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Spacecraft Computer SystemsOverviewOutlineDefinitionsDefinitions (continued)Definitions (continued)Computer System SpecificationComputer System SpecificationState Transition DiagramFunctional PartitioningEstimating Throughput and Processor Speed RequirementsEstimating Throughput and Processor Speed RequirementsEstimating Software Size and Throughput RequirementsComputer SelectionProcessing ArchitecturesTypical Spacecraft Data Processing SystemMemoryMass StorageMass Storage (continued)Mass Storage (continued)Mass Storage (continued)Input/OutputInput/OutputRadiation HardnessRadiation EffectsError DetectionFault ToleranceFault Tolerance (continued)Fault ToleranceHardware Reliability TechniquesError Detection and CorrectionIntegration and TestSoftware SafetyReferencesSpacecraft Computer SystemsColonel John E. KeeseeOverview• Spacecraft data processing requires microcomputers and interfaces that are functionally similar to desktop systems• However, space systems require:– Low power, volume, and mass– High reliability and fault toleranceOutline• Definitions• Computer system specification• Estimating throughput and processor speed requirements• Computer selection• Memory• Mass storage• Input/Output• Radiation hardness• Fault tolerance• Error detection and correction• Integration and testDefinitions• Embedded system – A built-in processor, providing real-time control as a component of a larger system, often with no direct user interface• Real-time processing – Handling or processing information at the time events occur or when the information is first createdDefinitions (continued)• Hard real-time – Requires precise timing to achieve correct results, where missing at time boundary has severe consequences• Soft real-time – Requires that the tasks be performed in a timely manner, where missing a time boundary results in degraded but continuous performanceDefinitions (continued)• Operating system software – Manages the computer’s resources, such as input/output devices, memory, and scheduling of application software• Application software – Mission-specific software which does work required by the user or the mission rather than in support of the computerComputer System Specification1. Allocate mission and system requirements to computer systems2. Define the computer system’s operational modes and states3. Functionally partition and allocate the computational requirements to space or ground, payload or spacecraft, individual systems, and to hardware or softwareComputer System Specification4. Evaluate internal and external interfaces5. Select the baseline architecture6. Form the baseline system specificationState Transition DiagramFunctional PartitioningPerform processing in spacePerform processing on groundPerform processing in hardwarePerform processing in softwareAllocate processing between spacecraft bus and payloadsDo not allocate processing between spacecraft bus and payloadsAllocate processing along organizational linesDo not allocate processing along organizational linesEstimating Throughput and Processor Speed Requirements• SMAD example– Operator types 100 words (600 characters)/min– 256 keyboard states implies an 8-bit data word• Input data stream = 4800 bits/min– Each character requires 10 instructions to process– Each word requires 100 instructions to process• 16,000 instructions/min = 267 instructions/secEstimating Throughput and Processor Speed Requirements• SMAD example continued– Each instruction takes five clock cycles to read and one to execute• CPU must operate at 96,000 cycles/min = 1600 cycles/sec = 1.6 kHz• To store input data and transfer 8-bit instructions to the CPU requires 80 bits/sec for data plus 2136 bits/sec for instruction fetches = 2216 bits/sec transfer rate• In 24 hours the system will store 6.9 Mbits of data in addition to its 880 bits of stored instructionsEstimating Software Size and Throughput Requirements• Control system tasks• System management tasks• Mission data software• Operating system software•SMAD Tables 16-13, 16-14, and 16-15•Analogy•Bottom up analysis•ParametricComputer Selection• Computation Rate– For Reduced Instruction Set Computer the rate is about 1.5 times the clock rate• Address space– 16-bit computers usually address 64 K words– 32-bit computers usually address 4 G words• Built-in hardware functions– Floating point and transcendental functions– Direct Memory Address• See SMAD Table 16-17Processing Architectures• Central Unit– Single processor or one of the processors is designated the master that coordinates all the others• Distributed processing system– Multiprocessor units where any one can assume the role of master, or where executive tasks are shared by all processors• Difficult to design• Tolerates faults wellTypical Spacecraft Data Processing SystemMemory• Read Only Memory– Programs and start-up instructions– Slow, not volatile• Random Access Memory– Volatile but fast• Special purpose memory– Cache, multi-port, fast multiply accumulate, othersMass Storage•Disks– Mechanical systems -> reliability concerns– Angular momentum and vibration• Digital tape– Highly reliable– Momentum, start/stop torques, vibration are concerns– Slow data accessMass Storage (continued)• Bubble memory– Solid state, non-volatile– Small cylindrical region of magnetization in a planar substrate (garnet) – Persists in a magnetic bias field– Patterns in magnetic permalloy and rotating magnetic field cause bubbles to move under read/write head– High mass, power dissipation, unwanted magnetic fields, but no moving partsMass Storage (continued)• Integrated Circuits (RAM)– High power, cost– High speed and density• Magneto-Optical disks– When the magnetization vector is pointed toward the light source reflected light is blocked by a polarization filter– Not blocked when vector is pointed away from light sourceMass Storage (continued)• Magneto-Optical disks (continued)– To erase or write a laser heats the spot and a magnetic field changes the direction of the vector– High storage capacity but not used in space yet– Disadvantages similar to disksInput/Output•Ports– Serial I/O ports– Parallel I/O ports– I/O mapped ports– Memory mapped ports• Direct Memory Access (cycle stealing)• Multi-port memoryInput/Output• Interrupts– Priorities– Context


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MIT 16 851 - Spacecraft Computer Systems

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