the detector, but only a fraction of its height;
JunoCam's filter strips are about 155 rows high. For JunoCam these
filter strips are scanned across the target by spacecraft rotation.
At the nominal spin rate of 2 RPM, frames are acquired about every 400
The spacecraft spin rate would cause more than a pixel's worth of
image blurring for exposures longer than about 3.2 milliseconds. For
the illumination conditions at Jupiter such short exposures would
result in unacceptably lower SNR, so the camera provides
Time-Delayed-Integration (TDI). TDI vertically shift the image one
row each 3.2 milliseconds over the course of the exposure, cancelling
the scene motion induced by rotation. Up to about 100 TDI steps can
be used for the orbital timing case while still maintaining the needed
frame rate for frame-to-frame overlap.
The pushframe imaging mode requires additional processing for image
reconstruction. First, each exposed frame is read out to the
spacecraft and the desired bands are extracted into 128-pixel-high
'framelets', editing out the unused lines between filters which may
suffer from spectral crosstalk. After optional summing and
compression, the framelets from all of the frames in an image are
transmitted to Earth. The Ground Data System then treats each
framelet as an individual image, using spacecraft attitude telemetry
to map-project it onto a planetary shape model. Finally, each
map-projected framelet is composited into an overall mosaic by spatial
location and bandpass to form an output map.