Vision-based navigation has become increasingly important in a variety of space applications for enhancing autonomy and dependability. Future missions, such as active debris removal for remediating the low Earth orbit environment, will rely on novel high-performance avionics to support advanced image processing algorithms with substantial workloads. However, when designing new avionics architectures, constraints relating to the use of electronics in space present great challenges, further exacerbated by the need for significantly faster processing compared to conventional space-grade central processing units. With the long-term goal of designing high-performance embedded computers for space, in this paper, an extended study and trade-off analysis of a diverse set of computing platforms and architectures (i.e., central processing units, multicore digital signal processors, graphics processing units, and field-programmable gate arrays) are performed with radiation-hardened and commercial off-the-shelf technology. Overall, the study involves more than 30 devices and 10 benchmarks, which are selected after exploring the algorithms and specifications required for vision-based navigation. The present analysis combines literature survey and in-house development/testing to derive a sizable consistent picture of all possible solutions. Among others, the results show that certain 28nm system-on-chip devices perform faster than space-grade and embedded CPUs by 1−3 orders of magnitude, while consuming less than 10 Watts. FPGA platforms provide the highest performance per Watt ratio. Vision-based navigation (VBN) endows robotic systems with the ability to guide themselves autonomously in an unknown environment by using active or passive image sensors to perceive their surroundings. Vision applications in space vary from rover exploration on Mars with simultaneous localization and mapping, to Moon landing with landmark tracking and hazard avoidance, and to spacecraft proximity operations with pose estimation of cooperative or uncooperative orbiting targets. However, the ever increasing computational demands of these enabling technologies are now challenging the classical on-board computing, both in terms of architecture and processing power. An example where this becomes especially apparent concerns active debris removal (ADR) scenarios, whose aim is to actively remove heavy debris objects by capturing them and then either disposing them by destructive re-entry in the atmosphere or moving them to graveyard orbits [1]. In future ADR missions, chaser spacecraft must rely, among others, on high-definition cameras and complex computer vision algorithms performing image feature extraction, matching, 3D reconstruction, tracking, etc., at high frame rates. Given the requirements for fast and accurate autonomous relative navigation, the gap between algorithmic complexity and conventional space-grade processors is quickly widening and calls for novel high-performance embedded computing in space. The Conventional Avionics Architecture 2 Classical on-board data handling [2] is based on a central on board computer (OBC) connected to various platform subsystems, such as the radio communication, attitude & orbit control (AOCS), payloads, etc. (Fig. 1 ). Most often, the internal communication network involves a serial bus of high data integrity, i.e., the military standard 1553 or the controller area network (CAN) bus at around 1 Mbps. The OBC has two primary functions, namely to decode telecommands and to serve as the central terminal of the spacecraft. In few cases, the OBC also handles payload data, e.g., for a remote sensing application which would otherwise require a separate processor. The peripheral terminals are stand-alone units or embedded within the instruments of the spacecraft. The more complicated of these instruments include their own embedded processor to support intelligent control units (ICUs), sophisticated remote bus interfaces, and specialized science data processing, which normally demands higher performance than that of the OBC [2]. Occasionally, the instrument data is collected by an independent "payload data-handling system" supporting relatively high data rates. Routine spacecraft tasks such as attitude control, housekeeping reports, command decoding and systems management, require very little processing power, which can be provided even by simple 8-bit microcontrollers [2]. More demanding tasks, such as payload software, intensive digital signal processing (e.g., time/frequency domain filtering, compression, feature extraction and matching), data formatting, error control, or encryption, are executed on more advanced microprocessors. Such tasks within payloads/experiments may have a dedicated controller interfacing with the platform, under real-time operation, to output data in source packet format. Their controller will typically be a standard microprocessor, such as the SPARC-based LEON, a digital signal processor (DSP) device, such as the TSC21020F, or, in more extreme cases, even an application-specific integrated circuit (ASIC). Peripheral supporting devices, like universal asynchronous receiver and transmitters (UARTs), dynamic random-access memory (DRAM) controllers, I/O controllers, and communications devices, have custom implementations on field programmable technology [2] . Until now, the aforementioned computers have predominantly been of space-grade technology and hence significantly slower compared to their contemporary terrestrial counterparts, lagging behind them by 1 − 2 orders of magnitude in performance [2] [3] [4] . Recent trends in the space industry push the on-board processing power and storage towards terrestrial industrial practices [2] . The goal is to support more complex data organization, autonomous decision-making, intensive signal processing and multitasking, and the coordination of large distributed development teams. Commercial-off-the-shelf (COTS) devices are competing with conventional rad-hard components by offering the advantages of higher performance, faster/cheaper development, ease of maintenance, off-the-shelf development systems and processing/communications libraries, built-in file handling, multitasking and inter-task communication, and software reliability [2] . Lately, the use of COTS in high-reliability space applications is on the rise and ranges from memory chips in flagship planetary explorers to various parts in pico-nano sats [4] . Operational constellations of the latter currently provide useful services based on COTS that have been selected 4 and tested to meet certain mission requirements [5] . ESA roadmaps [6] present an emerging trend towards COTS adoption, whilst the US is moving aggressively to accept COTS chips in space. This is also evident in institutional missions e.g. by ESA and CNES who have multiple activities with commercial Xilinx Zynq devices for processing. On the downside, when deployed in space, COTS devices can be severely harmed by the harsh environmental conditions of high-vacuum, extreme temperatures and high levels of ionizing radiation, or even by the vibrations during launch. Space users consider applying mitigation methods, e.g., redundancy, to handle the damage and increase the availability/reliability of these components in-flight [3, 6] . More specifically [2], although with very low probability, plastic materials may outgas under vacuum or cause electrostatic discharge. Thermal-wise, many COTS devices can only operate in a limited temperature range, e.g., −30 o to +70 o C, which must be moderated within a spacecraft. Radiation-wise, a non rad-hard COTS device will suffer from total dose effects (TDEs) and single event effects (SEEs) owing to the trapped particles in the Van Allen belts surrounding Earth, the galactic cosmic-rays originating from deep space and the solar-flare particles [7] . In general [2], avionics should be tolerant to 10−50 kRad total ionizing dose (TID) for low earth orbit (LEO) missions and 100−300 kRad for MEO/GEO missions. However, specifically for short duration missions in LEO orbit, e.g., as in an ADR scenario, the aforementioned radiation effects could be tolerable even for COTS due to the protection offered by the Earth's magnetic field and the limited exposure time.
The need for onboard mapping and terrain analysis capabilities is widely accepted but the resource limitations of space-grade processors have been hindering the implementation of these computationally demanding modules REF .
High-Performance Embedded Computing in Space: Evaluation of Platforms for Vision-Based Navigation
Journal of Aerospace Information Systems
Engineering