Re: [find_orb] Why keeping clicking in the same method yields different results?

Bill J Gray Apr 11, 2010

Hi Diego,

> I wonder why should I click several times in any given method
> (e.g. Full Step or even Auto) to obtain smaller RMS?
>
> Some cases, you even get oscillating values.

True. That usually happens when the orbit is poorly defined,
though sometimes in other cases.

Find_Orb has a lot of code that attempts to find where the
"real" minimum RMS is. Certain cases still confuse it; in some,
it doesn't even converge to the oscillating values you mention.

Best thing to do then is to switch to use of Herget steps.
These are somewhat more stable. Even then, all is not perfect;
you can get cases where things diverge badly.

Lately, I've been working a bit with a "downhill simplex"
method. (This is currently implemented only in the console version
of Find_Orb, which is where I frequently test things that eventually
make their way into the Windows GUI version of Find_Orb.) The math
behind this method is described at

http://www.fizyka.umk.pl/nrbook/c10-4.pdf

This appears to get good results in almost every case, even
when started with a completely stupid idea about what the orbit
might be. It's a little computationally intensive, though.

Probably a better method would be statistical ranging, but
I've been promising that one for a while now and just haven't
had the time to do it right.

-- Bill