raptop changed the topic of #kspacademia to: https://gist.github.com/pdn4kd/164b9b85435d87afbec0c3a7e69d3e6d | Dogs are cats. Spiders are cat interferometers. | Космизм сегодня! | Document well, for tomorrow you may get mauled by a ネコバス. | <UmbralRaptor> egg|nomz|egg: generally if your eyes are dewing over, that's not the weather. | <ferram4> I shall beat my problems to death with an engineer. | We can haz pdf
<egg|laptop|egg>
bofh: meow
<egg|laptop|egg>
bofh: hm, TLE-to-cartesian then integrate might not be ideal for finely synchronized satellites, apparently the uncertainties on TLEs are km-level
<egg|laptop|egg>
bofh: e.g. for FormoSat-2, I can get it nearly-phased if I multiply the velocity by (1 - 0.00014) from what I get out of the TLEs
<egg|laptop|egg>
(that's a bit below 1 m/s)
<egg|laptop|egg>
(it was probably phased since the TLE is before decommissioning)
<egg|laptop|egg>
bofh: meow
<bofh>
egg|laptop|egg: like the TLEs just before decommissioning are prolly complicated but also largely irrelevant, since they're prolly for when it's shifting into a graveyard/disposal orbit
<bofh>
as for book idées, uh
<egg|laptop|egg>
bofh: before you ask I have both Muller books
<egg|laptop|egg>
though probably first editions whereas they're on 2nd and 3rd apparently?
<egg|laptop|egg>
bofh: anything in numerics?
<bofh>
I get the feeling you have all the essentials? Like if you *don't* have a copy of Golub & van Loan and can get it cheaply, do so, but I swear you already do.
<egg|laptop|egg>
bofh: hm, I'm not sure I have Golub and van Loan
<egg|laptop|egg>
I have higham
<egg|laptop|egg>
bofh: have you tweeted your Golub and van Loan picture on main, I forget
<egg|laptop|egg>
bofh: there are some hairer books on geometric numerical integration
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<egg|laptop|egg>
bofh: do you know how much difference there is between the third and first editions of Muller's Elementary Functions?
<bofh>
I very highly doubt there's mich of a difference.
<bofh>
00:35:08 < egg|laptop|egg> bofh: have you tweeted your Golub and van Loan picture on main, I forget
<bofh>
I don't think so.
<bofh>
I prolly should at some point.
<egg|laptop|egg>
bofh: do eeeeeeet
<egg|laptop|egg>
bofh: also should I get Ignition! in hardcover or paperback
<bofh>
Either, but if you can afford the hardcover, why not?
<egg|laptop|egg>
yeah probably
<egg|laptop|egg>
bofh: the paperback is "Publisher: RUTGERS UNIV PR; None edition (30 April 2018)" whereas the hardcover is "Publisher: Rutgers University Press; None edition (23 May 2018)"
<egg|laptop|egg>
None edition! :-p
<bofh>
rofl None Edition
<bofh>
None Book with Left Edition
<egg|laptop|egg>
bofh: unfortunately the Capderou book only eggsists in paperback
<egg|laptop|egg>
even though it's 862 pages (whereas Ignition! is 302)
* egg|laptop|egg
stares in confusion
<egg|laptop|egg>
it does exist in hardcover in english on springer's website
<egg|laptop|egg>
... but not in french
<egg|laptop|egg>
the translator binds the book i guess
<egg|laptop|egg>
"Sorry, this item can't be sent to your selected address" fuuuuck
<bofh>
In Zürich?
<egg|laptop|egg>
wait they can't even send it to normandy?
<egg|laptop|egg>
they're a french bookstore O_o
<egg|laptop|egg>
"No sellers are currently delivering this item to Switzerland"
<egg|laptop|egg>
"No sellers are currently delivering this item to France - Mainland"
<egg|laptop|egg>
um
<egg|laptop|egg>
bofh: okay so for this book (in french!) there's one bookstore in france that ships to neither france nor switzerland, and the other that does ship to switzerland but sells it > twice the price of the other
<egg|laptop|egg>
bofh: I guess it's cheaper if you can't buy it
<UmbralRaptop>
None to mainland France, so corsica?
<egg|laptop|egg>
Hah
<bofh>
01:09:35 < egg|laptop|egg> "No sellers are currently delivering this item to France - Mainland"
<bofh>
what on *earth*?
<egg|laptop|egg>
bofh: no but the one that sells it for double the price does work, it's just amazon being bad at aggregating on the page for the book
<egg|laptop|egg>
but the fact that one of the sellers ships nowhere is weird
<bofh>
which is the other seller, out of curiosity?
<egg|laptop|egg>
bofh: well, it's a couple months before decommissioning
<egg|laptop|egg>
(re. the TLEs)
<egg|laptop|egg>
bofh: but also TLE-to-cartesian-then-integrate probably has more errors than TLE-evolved-by-SGP-4, because TLE-to-cartesian has large errors even at epoch
<egg|laptop|egg>
I'll request TLEs from 2010 to see what those look like
<bofh>
I wonder how much you can simplify SGP4 and still get reasonable results, particularly for systems we care about (since I know there are certain terms there that effectively are zero for most sats).
<egg|laptop|egg>
bofh: well I'm largely not interested in SGP4 anyway for principia porpoises
<egg|laptop|egg>
the only reason I'm touching TLEs is to use real satellites as test cases
<egg|laptop|egg>
the only thing principia does is numerical integration, no cheesy analytic models
<bofh>
I mean Principia *should* start using analytic models when numerical integration itsself stops sufficing, that would improve long-term accuracy :p
<egg|laptop|egg>
bofh: O_o
<egg|laptop|egg>
bofh: the analytic models (at least for artificial satellites) are eggstremely *short-term* in their validity
<bofh>
I can't see them being *worse* than just integrating the Keplerian elements, since the whole point is they're corrections *to* the integration of the Keplerian elements.
<egg|laptop|egg>
..... wat
<egg|laptop|egg>
bofh: when I say principia does numerical integration, I mean in the full-potential problem
<egg|laptop|egg>
bofh: which (ignoring atmospheric effects) is what SGP-4 and the like tries to analytically approximate
<bofh>
OH, right, in the full-potential problem, not the simple case I was thinking of.
<bofh>
Right.
<bofh>
Disregard. Yeah, that has questionable utility for Principia, then.
<egg|laptop|egg>
bofh: evolving a Kepler orbit hardly counts as integrating
<egg|laptop|egg>
I mean, you could do it by integration
<egg|laptop|egg>
also twitter informs me that i have had an account for five years
<bofh>
not bad
<bofh>
how old is mine I wonder
<egg|laptop|egg>
gah, Rosetta was 5 years ago?!
<bofh>
6 years this May, apparently.
<egg|laptop|egg>
wait, 6? I'm confused
<egg|laptop|egg>
ah you mean your twitter account not Rosetta
<egg|laptop|egg>
also I'm confused as to why I created this account
<egg|laptop|egg>
I thought it was to follow the rosetta coverage from elakdawalla, but the encounter was august
<kmath>
<jamesheathers> I'm going to write a browser extension that replaces "wellness guru" with "hammering twat" to make things like this… https://t.co/3X4DqSP8Gu
<bofh>
I created mine b/c following Oona Räisänen and Chris Hadfield by hand was getting annoying.
<bofh>
Don't know why I started posting on it tho.
<egg|zzz|egg>
bofh: it took me ages to actually follow people (and I still don't use the timeline), I just created the account to not get the reminder to do so :-p
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<egg|cell|egg>
Zzz
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* UmbralRaptop
>_<
<UmbralRaptop>
So, apparently 1) a 46 was almost passing my QM qual (cutoff was 50ish), and 2) only ~25% of students taking the QM qual this semester passed.
<SnoopJeDi>
jeez
<UmbralRaptop>
After additional searching (with my advisor), we determined that Sakurai does go over parity operators. But my edition carefully avoids showing this in the table of contents or the index. >_<
<bofh>
bloody hell, I have that exact edition, I think it was fixed in a later one.
<bofh>
(or had, rather)
<bofh>
also jfc
<bofh>
like, my hot take is a qual so difficult only a quarter of people pass is garbage.
<SnoopJeDi>
the very existence of quals tells me that a department is insecure about the coursework's ability to teach students what they're expected to know
<SnoopJeDi>
(insecurity in academia? why, I never!)
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<UmbralRaptop>
If anything, the flaw is in *newer* editions (advisor's copy is fine)
<SnoopJeDi>
i.e. the not-Sakurai ones?
<bofh>
Huh, that might make sense, I think mine was a 2nd-ed. I should check what's in the library copy of Sakurai.
<bofh>
(or just libgen all the editions and compare).
<SnoopJeDi>
bofh, that'd be a good for-nerds startup idea.
<SnoopJeDi>
some kind of system that kludges multiple versions together so you get a map of the text over time
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<SnoopJeDi>
bofh, Science Friday has started a second run of their etymology series, and TIL that "chocolate" is an indigenous word that for some reason never really got Anglicized
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<egg|zzz|egg>
meow
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<SnoopJeDi>
what the heck, 2014 MU69 is a pancake?!
<kmath>
<✔Alex_Parker> So if you already thought that 2014 MU69 looked a little ... odd? You haven't seen anything yet. Our latest data si… https://t.co/rYBdvXrkTC
<SnoopJeDi>
take the mental model you have and squish it perpendicular to the binary-contact axis o.O
<bofh>
egg|laptop|egg: ooh excellent, I need to try now.
<egg|laptop|egg>
bofh: you need piping bags to extrude the dough and filling, other than that no fancy equipment required
<egg|laptop|egg>
ok from those 2010 TLEs ιξιων computes κ = 14.000110
<egg|laptop|egg>
let's see what I get
<bofh>
egg|laptop|egg: yeah I think I even have some piping bags here somewhere; if not they're easy to acquire.
<UmbralRaptop>
Today in colloquia: Professor Nikolic (BSc from Belgrade) introduces Professor Nikolic (BSc from Belgrade).
<egg|laptop|egg>
UmbralRaptop: not Nikolić?
<UmbralRaptop>
yes that. Unicode is hard
<egg|laptop|egg>
hm, I get κ = +14.002761(89)
<SnoopJeDi>
UmbralRaptop, that professor appears to be degenerate
<egg|laptop|egg>
in both cases (2010 and 2016) ιξιων in much closer to the nominal κ
<egg|laptop|egg>
bofh: how are TLEs actually computed
<egg|laptop|egg>
as in, from what kind of observations
<bofh>
I'm not actually sure, I think it might strongly depend on the sat in question.
<bofh>
But their main purpose from what I gather is satellite ground tracking from ground stations.
<egg|laptop|egg>
bofh: what I'm seeing suggests that you get a much better estimate of, say, the anomalistic period from the TLEs than of the state vectors
<egg|laptop|egg>
so that if you compute the state vectors, integrate and determine the anomalistic period from that, you'll get higher errors
<bofh>
what do you mean by "state vectors" here?
<bofh>
TLEs are basically just Keplerian Elements
<egg|laptop|egg>
bofh: by state vectors I mean instantaneous position and velocity
<egg|laptop|egg>
bofh: "basically just Keplerian Elements" << yes, but *mean* elements following some theory
<egg|laptop|egg>
(Brouwer-Lyddane I think)
<bofh>
I mean yes, are there any other *sensible* types other than mean elements?
<egg|laptop|egg>
osculating
<egg|laptop|egg>
and there is an infinite supply of mean element theories, so "mean elements" covers many things
<bofh>
I mean yes, but I'm pretty sure just about any mean element theory still gives more useful Keplerian elements than using the osculating ones.
<bofh>
:p
<egg|laptop|egg>
depends what you want
<egg|laptop|egg>
mean elements are utterly useless if you want an instantaneous state, which is precisely the issue here
<bofh>
I mean yes, but you very rarely want an instantaneous state, you're usually concerned with a state propagating over some time interval of at the least, a few hours.
<egg|laptop|egg>
bofh: unless of course you're trying to have something interchangeable from one model to another
<egg|laptop|egg>
bofh: but my question is, since it seems that the actual periods (as in node-to-node times, or perigee-to-perigee times) can be recovered more accurately from the TLEs than by TLEs->instantaneous state vectors->numerical integration and period measurement, are the TLEs computed directly from such period measurements (rather than say, instantaneous ranging observations)?
<bofh>
(Also yes, it appears TLEs usually use Brouwer-Lyddane)
<bofh>
Generally, IME, yes.
<bofh>
Again, their principal use is ground station tracking of LEO sats. Most of those can't even really do instantaneous RF ranging b/c there's not really a use for it, your beamwidths are large enough you can tolerate a few arcseconds of pointing error.
<egg|laptop|egg>
bofh: OK, so the correct TLE-to-instantaneous-state transformation would be fitting the actually-observed quantities (which of course are undocumented and probably obscured somewhat in the Brouwer-Lyddane elements), rather than SGP4?
<bofh>
I *believe* so. Not 100% sure, but I'm fairly certain that is the correct approach.
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<egg|laptop|egg>
https://arxiv.org/pdf/1002.2277.pdf "Our method is essentially to use TLE data as “pseudo-observations” and to fit an orbit to these pseudo-observations using a high-precision special perturbations propagator and traditional batch least-squares differential correction."
<bofh>
rofl Constrained Least Squares Strikes Again
<egg|laptop|egg>
bofh: B787_300: UmbralRaptop: what is a typical order of magnitude for velocity corrections in stationkeeing
<egg|laptop|egg>
s/ee/eep/
<galois>
egg|laptop|egg meant to say: bofh: B787_300: UmbralRaptop: what is a typical order of magnitude for velocity corrections in stationkeeping
<B787_300>
we talking GEO or LEO
<egg|laptop|egg>
recurrent ground track LEO, but I'm interested in all of them
<egg|laptop|egg>
to clarify I mean the amplitude of a single correction, i.e. the precision to which the velocity is kept, rather than the cost over time
<B787_300>
oh dV for a single correction?
<B787_300>
a couple m/s
<B787_300>
for GEOs to a 10s of m/s for LEOs
<B787_300>
drag is a bitch
<bofh>
GEO corrections often aren't required too often due to the nature of GEO
<bofh>
and yeah, 10s of m/s for LEO
<egg|laptop|egg>
wait, even for repeat ground track applications? don't you want a fairly precise velocity there?