Andy Puckett Nov 11, 2014
On Tue, Nov 11, 2014 at 8:55 PM, Bill Gray pluto@... [find_orb] <find_orb@yahoogroups.com> wrote:Hi Peter, Tony,
Yup, you're right; Find_Orb is measuring the uncertainty position
angle in the wrong direction. I've checked the code, and I see
exactly where I got the negative sign in there, and have fixed it.
I'll post this in a bit... there are a few things I should clean up
before I post an update.
There will still be some ambiguity for more precise cases; as
Tony mentioned, when the uncertainty is small, the uncertainty
"ellipse" becomes more of an uncertainty "circle". There will also
sometimes be a disagreement of 180 degrees, because if the
uncertainty ellipse sticks out, say, a little north of due
east at position angle 73 degrees, it also sticks out a little south
of due west at position angle 253. Find_Orb always gives a value
between 0 and 180 degrees; MPC sometimes picks the westward-aiming
side of the uncertainty ellipse.
As Tony points out, you also get some differences between Find_Orb,
MPC and NEODyS because of different assumed uncertainties in the
observations, and because Find_Orb accounts for timing errors,
over-observing, and blunder management. As far as I know, NEODyS
has an over-observing scheme that gives lower weight to a station
that has submitted many observations in one night; but I don't think
they include timing errors or blunder management. I don't think MPC
includes any of these effects.
For objects with significant (more than a few arcseconds) uncertainty,
you'll mostly see disagreements in the magnitude of the uncertainty.
We'll usually have roughly the same position angle of uncertainty...
that is, we'll all tell you, "look a little north of due east and
a little south of due west", but we won't quite agree on just how far
out you'll need to look.
-- Bill
On 11/11/2014 03:31 PM, lists@... [find_orb] wrote:
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> Tony,
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> I probably shouldn't have confused things with 2014 VQ! All the other objects I've tried (single opp NEOs last observed a few years ago) like 2009 VZ have very stable PAs of uncertainty and as originally mentioned, MPC and NEODyS agree closely, FO doesn't.
>
> I think you're very likely right that FO is counting these uncertainty PAs clockwise from N, the opposite to the MPC, even though the direction of motion on the sky matches the MPC values and is counting anticlockwise from N.
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> Peter
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> ---In find_orb@yahoogroups.com, <tony.evans@...> wrote :
>
>
> Peter, I think the problem with a situation like this that the uncertainty is small (FO says 0.1") and the PA of the uncertainty is itself uncertain. It can vary hugely depending on exactly which set of observations FO/MPES are using. The frequent arrival of new observations and revisions of the elements will cause big changes in the uncertainty PA.
>
> When using FO, the exact settings will also cause differences - contents/use of sigma.txt file. Filter setting, time of download of obs DB etc.
>
> This morning (9:00 UT) I get an uncertainty PA of 23.0 from the MPES for J95 at 1:00 on 11/11.
>
> Using FO, and depending on which observations I select, sigma.txt, filter, I can get uncertainty PAs that range from 4 to 157 for J95 at 1:00 on 11/11.
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> The objects I originally tried had big uncertainties 2014 MQ67 and 2011 JU2 so the exact settings make little difference and MPES/FO should get near identical results.
>
> Tony
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>