timorl @timorl@qoto.org

How did spacecraft orient themselves using what was available in 1960s?

The obvious thing for things in Earth orbit is to find the Earth horizon and Sun; maybe find the direction the radio signals arrive from and/or their polarization.

If we are further from Earth, we need to figure it out before we can communicate, so the radio signals approaches don't work. Similarly, there's no horizon to speak of. Yet we managed to do it in 60s.

The first thing we can do is find Sun (simple: the brightest thing around) and Earth. How do we find Earth with no camera and no image processing? We know how far in angular distance we expect Earth to be from sun. So, we point an axis of our spacecraft at the sun and slowly rotate, while a photometer points at the correct angle off that axis. Once the photometer finds something bright enough (and not too bright), we stop the roll and keep tracking that bright thing, assuming it's Earth. We could (I don't know if that was ever done) instead use a radio receiver with an antenna that's angled in the same way.

What if the angular distance between Sun and Earth is too small (so that we wouldn't be able to get any sensible accuracy in the roll direction)? We do the same roll-and-observe-photometer trick to find Canopus, a very bright off-eclipctic star (it's in the southern hemisphere -- -50deg declination -- which is why e.g. I haven't ever knowingly seen it). It being off-eclipctic ensures that occlusions are rare, and it being _very_ bright makes it hard to confuse it with any other start (we discriminate by brightness and angular distance from the sun).

I knew about the concept of star trackers, but for a long time didn't know how they searched for the star. Picking the second brightest star (well, third) and constraining the sun-star angle is what I was missing.

Ref: https://ntrs.nasa.gov/api/citations/19750002096/downloads/19750002096.pdf (+ interesting details on how do we set up remote overrides for systems that are necessary for communication to happen using 60-70s technology

BTW This write-up ignores the questions of accuracy nearly completely; in part because I'm still confused how did the photoresistor-based sun trackers achieve an accuracy of a tenth of a degree (their fine alignment relied on driving the attitude so that two resistors' resistances were equal; I would expect their bias to creep due to some hard-to-predict differences in aging of different photoresistors, whether due to manufacting defects, or something as silly as different amounts of dust impacts, because the direction of motion wrt local dust was pointed in some slowly-changing direction wrt Sun and Canopus).

> In October 2018, Swedish ISPs were forced to block access to Sci-Hub after a court case brought by Elsevier; Bahnhof, a large Swedish ISP, in return soft-blocked the Elsevier website.

based

@humanetech

#Discourse also implementing #ActivityPub (joining the Fediverse) is great news! Thanks for sharing!

Here is a forum thread with more details on it. The linked post contains links to other related threads.

https://meta.discourse.org/t/federation-support-for-discourse/90921/7

.hg Happy Stanislav Petrov Day.

Math people either love or hate statistics.

What I have found that helps me is to think of the various theorems from the perspective of functions on spaces. Instead of the messy pseudo-math that intro courses present it as, there are underlying measure spaces. Past all the word salad, spaces are what is tossed around and modified to reason about reality.

.hg I love tooltips. It's like I'm just pointing to a UI control and asking /the software itself/ “what's this?” and /it just tells us/, and if we do that enough times with enough different controls we Learn How To Use The Thing

Important word thing

"Human" is a species
"Person" is a moral label
Not all people are necessarily humans
Stop treating these words as synonyms