5 ms·

I do not buy the power argument. A modern chip is orders of magnitude more power efficient, especially at low voltage. You could easily have multiples of these

I do not buy the power argument. A modern chip is orders of magnitude more power efficient, especially at low voltage. You could easily have multiples of these within the same power budget.

ilyt 4y ago
But smaller size increases the chance for event (which might be compensated) but also increases chance for fatal damage to happen. The radiation in space is not dangerous because it just produces a bit of current that can flip a bit; it is dangerous because it is strong enough to bump a bunch out of their position in silicon crystal, causing permanent damage that will accumulate over time. And when your feature size is starting to measure in <100 atoms it would be easy for single high energy event to break whole core.
shagie 4y ago
The modern chip is more power effect by using traces that are closer together and thinner. In the context of radiation hardening by design, this means that a cosmic ray strike could evaporate part of the 5nm trace or cause arcing to the next trace. A lower voltage chip can be more easily permanently damaged by a cosmic ray. That's why the RAD750 has its radiation resistance measured in grays. > The gray (symbol: Gy) is the unit of ionizing radiation dose in the International System of Units (SI), defined as the absorption of one joule of radiation energy per kilogram of matter. This is the total absorbed dose - not the flux. Additionally, this is for the entire system - not just the CPU. It's not just "here is a redundant CPU" but also "here is the entire system that the CPU is part of" that is redundant. The JWST is using a RAD750 with 44 MB of ram... and yea, that's not a lot, we're not trying to ship super computers out there. The alternative that you're suggesting is suggesting putting those all in multiple out in space and checking them against each other constantly while the system is in use collecting data. The power budget is slim and also coupled to the heat dissipation budget. The JWST has a tighter heat dissipation budget than power budget... and the Perseverance has a tighter power budget (total rover power budget is 110 watts). The challenge I'd pose is "what other processor arrangement fits within its power budget?" I'm also going to point out that I got the numbers wrong for Perseverance's power budget for its CPU. The single board is 10 watts, the cpu is 5 watts. https://en.wikipedia.org/wiki/RAD750 https://en.wikipedia.org/wiki/RAD750 > The CPU can withstand an absorbed radiation dose of 2,000 to 10,000 grays (200,000 to 1,000,000 rads), temperatures between −55 °C and 125 °C, and requires 5 watts of power. The standard RAD750 single-board system (CPU and motherboard) can withstand 1,000 grays (100,000 rads), temperatures between −55 °C and 70 °C, and requires 10 watts of power. https://mars.nasa.gov/mars2020/spacecraft/rover/brains/ https://mars.nasa.gov/mars2020/spacecraft/rover/brains/ > Unlike people and most animals, the rover's brains - its computer - are in its boxy body. The computer module is called the Rover Compute Element (RCE) - there are actually two identical RCEs in the body so there is always a spare "brain." Also - https://space.stackexchange.com/questions/50470/what-makes-insights-rad750-processor-so-radiation-resistant-compared-to-1998 https://space.stackexchange.com/questions/50470/what-makes-i... These are modern chips that are designed for that efficiency - it's just that there are other constraints too. You are unlikely to find a lower wattage processor that you can run with redundancy that is able to tolerate space travel. https://link.springer.com/article/10.1007/s12567-016-0138-0 https://link.springer.com/article/10.1007/s12567-016-0138-0 Radiation Effects and COTS Parts in SmallSats https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2934&context=smallsat https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2... Note that the cubesat has an average lifespan of about a year.
naikrovek 4y ago
i guarantee the people building the spacecraft and choosing the CPUs have thought about this more than you, and have much more knowledge about the problem space than anyone here questioning their decision-making processes. if it was a good idea to have multiple redundant CPUs on these spacecraft, the spacecraft would have multiple redundant CPUs.
- shagie 4y ago
  I will note that they do have multiple, redundant CPUs... they just aren't all active at once. https://www.jpl.nasa.gov/news/press_kits/mars_2020/landing/mission/spacecraft/perseverance_rover/ https://www.jpl.nasa.gov/news/press_kits/mars_2020/landing/m... > Just like Curiosity, Perseverance has two main computers, or “brains”: One is active at any given time, while the other serves as a backup. Both use radiation-hardened RAD750 computers, and together, they’re referred to as Rover Compute Element A and B. > Another RAD750 serves as a third “brain,” on Perseverance. Called the Vision Compute Element, it is equipped with a special card for analyzing images. This card has a Virtex-5 field programmable gate array (FPGA) and is the driving force behind Terrain-Relative Navigation and the Lander Vision System, which analyzes images of the landing site during descent and compares them to an onboard map to determine the rover’s position relative to the ground. That "Virtex-5 FPGA" is an interesting one too... https://www.xilinx.com/products/silicon-devices/fpga/virtex-5qv.html https://www.xilinx.com/products/silicon-devices/fpga/virtex-... Where it is listed as "Space-grade Virtex-5QV FPGA"
- incrudible 4y ago
  I said I do not buy the power argument and I received two high quality comments explaining issues beyond power consumption. Your appeal to authority adds nothing of value.

I do not buy the power argument. A modern chip is orders of magnitude more power efficient, especially at low voltage. You could easily have multiples of these within the same power budget.

ilyt 4y ago

But smaller size increases the chance for event (which might be compensated) but also increases chance for fatal damage to happen. The radiation in space is not dangerous because it just produces a bit of current that can flip a bit; it is dangerous because it is strong enough to bump a bunch out of their position in silicon crystal, causing permanent damage that will accumulate over time. And when your feature size is starting to measure in <100 atoms it would be easy for single high energy event to break whole core.

shagie 4y ago

The modern chip is more power effect by using traces that are closer together and thinner. In the context of radiation hardening by design, this means that a cosmic ray strike could evaporate part of the 5nm trace or cause arcing to the next trace. A lower voltage chip can be more easily permanently damaged by a cosmic ray. That's why the RAD750 has its radiation resistance measured in grays. > The gray (symbol: Gy) is the unit of ionizing radiation dose in the International System of Units (SI), defined as the absorption of one joule of radiation energy per kilogram of matter. This is the total absorbed dose - not the flux. Additionally, this is for the entire system - not just the CPU. It's not just "here is a redundant CPU" but also "here is the entire system that the CPU is part of" that is redundant. The JWST is using a RAD750 with 44 MB of ram... and yea, that's not a lot, we're not trying to ship super computers out there. The alternative that you're suggesting is suggesting putting those all in multiple out in space and checking them against each other constantly while the system is in use collecting data. The power budget is slim and also coupled to the heat dissipation budget. The JWST has a tighter heat dissipation budget than power budget... and the Perseverance has a tighter power budget (total rover power budget is 110 watts). The challenge I'd pose is "what other processor arrangement fits within its power budget?" I'm also going to point out that I got the numbers wrong for Perseverance's power budget for its CPU. The single board is 10 watts, the cpu is 5 watts. https://en.wikipedia.org/wiki/RAD750 https://en.wikipedia.org/wiki/RAD750 > The CPU can withstand an absorbed radiation dose of 2,000 to 10,000 grays (200,000 to 1,000,000 rads), temperatures between −55 °C and 125 °C, and requires 5 watts of power. The standard RAD750 single-board system (CPU and motherboard) can withstand 1,000 grays (100,000 rads), temperatures between −55 °C and 70 °C, and requires 10 watts of power. https://mars.nasa.gov/mars2020/spacecraft/rover/brains/ https://mars.nasa.gov/mars2020/spacecraft/rover/brains/ > Unlike people and most animals, the rover's brains - its computer - are in its boxy body. The computer module is called the Rover Compute Element (RCE) - there are actually two identical RCEs in the body so there is always a spare "brain." Also - https://space.stackexchange.com/questions/50470/what-makes-insights-rad750-processor-so-radiation-resistant-compared-to-1998 https://space.stackexchange.com/questions/50470/what-makes-i... These are modern chips that are designed for that efficiency - it's just that there are other constraints too. You are unlikely to find a lower wattage processor that you can run with redundancy that is able to tolerate space travel. https://link.springer.com/article/10.1007/s12567-016-0138-0 https://link.springer.com/article/10.1007/s12567-016-0138-0 Radiation Effects and COTS Parts in SmallSats https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2934&context=smallsat https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2... Note that the cubesat has an average lifespan of about a year.

naikrovek 4y ago

i guarantee the people building the spacecraft and choosing the CPUs have thought about this more than you, and have much more knowledge about the problem space than anyone here questioning their decision-making processes. if it was a good idea to have multiple redundant CPUs on these spacecraft, the spacecraft would have multiple redundant CPUs.

shagie 4y ago
I will note that they do have multiple, redundant CPUs... they just aren't all active at once. https://www.jpl.nasa.gov/news/press_kits/mars_2020/landing/mission/spacecraft/perseverance_rover/ https://www.jpl.nasa.gov/news/press_kits/mars_2020/landing/m... > Just like Curiosity, Perseverance has two main computers, or “brains”: One is active at any given time, while the other serves as a backup. Both use radiation-hardened RAD750 computers, and together, they’re referred to as Rover Compute Element A and B. > Another RAD750 serves as a third “brain,” on Perseverance. Called the Vision Compute Element, it is equipped with a special card for analyzing images. This card has a Virtex-5 field programmable gate array (FPGA) and is the driving force behind Terrain-Relative Navigation and the Lander Vision System, which analyzes images of the landing site during descent and compares them to an onboard map to determine the rover’s position relative to the ground. That "Virtex-5 FPGA" is an interesting one too... https://www.xilinx.com/products/silicon-devices/fpga/virtex-5qv.html https://www.xilinx.com/products/silicon-devices/fpga/virtex-... Where it is listed as "Space-grade Virtex-5QV FPGA"
incrudible 4y ago
I said I do not buy the power argument and I received two high quality comments explaining issues beyond power consumption. Your appeal to authority adds nothing of value.