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.
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)?
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.
RAD5500?
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.
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.
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.
- You want a travel speed of 0.5 m/s - Worst-case round trip time from command to result is 40 minutes, or 2400 seconds - Max distance covered is 1200 meters - Assume you can drive over any rock 10cm or less in size (the rover can do better, but at a reduced speed)
So you need to scan an area 1200m x 1200m, with range accuracy of, say, +/-5cm. Forget the camera boom height, or the time it might take to scan and process, there's no sensor that will give you that kind of accuracy. The stereo baseline would be huge, and leave you entirely at the mercy of whatever texture or not a given part of the martian surface has to offer. LIDAR is OK if you definition of "long range sensing" is larger objects at 200m. The time cost for shipping back all the raw data for processing would kill performance as well, and if you wanted to process it locally the compute requirements would be just as high as doing the planning locally as well.
Onboard, or at least much closer,compute is the only way forward in autonomy. Honestly, the best bet on improving local compute would be to send a robot bulldozer, some C4, a rack full of milspec servers, and a big RTG. Blow a nice crater, push the servers to the bottom, and bury them in dirt for shielding. If you get really lucky, you could find some old cave or lava tube.
One drawback of the remote A* approach is that you end up using more energy as the rover would have to be constantly communicating with its onboard antenna. Its relay satellites are only in range for a limited period each day. Fine grained maneuvers (like drive around that big rock to get to this small rock) would also prove difficult because of likely errors in the rover's inertial-navigation system.
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.
I remember once hearing that Curiosity, flat out, could do 1km a day.
Also it seems to be doing sometype of image processing to identify a suitable landing spot and guiding on to that point.
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.
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.
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.