Instrument Overview
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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 massresources.

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 kmlate 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 thevertical 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 thedegradation of the instrument to be graceful.

For further information about the JunoCam instrument, please seeHANSENETAL2014.

Detailed instrument description
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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 imagebuffer, 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 subsystemscommunicate via a spacecraft-provided wiring harness.

Camera head
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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 verticalsubframing 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 doublesampling (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. Thetotal mass of the CH is about 2.6 kg.

Filters
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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.

Optics
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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 opticsto facilitate precision mounting on the spacectaft.

The JunoCam lens was fabricated by Rockwell-Collins Optronics.

JDEA

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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-bitparallel 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, sinceconsiderable radiation shielding is provided by the avionics vault.
The JDEA mass is about 1 kg.

Flight software
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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 isotherwise unoccupied.

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 lossytransform-based or lossless predictive image compression.

Pushframing
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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 thesefilter strips are scanned across the target by spacecraft rotation.
At the nominal spin rate of 2 RPM, frames are acquired about every 400milliseconds.

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 neededframe 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 spatiallocation and bandpass to form an output map.