Speaker
Description
We present the Data Processing Units (DPUs) based on Commercial Off-The-Shelf (COTS) electronic devices for two different instruments aboard of the Sunrise-III stratospheric balloon solar observatory. The DPU performs the entire high-level control of each instrument and carries out the on-board data processing which consists in a very demanding image acquisition and processing real-time pipeline. We explain the main design milestones of this DPU approach and we compare it with our previous DPU development for both balloon and space instruments. Special interest is devoted to design features for dealing with the on-board data processing: hardware devices, software tools, availability and affordability of both of them.
Sunrise-III is the third edition of a balloon mission that takes a one-meter solar telescope to the stratosphere. It will be launched from Kiruna (Sweden) in the summer of 2021. Sunrise-III will fly to Canada above the Arctic Ocean during six days approximately. Such trajectory and altitude enable a continuous observation of the Sun during 24 hours a day.
In this work we present the Data Processing Unit (DPU) design for two payload instruments aboard Sunrise-III. On one hand, IMaX+ (Imaging Magnetograph eXperiment Plus) will study solar magnetic fields at high spatial resolution (100 km on the solar surface). It makes images of the solar surface magnetic field by alternatively measuring the state of polarization of light within two selected spectral lines. On the other hand, SCIP (Sunrise Chromospheric Infrared spectro-Polarimeter), a spectrograph based instrument, is designed to observe two spectral regions at once. By combining these spectral regions, it can cover the photosphere and the chromosphere and obtain the 3D magnetic and velocity structure of the solar atmosphere.
Both instruments are very complex which involves several mechanisms, optical elements, and hardware devices for carrying out the scientific observation modes. The DPUs perform the high-level control of the entire instruments and carry out the on-board data processing. Each instrument uses one instance of the same DPU design with some particular features for addressing specific scientific aims. It performs the real-time acquisition, processing, and management of the images. The telemetry is critical during the flight. We can barely communicate with the DPU from ground. Only a few telecommands can be sent and some status information received. This implies a complete processing and compression of the collected images and other data prior to being stored on board. Finally, the gondola, the payload instruments, and the valuable data are retrieved after a controlled landing.
Basically, the ad-hoc DPU design is split into two main processing blocks: an NVIDIA Jetson TX2 module, which acts like the system controller, and a Xilinx FPGA (XCKU040) which behaves as a frame grabber. Both devices are connected using a high bandwidth PCI-express bus. The NVIDIA device contains an ARM multiprocessor architecture and a complete Linux-based operating system. It is suitable for carrying out tasks like storage management and communications with the balloon platform and with the ground support software. The frame-grabber FPGA has to control the most intensive image processing tasks as the acquisition, polarization demodulations, accumulations, and lossless compression of the images. It is connected directly to the cameras: 3 cameras in the SCIP case and 2 cameras in the IMaX+ one. The links are based on CoaxPress with a throughput of 3.125 Gbps per camera that come from up to 48 frames per seconds of 2,048 x 2,048 pixels. Our proposed specific FPGA architecture uses two DDR4 memory banks for dealing with the image streams in real time. Using this DPU approach we can reduce the data down to almost 10% for the SCIP instrument and 4% for IMaX+ by means of the on-board processing and without sacrificing polarimetric information in the data.
Since the budget for this type of mission is only a fraction of a space one, we use a design based on COTS devices. We do not use a redundant design but our electronics is protected in a pressurized and temperature-controlled box. The reason is twofold: on the one hand, we can use commercial devices without any concern about vacuum; on the other hand, we can manage the necessary high-voltage power supply. The latter would not have been possible under the stratospheric pressure conditions (of a few millibars). The development time is another important factor to have into account: the use of general-purpose, modern devices together with state-of-the-art software tools sharply reduces the engineering efforts.
This is the second DPU generation for the Sunrise missions. This time we have designed our specific DPU board meanwhile in previous flights we used a DPU composed entirely by commercially developed boards. Our current and modern approach reduces the number of final devices from a set of several DSPs, FPGAs and processor boards to a unique board containing the NVIDIA and Xilinx devices. In this talk, we will explain how we have taken advantage of the previous expertise and of the new technology and tools during this new development.
Additionally, after the first two flights of Sunrise, we have been developing another instrument. This is the Polarimetric and Helioseismic Imager (PHI) instrument aboard the Solar Orbiter mission, where the scientific targets are similar but the electronic development is completely space focused. During the presentation of our DPU design we will constantly compare the differences between the space and the COTS-based designs and will highlight the design trade-offs and the main lesson learned about affordability and availability.
| Paper submission | Yes |
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