Selecting SSDs for aviation, space and defence applications
As more electronics-based hardware is used for aviation and space applications, whether for civilian or defence purposes, the demands placed on data storage become significant. Trends include using constellations of microsatellites for earth observation purposes such as remote sensing of extreme weather events and humanitarian-based surveillance of rogue nations. Whether deployed into the stratosphere or beyond, any form of memory, such as a solid-state drive (SSD), will experience extremes of temperature, radiation and mechanical stress yet still need to operate with 100% reliability. Data stored on the SSD may be valuable intellectual property such as firmware, critical operation information, and sensitive commercial or political information. They will require encryption, safeguarding, and data management capabilities.
Aviation and defence applications for SSDs
As the storage density and reliability of NAND memory technology has advanced, SSDs have become immensely popular for laptops and desktops. Demand for mechanically prone hard disk drives (HDD) is waning in most consumer devices, but HDDs still have a crucial role in enterprise storage applications.
SSDs, without the mechanical robustness limitations of HDDs, are ideal storage candidates for use in aviation, space, and defence (A&D) applications. However, despite their attractive pricing and ease of availability, the development and production goals of commercially available (commercial off-the-shelf - COTS) SSDs are often skewed more towards price and performance than a broader set of criteria, including robustness, reliability, and security demands of A&D.
Figure 1 highlights the salient differences between COTS grade SSDs and those for A&D applications.
Perhaps unknown to many commercial users, many manufacturers of SSDs may implement firmware that tracks the amount of data written to NAND media in addition to the power-on time of the SSD. Firmware algorithms can then detect if excessive NAND writes take place against the recorded operational hours. Suppose it appears that excessive use occurs such that the SSD will not meet its stated warranty specifications. In that case, firmware intervenes to slow down NAND writes to fulfil its warranty expectations. For commercial applications, such functions prolong media life, but for A&D applications it can present a significant limitation. Aviation, space, and defence applications may experience many periods where high-speed data acquisition occurs.
Figure 1 - A comparison of the design and development goals of SSDs designed for COTs against those for aviation and defence applications. Source: Mercury Systems (click to enlarge)
As NAND memory technology has evolved, multi-level (MLC) and triple-level (TC) fabrication methods have increased. These cell adaptions improve storage density capabilities above the original single-level cell (SLC) approach but at the risk of reducing the operating temperature range. Again, manufacturers employ firmware techniques to reduce read and write speeds to limit internal temperature rises. By comparison, SSDs used in aviation and defence applications typically use SLC flash, industrial-grade components, and utilise enhanced thermal management methods to allow high-temperature operation well beyond those offered by COTS SSDs.
Another critical aspect of A&D-grade SSDs is that the ruggedised construction methods used in their manufacture make them more robust and able to survive mechanical shocks and continued vibration during operation.
Perhaps the most significant differences between COTS and A&D SSDs involve provisioning comprehensive security features.
SSD security
As highlighted in Figure 1, COTS SSDs fall well short of the minimum security requirements involved in handling the data typically gathered by A&D applications. Two key aspects of provisioning SSD data security are data protection and elimination.
Data protection
Data encryption must occur within the SSD to internationally recognised aviation and defence standards. Typically, encryption 256-bit AES asymmetric encryption functions encrypt the data during write cycles. Most importantly, the encryption process protects the data, and the keys generated also need secure storage. SSDs that manage the whole encryption process independent of the attached host are termed self-encryption drives (SEDs). Relevant standards for data encryption include the National Institute of Standards and Technology (NIST), FIPS 197 and FIPS 1402. The Trusted Computing Group (TCG), an open standards and security specifications organisation, specifies the OPAL encryption and decryption standard for the always-on security of SED data.
The US Government's National Security Agency (NSA) also stipulates the Commercial Solutions for Classified (CSfC) specification for secure data storage.
Data elimination
From time to time, particularly for defence applications, a threat situation may occur that necessitates the immediate deletion of all data on the SSD. This declassification process applies to the encrypted data and all keys, firmware, and program code. A set of different military standards stipulate the degree to which data is purged, cleaned, or sanitised - see Figure 2. In addition to fast erase cycles, one or more data write cycles using defined data strings achieve high levels of data elimination.
The classification category of the data and keys stored on the SSD dictates the extent of the data purging and the number of write cycles required to achieve compliance.
Note that the process of eliminating data from an SSD is considerably easier and quicker than degaussing the magnetic platters of a hard disk drive.
Selecting an SSD for aviation, space, and defence applications
In addition to the environmental, data encryption and data elimination specifications mentioned above, engineers selecting an SSD for A&D applications also should review the SSD form factor, storage capacity, and host interface requirements.
Also, the high-altitude conditions of space applications further necessitate SSDs resistance against the long term effects of radiation exposure, a process termed radiation hardening.
Figure 2 - Standards for data elimination. Source: SMART High Reliability Solutions (click to enlarge)
Examples of SSDs include those available from Mercury Systems and Smart Modular Technologies and Greenliant. Prevalent SSD form factors for space-constrained systems include mSATA (mini SATA), M.2 and BGA (ball grid array). Legacy applications may dictate an SSD that can emulate a traditional 2.5-inch HDD. Host interfaces include SATA, PCI Express, and NVMe.
Defence grade SSDs paramount for aviation, space, and defence applications
COTS SSDs are not suitable for use in demanding aviation, space, and defence applications. They are not robust and rugged enough to endure the mechanical and environmental rigours encountered. Also, their overall lack of security features leaves them open to compromise and sensitive, confidential data at risk for such applications.
Although COTS SSDs and defence-grade SSDs may provide similar essential read and write storage capabilities, only defence-grade SSDs can accommodate the harsh environmental conditions and offer vital data security protection functions.
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About Author
Paul Leys
Paul Leys is the Market Segment Manager for the Aerospace and Commercial Avionics business at Avnet ...
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