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Protecting Satellite Image Integrity from Radiation

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WHITE PAPER Rad-tolerant storage mrcy.com 3 enough energy to cause an effect in a device. A common example of this is when impact on a memory chip causes a 'bit flip', changing a stored value Electronics in space face continual cosmic bombardment. This ongoing exposure to radiation has caused not just imaging issues but also numerous instrument failures and sometimes complete operational loss of a satellite, such as the Hipparcos in 1993, Galaxy 15 in 2010, SumbandilaSat in 2011 and Intelsat 29e in 2019. While the problem is now fairly well understood, the solution options are still limited. Satellite system designers have three types of tools for dealing with the radiation challenge. Shielding: Surrounding electronics with radiation-absorbing material can be very effective but comes with the significant downside of added weight, a factor affecting both satellite launches and maneuvers in space. Aluminum is often used, with thicknesses varying based on trade-offs between weight and the risk of radiation-generated errors. Rad-tolerant components: Sometimes referred to as 'rad-hardened,' these are microprocessors, controllers and other components specifically designed to function reliably in a high-radiation environment. Techniques include using special substrates and designing redundant functional elements within the chips. ECC: This approach is specific to memory and data storage components. With ECC, a data storage unit uses an error correction code to first detect and then correct data corruption. It overcomes 'bit flip' issues, ensuring the data read from storage is the same as the data that was input. Most ECC implementations can effectively correct single-bit errors; more sophisticated methods can correct multiple errors across blocks of storage. RAD TOLERANCE THROUGH PURPOSE-BUILT INNOVATION Several years ago, Mercury began receiving customer requests for high-capacity, rad-tolerant storage. Space imaging application designs were moving to higher-resolution sensors and the existing storage options couldn't keep up. A team of Mercury 's real-time storage experts went to work, focused on meeting the customer requirements while remaining open to all components and concepts to find the right solution. The Mercury team agreed upon a design approach that could be expanded and extended as technology evolves. The resulting family of storage subsystems, known as solid state data recorders (SSDR), uses rad-tolerant components — to levels greater than 100 krad — combined with advanced ECC and NAND flash, packaged in open standard form factors. In satellites, especially small LEO satellites, storage subsystems must be able to operate on a low-power budget. This requirement makes DRAM chips unsuitable because they require constant power to retain data. Flash memory, typically NAND flash, is the component type now used for almost all satellite image storage, since it does not require constant power and can turn off power to data storage between the time an image is collected and when it is transmitted to Earth, significantly reducing power budget. Satellite designers are also seeking components and subsystems that are packaged in open standard form factors. Using these types of off-the-shelf solutions translates into cost savings as well as compressed design schedules. Moreover, unique to systems in space, image data storage must be able to withstand an endless stream of cosmic radiation at levels that are always dangerous to electronics and often move even higher based on solar events. The accuracy of every stored image must remain uncompromised by that radiation for the operational life of the satellite. BUILDING IN RAD TOLERANCE The term 'cosmic rays' is somewhat inaccurate. When cosmic radiation was first detected in the early 20th century, it was believed to be electromagnetic. Actually, it consists of very high-energy particles, largely protons but also heavier ions (He, Fe, etc.) and high-energy electrons. Most of the particles impacting Earth-orbiting satellites are generated by the sun at levels varying with the solar activity cycle associated with the volume of sunspots. There is also a low flux level of radiation found across free space referred to as galactic cosmic rays (GCRs), which consists of very high- energy heavy ions moving at almost the speed of light. The atmosphere absorbs almost all of these high-energy particles before they reach Earth's surface, which is a very good thing for terrestrial electronics because when the particles do strike electronics, they cause performance problems, usually separated into two general areas. First, high-energy particles moving through electronics will cause ionization in semiconductor material. These radiation-induced trapped charges, which build up over time, can cause failures, such as logic gates being unable to close. Second, there are single-event effects (SEEs) where an individual ionizing particle deposits Bit flip 1 0 1 0 1 Data Storage Before Data Storage After 1 0 1 1 1 Bit flip Cosmic radiation

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