In recent years there has been a trend towards the wider use of COTS (Commercial Off The Shelf) equipment in space missions. This trend has been mainly driven by the restrictions in R&D budgets and a growing demand for shorter design cycles. Funding Agencies are encouraging designers of spacecraft systems to identify and overcome the obstacles that previously prevented the use of COTS products for space missions.
When it comes to space vehicle engineering, the tolerance of onboard electronics to radiation effects can be one of the most challenging aspects of the system design. The risk of failure for avionics equipment on-board spacecraft due to radiation exposure is determined by the vehicle’s orbit trajectory and flight duration, during which the vehicle is exposed to trapped radiation as well as solar and cosmic radiation sources.
The overall impact at equipment level is determined by a complex interaction of shielding, circuit design, device technology and particle energy spectra and the tolerance to radiation effects is one of the key criteria for selecting the onboard equipment and subsystems. A novel approach to radiation mitigation at the board level, as opposed to component level, enables the use of existing high-performance COTS electronics systems in a space radiation environment, thereby lowering the cost involved in the design, certification, manufacture and deployment of such a system. This new approach, called “Radiation Tolerant Space COTS”, is based on a new type of backplane called the “Smart Backplane”. The Smart Backplane enables integrators to use proven, affordable COTS modules in a space radiation environment. In addition to faster time to deployment, the numerous benefits of this game-changing design strategy include significantly lowered costs for deployment, design, certification, and manufacture of space avionics.
Radiation events can result in temporary or permanent disturbances to the function of a device, a phenomenon known as a Single Event Effects (SEE). Over the years, as the density of ICs has increased, the size of elementary semiconductor structures has shrunk to the level where a spurious current spike produced by a single radiation particle can result in SEEs capable of disrupting the circuit’s operation. Two types of SEE are most relevant to protecting spacecraft avionics:
Single Event Upset (SEU) - occurs when a radiation-induced current causes a memory structure to change its state. This results in a temporary error in device output or its operation and is commonly referred to as “soft error”. In the case of an SEU, the device is not damaged and will function properly in the future, but the data processed by the device can be corrupted.
Single Event Latch-Up (SEL) - occurs when a radiation-induced current activates a parasitic structure (e.g. transistor), which forms an undesired low-impedance path in the semiconductor structure. It disrupts proper functioning of the circuit, and if not corrected, can possibly even lead to its destruction due to overcurrent. The circuit typically remains latched up until it is powered off and afterwards it may continue to function properly.
The Space COTS Approach
While the lowest-risk method for preventing radiation damage is the use of hardened components throughout the system design, this approach is very costly and time-consuming. The Radiation Tolerant Space COTS approach provides an alternative method for mitigating against SEUs and SELs that ensures the reliability and mission assurance requirements of the system while respecting the program’s budget and schedule requirement constraints.
In order to mitigate against SEUs and SELs, Curtiss-Wright has designed the ‘Smart Backplane’ chassis, a rugged 12- user slot chassis for data acquisition in a radiation-intensive environment. Based on the Acra KAM-500, a modular rugged data acquisition unit (DAU) and recording system designed for use in flight test market, the Smart Backplane enables the use of COTS data acquisition plug-in modules while at the same time preventing against the harmful effects of ionizing radiation. The system’s unique design enables the use of standard plug-in COTS modules in a space environment. This eliminates the need for modules to be built using radiation-hardened components. This minimizes the cost of the overall system and enables the space system designer to leverage over 100 plugin modules already designed for data acquisition in aircraft flight testing.
The DAU operates as a collection of synchronized FPGA based state machines. Once per acquisition cycle, the RAM is refreshed. Therefore, any SEUs that occur in RAM are overwritten within one acquisition cycle time. An acquisition cycle time can be anywhere from 100 microseconds up to 2 seconds in length.
The plug-in COTS modules used in the system are manufactured with commercial components but are protected by the radiation hardened Smart Backplane. The Smart Backplane can detect an SEL event on one of the modules in real-time and correct for that event before any damage can be done, thereby ensuring normal data acquisition is resumed without component damage and with minimal data loss. The system then recovers from the SEE, and normal operation of the entire data handling subsystem is uncompromised. The breakthrough concept behind the system design is that instead of seeking to prevent radiation events, the Smart Backplane instead quickly detects and then corrects for these events in real-time when they happen, with no damage to the equipment and most importantly, it ensures that the mission assurance, safety and reliability requirements are met.
The radiation tolerant Smart Backplane KAM-500 data acquisition system has already been selected for deployment in mission critical spacecraft avionics systems for manned and unmanned re-entry vehicles as well as launcher upper stages and is being considered for the instrumentation system on future planetary re-entry vehicles and as the basis for low cost COTS-based small satellite avionics systems.
In addition to supporting space applications aboard the International Space Station and the European Space Agency’s IXV (Intermediate Experimental Vehicle) launch vehicle, Smart Backplane was selected by The Boeing Company to supply rugged data handling avionics for use in the Crew Space Transportation (CST)-100 spacecraft. The Boeing CST-100 spacecraft will provide transportation for up to seven passengers or a mix of crew and cargo to low-Earth orbit destinations such as the International Space Station (ISS) and the Bigelow planned station.
More recently, the Smart Backplane has also been selected by Rocket Lab for use on the Electron launch vehicle. The DAU will acquire data from various analog and digital sensors onboard the Electron, Rocket Lab’s dedicated vehicle for launching small satellites and other pay-loads to Low Earth Orbit. The two-stage Electron vehicle is a dedicated launch service for small satellites to Low Earth Orbit. Its innovative design uses advanced carbon composites for a strong and lightweight flight structure. The Electron is the first oxygen/hydrocarbon engine to use 3D printing for all primary components. Rocket Lab recently successfully launched the first test flight of Electron from its orbital launch site on New Zealand’s Mahia Peninsula.
As space vehicle system designers look to lower the cost of space flight through the use of COTS electronics equipment, those solutions must be provably able to survive in demanding space radiation environments. Fully radiation hardened designs may yield the most protection, but they are also the most expensive option. The Smart Backplane chassis allows the use of high-performance COTS data acquisition user-modules in radiation-intensive space applications, lowering the cost of such a system by up to 75% while still meeting with the mission reliability requirements and minimizing the loss of telemetry data due to radiation events to less than 2% for a typical low Earth orbit (LEO) application.
This article was written by Daniel Gleeson, Space Business Development Manager, Curtiss-Wright (Dublin, Ireland). For more information, Click Here .