4 ms·
I struggle to think that JPL didn’t think of this already. That in the distance/time code they didn’t take timestamps of each successful frame vs just assuming
by SV_BubbleTime 5y ago
I struggle to think that JPL didn’t think of this already.
That in the distance/time code they didn’t take timestamps of each successful frame vs just assuming it’s a 30hz camera so surely each frame will be 30hz!
It seems to me, as someone who does similar embedded work, that if I would account for a missing frame failure, they surely would. I am a professional in embedded timing systems - I am guessing this isn’t the full story or it’s been paraphrased for normal people and something important was lost.
EDIT: Better info below. The photo was lost but it’s timestamp was not. They apparently didn’t tightly couple the image and it’s metadata together (hashes... what are they for!?) so there was some sort of mess up there. Knew there was more to the story.
- dmurray 5y agoI wonder if hashes are discouraged at JPL due to a historical focus on "correct, and provably correct" code. Hash collisions happen. Yes, you can make collisions arbitrarily unlikely, and there are so many situations where they come in useful, but I can imagine an engineering culture where they are just never an easy tool to reach for.
- dnautics 5y agoYou can axiomatize uniqueness of hashes in proof systems.
- ethbr0 5y agoThis would require a hash input (or combination of inputs) to be unique over a limited / expected domain, right?
- ncallaway 5y agoI mean, you could take anything as an axiom so there are no real requirements for it. I would imagine you would want to demonstrate the P(collision|domain+operating lifetime) << acceptable level or risk, and after you’d done that, take the uniqueness of hashes as a axiom for the proof system? That’s just speculation on my part, since I haven’t done much with formal proofs of working code
- dnautics 5y agoNo you just special case the function call and tell the proof analyzer "this never happens". Strictly speaking, a lie, but probably a sophisticated prover could even be rigged to keep track of those conditions (not in x years, e.g.) if you so chose
- SV_BubbleTime 5y ago> arbitrarily unlikely IDK, I would call more possibilities than grains of sand in the universe a little more than “arbitrarily unlikely”.
- twhitmore 5y agoIt's not actually a time "stamp" if you don't keep it on or tightly associated.. A struct or pair of {Image, Timestamp} would be the simplest & most robust solution.
- richardfey 5y agoExactly my thoughts. This is something I would expect covered by the testing process. I wonder what the Apollo engineers would say about this.