PS6 Flashcards
A spacecraft has a solid-state memory system constructed by standard commercial-off-the-shelf CMOS static RAMs, protected by a Hamming-coded error-detection and correction (EDAC) circuit. The memory data-bus is 8-bits wide. Explain how a Hamming (12,8)code EDAC circuit may be used to mitigate the effects of single-event upset (SEU) in this memory system.
The Hamming (12,8) code uses a 12-bit word to encode an 8-bit byte (i.e. the 8-bits of data are stored alongside an extra 4-bits of code information.
The code is constructed such that if any single bit of the 12-bit word is changed, it can be detected and corrected. If any 2 bits are altered, this can be detected, but no correction is possible. If 3 or more bits change, the code may not detect the error.
In a Hamming (12,8)-coded EDAC protected memory, 8-bit data are written to, and read from the memory via a Hamming codec. The memory is arranged to store the 4-bit code overhead as well as the 8-bit data. The 4 code bits are automatically calculated when the data is written to memory.
When data are read back, the Hamming codec re-calculates the appropriate 4-bit code, and compares it to that stored. If there is a discrepancy, correction takes place (provided only 1-bit is in error), and the corrected data are passed out of the codec.
However, the corrupted data are still stored in the memory, so in order to prevent the accumulation of bit-errors, the memory must be washed. That is the data at each address must be periodically read and re-written to memory. Thus, if an SEU causes any single-bit to be corrupted in a Hamming word, the Hamming codec corrects the memory output, and the action of memory washing ensures that the stored data are also corrected
Briefly explain what a Reed-Solomon code is, and what is meant by the (255,252) suffix. Why is such an error-protection system needed in a spacecraft memory?
A Reed-Solomon code is a block error-correcting code, which operates on bytes as symbols.The RS (255,252) code uses a 24 bit code word (3-bytes) to encode the information in 252 data bytes. All 255 bytes can be stored together as the code is self-protecting. Any single byte in error in the 255 byte-block can be detected and corrected by the RS decoder
State, with reasons, which of these architectures (MIL-STD 1750 SOS or COTS microcontroller) would be most suitable for controlling the propulsion system of a space probe designed to enter Marsorbit.
The SOS computer would be best as the application is not computationally demanding, yet it is critical that the OBC works when needed, so it must be rad-hard
Briefly state the differing philosophies behind the reduced instruction set computer (RISC) and the complex instruction set computer (CISC) in achieving high performance
RISC computers execute single (simple) instructions per clock cycle and achieve performance by running at very high clock speeds. CISC computers execute complex (powerful) instructions, but usually require many clock cycles to parse each instruction
Contrast the advantages and disadvantages of implementing an OBC using static-random-access-memory (SRAM) field-programmable gate-array (FPGA) or antifuse FPGA technology
SRAM FPGAs have the advantage of being readily re-configurable, which makes system development easier and could allow system flexibility in flight. The disadvantage as the SRAM is potentially susceptible to SEU, thus the configuration memory could change unexpectedly. SEU mitigation (e.g. scrubbing, TMR) is needed.
Antifuse FPGAs are one-time programmable so they are less convenient to work with during system development, and they cannot be re-configured in flight. However, they are robust against SE
The ADA software language has often been used for program spacecraft on-board computer systems. State three benefits of using ADA over the C/C++programming languages, and explain why, despite these advantages, C/C++is more often used in spacecraft system
ADA is a modular programming language that has a number of benefits over C/C++:It is designed specifically to operate in a real-time environment, and has structures such as semaphores to allow tasks to operate in parallel.
It is a MIL-STD language, and therefore ADA compilers are less subject to the variation associated with commercial software. Verified compilers are available.
It is a strongly-typed language (unlike C/C++), and therefore is less likely to exhibit the unexpected responses caused by mixing variable types.
The C/C++ languages are very common in the commercial environment, and so it is easier to find programmers used to these languages as opposed to the rather specialist ADA language. Therefore, “best commercial practice” is now accepted for space-based software.
Explain why a spacecraft TT&C system requires a low-gain omni-directional antenna on the spacecraft and a corresponding high gain antenna at mission operations the ground-station
The spacecraft must be capable of being operated at all times, regardless of its attitude with respect to the ground-station. Thus, the spacecraft antenna needs to be omni-directional (and thus, low-gain). To maintain the link budget, the ground-station antenna has to be of high gain, so that the large free-space path loss can be overcome
Explain why dish antennas are unsuitable for VHF or UHF TT&C links. Name and sketch a suitable high gain antenna for a TT&C ground-station
High gain dish antennas are impractical for radio wavelengths in the VHF (~2m) or UHF (~70cm) bands as the gain goes as /D, thus the dish diameter would need to be very large to achieve significant gain. Thus, “wire” type antennas such as multi-element Yagi or multiturn helical antennas are used. The more elements or the more turns the higher the gain
Explain why circular polarised antennas are to be preferred over linear polarised antennas in a VHF or UHF TT&C lin
However, if the attitude of the spacecraft rotates such as its antenna is pointing the opposite way to the ground-station antenna, then the polarisation is effectively reversed (like looking into a mirror) and so again we would get a deep fade. To get around this, we can use a linearly polarised antenna on the spacecraft and a circular polarised antenna on the ground –this reduces the chance of a deep fade due to polarisation mis-match, but at the cost of losing half the signal power(i.e. losing 3dB) from the radio link.
