Radiation Tolerance in Space Electronics: Exploring the CAVU OBC Product Line

In the unforgiving environment of space, electronic systems must withstand a barrage of radiation-induced challenges, from cumulative degradation to sudden faults. At CAVU AEROSPACE UK , our OBC product line—featuring the OBC-Cube-104 (SmartFusion2), OBC-Cube-Polar (non-RT PolarFire SoC), and OBC-Cube-RT-Polar (RT PolarFire SoC)—is designed to meet these demands with a blend of inherent device resilience and system-level engineering. This article delves into the radiation effects impacting space electronics, our mitigation strategies, and how we tailor these solutions to diverse mission profiles, offering a cost-effective yet reliable approach for aerospace applications.

Understanding Space Radiation Effects on Electronics

Space exposes electronics to multiple radiation-induced faults, each requiring specific countermeasures: 

1. Total Ionizing Dose (TID):

• TID is the cumulative effect of radiation exposure over time. It causes permanent degradation in semiconductor devices by slowly damaging the transistors until they can no longer switch properly, leading to device failure.
• No semiconductor is completely immune to TID, even radiation-hardened (Rad-Hard) versions have a maximum TID tolerance before they fail.
• Commercial off-the-shelf (COTS) semiconductors such as Microchip’s SmartFusion2 and non-RT PolarFire FPGAs, typically tolerate 30kRad (Si), making them suitable for Low Earth Orbit (LEO) missions of 3–5 years. Shielding can extend this tolerance significantly.

2. Single-Event Effects (SEE), including Single-Event Upsets (SEU):

• SEEs occur when high-energy particles (e.g., cosmic rays, solar activity) interact with semiconductors, causing temporary or permanent errors.
• Single-Event Upsets (SEU) lead to bit-flips in memory or logic circuits. These are temporary but can affect mission-critical operations if not mitigated.
• Single-Event Latchup (SEL) is more severe, creating short circuits that can overheat and damage devices without control measures.
• Both SmartFusion2 and PolarFire leverage flash-based architectures, which are inherently immune to SEU in their configuration memory—meaning no bit flips occur in the programmed logic. Transactional data (e.g., registers) still requires error monitoring, which we implement via FPGA fabric techniques like Triple Modular Redundancy (TMR) and voting circuits. Their SEE immunity is rated at ~60MeV·cm²/mg, a Linear Energy Transfer (LET) threshold indicating the energy needed to cause an upset. This is robust for LEO, where most particles are below this level, though higher-energy events in GEO or deep space could pose risks depending on mission duration.

3. Latchup Protection:

SELs can lead to temporary or permanent failures. We employ Latching Current Limiters (LCL), watchdogs, and power redundancy to detect and recover from latchup events, preventing system-wide compromise.

Each fault type demands distinct hardware and system-level strategies, shaping our approach to radiation tolerance.

Radiation-Hardened vs. Radiation-Tolerant Devices

In space electronics, we categorize devices into two types based on their radiation performance:

• Radiation-Hardened (Rad-Hard): These devices are designed and fabricated from the ground up to withstand extreme radiation levels (e.g., TID > 100 kRad, SEL immunity > 100 MeV·cm²/mg). They undergo specialized manufacturing processes (e.g., hardened oxides, substrate engineering) and rigorous testing to ensure reliability in harsh environments like GEO or deep space. Examples include Rad-Hard ASICs or FPGAs qualified to QML-V standards.
• Radiation-Tolerant (RT): These devices offer improved radiation performance over commercial-grade parts but aren’t as extensively hardened as Rad-Hard solutions. They often leverage inherent design features (e.g., flash-based architectures) and additional screening to achieve TID tolerance of 30–100 kRad and moderate SEE immunity. The RT PolarFire SoC falls into this category, balancing cost and performance for missions like LEO or Lunar orbits.

Radiation-Tolerant Systems: By Design vs. By Device

We also distinguish between two approaches to system-level radiation tolerance:

• Radiation-Tolerant by Device: Relies on using Rad-Hard or RT components throughout the system. This ensures each part meets a high radiation threshold but can increase costs significantly.
• Radiation-Tolerant by Design: Uses a mix of commercial or RT devices with system-level mitigation (e.g., shielding, redundancy, error correction) to achieve tolerance without requiring every component to be Rad-Hard.

At CAVU AEROSPACE UK , we combine these approaches to create subsystems that are radiation-tolerant overall. For instance, we might pair a SmartFusion2 (non-RT) with design-level hardening or use an RT PolarFire SoC with additional board-level protections, tailoring the solution to mission’s needs and budget.

Mitigation Techniques for Radiation Effects

To ensure our subsystems perform reliably in space, we employ a range of mitigation techniques tailored to each radiation fault type. Our agile approach allows us to adapt these strategies to mission’s specific requirements, balancing performance, cost, and complexity. Below are the key methods we use:

TID Mitigation

Shielding (e.g., 2–5 mm aluminum or tantalum) significantly boosts the 30 kRad TID tolerance of SmartFusion2 or non-RT PolarFire. The RT PolarFire SoC, with a >100 kRad baseline, benefits further in harsh orbits.

SEE/SEU Mitigation

For Single-Event Effects, including SEUs, we leverage both the inherent strengths of our FPGAs and active design techniques:

• Flash-Based Architecture: The SmartFusion2 and PolarFire SoCs’ flash technology ensures zero configuration upsets, as the programmed logic remains intact even under particle strikes. This eliminates the need for constant reprogramming seen in SRAM-based FPGAs.
• Error Detection and Correction (EDAC): For transactional data in registers or external memory, we implement EDAC within the FPGA fabric. Techniques like Hamming codes or Reed-Solomon encoding detect and correct single-bit errors in real time, while multi-bit errors trigger system-level responses.
• Triple Modular Redundancy (TMR): Critical logic paths are triplicated with voting circuits to ensure the correct output prevails, even if one path experiences an SEU. This is particularly effective for mission-critical operations.
• Radiation-Resistant Storage: Where large onboard storage is required, we can integrate magneto-resistive RAM (MRAM), FRAM or radiation-tolerant NAND flash, both of which are highly resistant to SEUs compared to standard DRAM or SRAM.
• System-Level Redundancy: We often employ redundant subsystems (e.g., dual processing chains) to maintain functionality if an SEE impacts one path, with automatic failover mechanisms (e.g. CDH-FS).

SEL Mitigation

To protect against latch-ups, we implement both preventive and recovery strategies:

• Latching Current Limiters (LCL): These circuits monitor current spikes indicative of an SEL and cut power to the affected component within microseconds, preventing overheating or damage.
• Watchdog Timers: Software and hardware watchdogs reset the system if a latch-up stalls processing, ensuring rapid recovery without manual intervention.
• Power Redundancy: Dual power rails or isolated power domains ensure that a latch-up in one section doesn’t bring down the entire board. Current-limiting fuses or regulators further isolate faults.
• Component Selection: The RT PolarFire SoC’s SEL immunity (>80 MeV·cm²/mg) reduces latch-up risk significantly, while SmartFusion2 relies on our system-level protections to achieve similar reliability.

OBC Products: FPGA Choices and Radiation Performance

Why Use SmartFusion2 and PolarFire for Space?

Our choice of SmartFusion2 and PolarFire SoCs isn’t driven by their status as radiation-hardened (Rad-Hard) devices—neither is inherently Rad-Hard—but by their architectures, which are naturally radiation-tolerant to a significant degree. Here’s why they’re well-suited for space:

• TID Tolerance: Both SmartFusion2 and the non-RT PolarFire SoC can withstand up to 30 kRad (Si) of Total Ionizing Dose, sufficient for many Low Earth Orbit (LEO) missions lasting 3–5 years. With shielding, this can be extended further.
• SEU Immunity: These FPGAs use flash-based technology, making their configuration memory immune to Single-Event Upsets (SEU). Unlike SRAM-based FPGAs, which suffer bit-flips in their configuration and require constant reprogramming, SmartFusion2 and PolarFire maintain their programmed logic intact under radiation exposure.
• SEE Threshold: Their Single-Event Effect (SEE) immunity is rated at ~60 MeV·cm²/mg (Mega-electron volts per square centimeter per milligram). This Linear Energy Transfer (LET) threshold indicates the energy a particle must deposit to cause an upset. In LEO, most radiation particles fall well below this level, making these FPGAs robust for typical orbits. However, in higher-radiation environments like GEO or deep space, rare high-energy particles could exceed this threshold, necessitating additional mitigation.

These FPGAs’ flash architecture, TMR, and ECC storage make them reliable for LEO without Rad-Hard costs, while RT PolarFire extends this to harsher environments.

Choosing the Right Solution

Selecting the right solution for your mission hinges on balancing radiation requirements, performance needs, and budget constraints. Our agile approach ensures subsystems are tailored to specific mission profiles, guided by the following key parameters:

Orbit: Radiation levels vary dramatically by orbit:

• LEO: Typically experiences 1–5 kRad/year TID, with most particles below 60 MeV·cm²/mg LET, making SmartFusion2 or non-RT PolarFire viable with shielding.
• MEO/GEO: TID can exceed 10 kRad/year, and higher-energy particles are more common, favoring the RT PolarFire’s >100 kRad TID and >80 MeV·cm²/mg SEL immunity.
• Lunar/Deep Space: Extreme radiation (e.g., solar flares, cosmic rays) demands the RT PolarFire and full board-level RT components. But some shorter missions could also benefit from non-RT variants.

Mission Lifetime: Longer missions amplify TID accumulation:

• 3–5 Years: The 30 kRad TID tolerance of SmartFusion2 or non-RT PolarFire suffices in LEO with shielding, covering many missions.

• 5+ Years: The RT PolarFire’s >100 kRad TID becomes critical, especially in higher orbits or during solar maximum.

Launch Window and Solar Activity: Solar cycles affect radiation intensity:

• Solar Minimum: Lower particle flux favors non-RT solutions like SmartFusion2 or non-RT PolarFire.
• Solar Maximum: Increased solar events may push end-of-life TID and SEE risks, tilting toward RT PolarFire.

Processing Budget: A key parameter in architecture selection, processing needs dictate computational resources:

• SmartFusion2 integrates a modest ARM Cortex-M3, suitable for simpler tasks with limited processing demands.
• Non-RT PolarFire offers RISC-V cores and expanded FPGA resources, supporting more complex computations.
• RT PolarFire matches non-RT PolarFire’s capabilities but with radiation-enhanced reliability for high-performance missions.

Budget: Costs scale with radiation tolerance, architecture and screening:

• SmartFusion2 and PolarFire: These non-RT solutions offer a cost-effective range, starting at a couple of tens of thousands and potentially reaching tens of thousands, depending on the chosen architecture and interfaces and board-level redundancy features.
• RT PolarFire: A premium solution, with costs escalating to a couple of hundred thousand due to the use of radiation-tolerant components, enhanced architecture, and QML-V/Y screening requirements.

Screening and Qualification Levels

Component screening also impacts reliability and cost:

• QML-Q: Offers moderate tolerance, suitable for less demanding environments like LEO, and serves as a cost-effective screening option, typically applied to commercial or early engineering samples.
• QML-V: Provides high tolerance and space-grade qualification, meeting rigorous standards for demanding missions. The RT PolarFire SoC, when fully screened and qualified (e.g., RTPF500ZT, RTPF500ZTL, RTPF500ZTS, RTPF500ZTLS), aligns with this level, supporting TID tolerance up to 300 kRad (Si) and SEL immunity >80 MeV·cm²/mg as per NEPP 2021 testing.
• QML-Y: Represents advanced radiation-tolerant features and the highest qualification grade, tailored for the most extreme space environments with enhanced screening and testing.

For the RT PolarFire SoC used in flight-qualified applications, we align with QML-Q or QML-V based on mission certification needs. In contrast, RT PolarFire MS (Military Temperature Engineering Silicon) devices, intended for hardware functional verification only, lack QML certification, radiation performance testing, and are unsuitable for space flight.

Modeling and Customization

Engineers model radiation exposure using tools like SPENVIS or CREME96 to confirm TID accumulation and SEE rates, ensuring the chosen solution meets reliability targets within budget—whether through a fully RT subsystem or a hardened COTS design. This process adapts to specific orbits, lifetimes, and technical requirements (e.g., Processing, Interfaces, storage).

Looking Ahead: Advancing OBC Solutions

CAVU AEROSPACE UK ’s OBC product line harnesses the radiation-tolerant strengths of SmartFusion2 and PolarFire SoC FPGAs, delivering adaptable solutions through a blend of device resilience and system-level engineering. From cost-effective designs for LEO missions to robust RT configurations for extreme environments, our agile methodology aligns performance with mission demands. As the frontiers of space exploration expand, we continue to innovate, ensuring our onboard computers empower the next generation of orbital and deep-space endeavors.

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