10 ms·

The RAD750 (edit - the whole RAD family, there are newer models available) remains the standard because it's the highest performance rad-hard design available,

The RAD750 (edit - the whole RAD family, there are newer models available) remains the standard because it's the highest performance rad-hard design available, period. If you're putting an expensive satellite in orbit for 5,10 years, the cost of the processors is insignificant compared to everything else. The real problem is that we don't have good solutions for improving the performance of rad-hard designs, so we're stuck with older, larger process sizes that limit what can be implemented. Look at the lengths involved in getting A* to run on Curiosity, and you see just how limiting the hardware is. Everyone, Nasa especially, wants more compute available. In low earth orbits and shorter mission durations, you can get away with redundant hardware instead of rad-hard. Most of the damage done by radiation is upsets, so you can reboot the affected hardware and keep going. But on an unprotected design some of the damage can be permanent, and thus redundancy alone isn't enough for longer/farther missions.

formerly_proven 6y ago
> The RAD750 remains the standard because it's the highest performance rad-hard design available, period. RAD5500?
- cpgxiii 6y ago
  You're right, the RAD5500 and family are available. I should have said the whole BAE RAD family. The reality hasn't changed much, though, there's really only one game in town for high rad-hard performance, and it's still well behind conventional processors.
runeb 6y ago
Could they not offload a lot of compute to ground based computers and submit results back via radio? Or are these real-time applications?
- senkora 6y ago
  For Mars, at least, that would be tens of minutes round trip because of the speed of light. It works for some things, but for pathfinding it isn’t a great fit. The other issue is bandwidth between the craft and Earth, which is quite limited. Maybe there would be benefits to a “orbiting datacenter” around Mars carrying a bunch of rad-hardened compute? I assume NASA has considered this and decided it would be a bad idea.
- deleted 6y ago
  [deleted]
- arjun-menon 6y ago
  I was thinking, instead of that, what if you had a separate isolated tiny computer on spacecraft, that was powered by its solar panels (so there's no electrical wiring, or other connection to it), and have its own radio. And this separate computer could use the latest bleeding-edge CPU, and be encased in a radiation-hardened shell. It would use its radio to talk to the slower main computer, and do math really fast locally, and if need be, beam the results to Earth, or to a nearby orbiting satellite.
  - angry_octet 6y ago
    Unfortunately there isn't really any practical way to have a radiation hardened shell that is sufficiently effective. Eg. 5cm of aluminium stops only 30% of the galactic radiation. (Heavier elements (e.g. gold) are scattered by incoming particles causing incoming heavy ions which cause even more damage.) So practically it would still experience significant radiation. But having the main compute for Mars remain in orbit with the relay isn't a bad idea.
- cpgxiii 6y ago
  The whole point of implementing A* on Curiosity was to give it some navigation autonomy. The time delay in getting sensor data back to earth, coming up with a motion plan, then sending the plan back to be executed imposes tight limits on how fast the rover can drive, what kinds of terrain it can cover, and ultimately how much science can be done. Local autonomy for basic "go over to than weird-looking rock" tasks is a major improvement.
  - gorgoiler 6y ago
    You could A* your way around the whole planet by using an Earth based computation but only if you knew where every rock was. There must be some equation of motion in space robots that combines terrain difficulty, robot speed, round trip time to Earth, and how far ahead you’d need to be able to see. Curiosity moves about as fast as a Roomba. The ping is (min/avg/max) 10’/24’/40’. Ergo, it needs to be able to see X yards ahead of itself to plan A* from Earth, requiring a camera boom Y feet tall producing images with Z megapixels of resolution. I wonder what X, Y and Z are.
    - throwaway189262 6y ago
      Does curiosity really move that fast? Roomba is maybe 2 mph. I thought curiosity was closer to 0.1 mph
      - JohnJamesRambo 6y ago
        His or her estimate seems wildly off. I had the same response because my Roomba moves fast! Some quick googling says- Curiosity max speed equals 0.08699 mph. Roomba equals a foot per second which is 0.682 mph. Off by about 7.82x.
        gorgoiler 6y ago
        Thanks: my estimate of Roomba slowness was way off. I’ve only ever seen them on TV. I remember once hearing that Curiosity, flat out, could do 1km a day.
- throwaway189262 6y ago
  Shannon limit implies a linear relation between bit rate and transmit power. The only way to get fast enough transfer would be to spend tons of the power budget on radio
kevin_thibedeau 6y ago
I had to select a processor that controls the camera in the GOES-R ABI. The image processing is all done by custom hardware so all that was needed was microcontroller level performance. It turns out there are very limited options in this space and all of them are quirky outdated architectures with limited available tooling. The RAD750 in particular is a bit of a nightmare because of the high pin count, need for a support chip, and the 32-bit bus forces the use of more RAM and ROM than a smaller micro would need. I took a pass on that. I also never have liked IBM's reverse bit numbering and the implications it has on SRAM power consumption.
- trzy 6y ago
  How does the PowerPC’s reverse bit numbering impact power consumption?
  - kevin_thibedeau 6y ago
    If you wire the bits as numbered to a conventional memory device designed with LSB as bit-0, the internal address bus will induce more switching from sequential access than normal. The internal row and column decoders will be working overtime consuming more power than necessary. Reversing the bus to deal with that isn't always straightforward on a space constrained board.
    - trzy 6y ago
      If I understand correctly, you are saying that with the reversed bit numbering, bit 31 (in a 32-bit address bus) changes most frequently with sequential accesses because it is the LSB but when wired to the MSB of SRAM, it causes switching in the column decoder for every single access. That makes sense but I didn’t realize that it was difficult to simple swap the wiring. Are the physical pins ordered backwards as well (that is, do PowerPC’s A31 and A30 appear where A0 and A1, respectively, would be on a “normal” system)?
      - yholio 6y ago
        I have absolutely no idea why someone would connect the upper bits of the CPU address bus to the lower bits of the memory, if this is what the GP refers to. Their naming scheme seems irrelevant. Almost all modern memory is built in a large matrix where the upper bits select the row into a buffer and the lower bits control a multiplexer that selects a slice of that row. Scanning incrementally through the memory will hit the fast multiplexer path and result in much faster access. Propagating into the whole matrix at each increment is not only a power draw but a massive slowdown.
nzentzis 6y ago
> Look at the lengths involved in getting A* to run on Curiosity Can you link to something that goes into detail? Googling it doesn't turn up anything relevant, but it sounds like it'd be interesting to read about.

formerly_proven 6y ago

> The RAD750 remains the standard because it's the highest performance rad-hard design available, period. RAD5500?

cpgxiii 6y ago
You're right, the RAD5500 and family are available. I should have said the whole BAE RAD family. The reality hasn't changed much, though, there's really only one game in town for high rad-hard performance, and it's still well behind conventional processors.

runeb 6y ago

Could they not offload a lot of compute to ground based computers and submit results back via radio? Or are these real-time applications?

senkora 6y ago
For Mars, at least, that would be tens of minutes round trip because of the speed of light. It works for some things, but for pathfinding it isn’t a great fit. The other issue is bandwidth between the craft and Earth, which is quite limited. Maybe there would be benefits to a “orbiting datacenter” around Mars carrying a bunch of rad-hardened compute? I assume NASA has considered this and decided it would be a bad idea.
deleted 6y ago
[deleted]
arjun-menon 6y ago
I was thinking, instead of that, what if you had a separate isolated tiny computer on spacecraft, that was powered by its solar panels (so there's no electrical wiring, or other connection to it), and have its own radio. And this separate computer could use the latest bleeding-edge CPU, and be encased in a radiation-hardened shell. It would use its radio to talk to the slower main computer, and do math really fast locally, and if need be, beam the results to Earth, or to a nearby orbiting satellite.
- angry_octet 6y ago
  Unfortunately there isn't really any practical way to have a radiation hardened shell that is sufficiently effective. Eg. 5cm of aluminium stops only 30% of the galactic radiation. (Heavier elements (e.g. gold) are scattered by incoming particles causing incoming heavy ions which cause even more damage.) So practically it would still experience significant radiation. But having the main compute for Mars remain in orbit with the relay isn't a bad idea.
cpgxiii 6y ago
The whole point of implementing A* on Curiosity was to give it some navigation autonomy. The time delay in getting sensor data back to earth, coming up with a motion plan, then sending the plan back to be executed imposes tight limits on how fast the rover can drive, what kinds of terrain it can cover, and ultimately how much science can be done. Local autonomy for basic "go over to than weird-looking rock" tasks is a major improvement.
- gorgoiler 6y ago
  You could A* your way around the whole planet by using an Earth based computation but only if you knew where every rock was. There must be some equation of motion in space robots that combines terrain difficulty, robot speed, round trip time to Earth, and how far ahead you’d need to be able to see. Curiosity moves about as fast as a Roomba. The ping is (min/avg/max) 10’/24’/40’. Ergo, it needs to be able to see X yards ahead of itself to plan A* from Earth, requiring a camera boom Y feet tall producing images with Z megapixels of resolution. I wonder what X, Y and Z are.
  - throwaway189262 6y ago
    Does curiosity really move that fast? Roomba is maybe 2 mph. I thought curiosity was closer to 0.1 mph
    - JohnJamesRambo 6y ago
      His or her estimate seems wildly off. I had the same response because my Roomba moves fast! Some quick googling says- Curiosity max speed equals 0.08699 mph. Roomba equals a foot per second which is 0.682 mph. Off by about 7.82x.
      - gorgoiler 6y ago
        Thanks: my estimate of Roomba slowness was way off. I’ve only ever seen them on TV. I remember once hearing that Curiosity, flat out, could do 1km a day.
throwaway189262 6y ago
Shannon limit implies a linear relation between bit rate and transmit power. The only way to get fast enough transfer would be to spend tons of the power budget on radio

kevin_thibedeau 6y ago

I had to select a processor that controls the camera in the GOES-R ABI. The image processing is all done by custom hardware so all that was needed was microcontroller level performance. It turns out there are very limited options in this space and all of them are quirky outdated architectures with limited available tooling. The RAD750 in particular is a bit of a nightmare because of the high pin count, need for a support chip, and the 32-bit bus forces the use of more RAM and ROM than a smaller micro would need. I took a pass on that. I also never have liked IBM's reverse bit numbering and the implications it has on SRAM power consumption.

trzy 6y ago
How does the PowerPC’s reverse bit numbering impact power consumption?
- kevin_thibedeau 6y ago
  If you wire the bits as numbered to a conventional memory device designed with LSB as bit-0, the internal address bus will induce more switching from sequential access than normal. The internal row and column decoders will be working overtime consuming more power than necessary. Reversing the bus to deal with that isn't always straightforward on a space constrained board.
  - trzy 6y ago
    If I understand correctly, you are saying that with the reversed bit numbering, bit 31 (in a 32-bit address bus) changes most frequently with sequential accesses because it is the LSB but when wired to the MSB of SRAM, it causes switching in the column decoder for every single access. That makes sense but I didn’t realize that it was difficult to simple swap the wiring. Are the physical pins ordered backwards as well (that is, do PowerPC’s A31 and A30 appear where A0 and A1, respectively, would be on a “normal” system)?
    - yholio 6y ago
      I have absolutely no idea why someone would connect the upper bits of the CPU address bus to the lower bits of the memory, if this is what the GP refers to. Their naming scheme seems irrelevant. Almost all modern memory is built in a large matrix where the upper bits select the row into a buffer and the lower bits control a multiplexer that selects a slice of that row. Scanning incrementally through the memory will hit the fast multiplexer path and result in much faster access. Propagating into the whole matrix at each increment is not only a power draw but a massive slowdown.

nzentzis 6y ago

> Look at the lengths involved in getting A* to run on Curiosity Can you link to something that goes into detail? Googling it doesn't turn up anything relevant, but it sounds like it'd be interesting to read about.