Displaying orbits

[ajtribick]

Having played around with the code, I still have to say I still don't like the effect for nearly elliptical orbits, though it would certainly be useful for things like orbits of moons displayed in a heliocentric frame. Having the fade effect ending at a different point to the planet looks wrong too: I thought it was a bug until I looked more carefully at the code and saw the parameter.

One reason I decided to release the Gliese 876 add-on was to provide an example of a rapidly-evolving system to investigate how well the fade effect represents it: unfortunately no fading is applied to the xyzv trajectory (no ability to specify an orbital period here).

[chris]

Having played around with the code, I still have to say I still don't like the effect for nearly elliptical orbits, though it would certainly be useful for things like orbits of moons displayed in a heliocentric frame. Having the fade effect ending at a different point to the planet looks wrong too: I thought it was a bug until I looked more carefully at the code and saw the parameter.

One reason I decided to release the Gliese 876 add-on was to provide an example of a rapidly-evolving system to investigate how well the fade effect represents it: unfortunately no fading is applied to the xyzv trajectory (no ability to specify an orbital period here).

It's not a problem to enable fading for non-periodic trajectories provided that there's some way for either users or add-on creators to specify an appropriate time window. We should add a Period parameter to SampledTrajectory for cases when an xyzv file is used for an approximately periodic orbit like in your Gliese 876 add-on. For non-periodic trajectories, I'm not sure... One possibility would be to offer provide the user with two settings: a duration of visibility for all non-periodic orbits, and a boolean setting that can override the duration of visibility and show the entire orbit. A more comprehensive plotting UI would also be useful, but it seems like we should include some easy-to-use settings that cover common usages.

--Chris

[abramson]

Hi. I have noticed that the code for orbit fading is included in the latest SVNs. How can activate it for testing? Is it a switch in the compiler? (I'm using VC++ 2008 Express, Windows 7). Thanks.
Guillermo

[chris]

Hi. I have noticed that the code for orbit fading is included in the latest SVNs. How can activate it for testing? Is it a switch in the compiler? (I'm using VC++ 2008 Express, Windows 7). Thanks.

Guillermo,

At the moment, you have to edit render.cpp. There are are three constants you can adjust, all pretty well documented in the code starting at line 1953. For a start, try setting OrbitWindowEnd to 0.0 and LinearFadeFraction to 0.5.

Then, if you want to visualize orbits over multiple revolutions, adjust the setting of OrbitalPeriodsShown to a value greater than 1.0. When viewing multiple orbits, I've found that it looks better when fading occurs over the whole path, so set LinearFadeFraction to 1.0.

--Chris

[abramson]

Thanks. Done. Beautiful. It also serves to see if you are watching from the north or from the south.
Guillermo

[duds26]

I would like to see a tabbed render menu.
With tabs like general, orbits.
Because the options will only increase in the future.
I also second Guckytos opinion about general rules and per-object override.

[BobHegwood]

I would like to see a tabbed render menu.
With tabs like general, orbits.
Because the options will only increase in the future.
I also second Guckytos opinion about general rules and per-object override.

For whatever it's worth here, I agree with duds, especially concerning the tabbed render menu.

Thanks, Brain-Dead

[lithium3]

I cant find the *.CPP

and i get a run time error if i pout it in the *.cfg file it loocks so cool i just cant see it
eney help?

I'm not a Developer of celestia i only do *.SSC textures 3DS and scripts.

[abramson]

I'm not a Developer of celestia i only do *.SSC textures 3DS and scripts.

Li, that explanation was to make a custom compilation of Celestia, for which you know all the source files (the cpp's, h's, etc), plus a C++ compiler. If you are not into it, you can safely wait a little and the feature will come to a public release. The fading orbits are nice, but you can definitely wait for them.

Guillermo

PS: in the wikibook there are instructions to compile celestia yourself as well as in my page (for windows).

[chris]

Another orbital evolution example... 20 orbits of the International Space Station are shown here, with fading enabled to indicate time elapsed. The precession of ISS's inclined orbit is very apparent here.

iss-orbits.jpg

Note that for this image, I did not use the Celestia's default ISS orbit, which is just a simple ellipse. I'm testing some new code that calculates object positions from NORAD two-line elements. You can see that simple elliptical elements are inadequate for determining the location of satellites in low Earth orbit--the 20 orbits shown in the image are the trajectory of ISS over 30 hours.

--Chris

[Hungry4info]

What causes that precession of ISS's orbit? Oblateness of Earth?

[chris]

What causes that precession of ISS's orbit? Oblateness of Earth?

Yes, the Earth's oblateness. The rate of precession is dependent on the inclination and semi-major axis. This is to good effect in sun-synchronous orbits, where the satellite's orbit precesses at the same rate the Earth orbits the Sun.

--Chris

[CAP-Team]

I've been experimenting with orbits of spacecraft, for example, Cassini.

For each spice kernel, I entered the orbital period in days. Celestia now always wants to close the orbit, while displaying just the part the kernel covers would be nicer.
Any chance this can be improved (or use a switch or something)

Another idea would be to draw the orbit of a body a selectable amount of days in front and behind a body.

[MKruer]

Chris,

I really like idea of fading, however I was also wondering if its possible to add ticks to the line. Something to see that the body is moving in addition to the fade.

ticks depending on distance viewable could be in second, minute, hour, day, year, decade, etc...

Just an idea

[scroll]

How do you turn fading on/off? I have solid line orbits and can find no reference to fading and orbit paths in 1.6.0. Thanks.

[abramson]

How do you turn fading on/off? I have solid line orbits and can find no reference to fading and orbit paths in 1.6.0. Thanks.

Hi, Scroll. Fading orbits are still not in the official release version of Celestia. You will have to wait until the next version.

You can try this feature by compiling Celestia yourself from the development sources. If you use Windows and you don't know how to compile Celestia, you can use one of my pre-compiled ones. Download from here the most recent one and unpack inside your Celestia folder (take the precaution of renaming the executable, so you do not overwrite the celestia.exe you already have there; any name will do).

Guillermo

[scroll]

How do you turn fading on/off? I have solid line orbits and can find no reference to fading and orbit paths in 1.6.0. Thanks.

Hi, Scroll. Fading orbits are still not in the official release version of Celestia. You will have to wait until the next version.

You can try this feature by compiling Celestia yourself from the development sources. If you use Windows and you don't know how to compile Celestia, you can use one of my pre-compiled ones. Download from here the most recent one and unpack inside your Celestia folder (take the precaution of renaming the executable, so you do not overwrite the celestia.exe you already have there; any name will do).

Guillermo

Thanks Guillermo. It makes sense now.
You wouldn't happen to know how to sync a real-time orbit with that awesome ISS model that's available, woud you? (viewtopic.php?f=6&t=13360&start=120#p122203)
It's .ssc file is not accepting the usual edit...

[abramson]

how to sync a real-time orbit with that awesome ISS model that's available, woud you? (viewtopic.php?f=6&t=13360&start=120#p122203)
It's .ssc file is not accepting the usual edit...

If you mean making the simulated movement of the model ISS reflect the actual position of the station, you can't do it just within Celestia. The orbit of the ISS is very variable and you can't have a fixed set of parameters represent it reliably. The programs that do it (Cartes du Ciel, for example) need to update the data for the orbit frequently to do it succesfully.

Of course, you could do it in principle: you need to donwload these data (in a form usually called "two-lines elements" (easily available) and convert them to a keplerian orbit for use in Celestia. That's perfectly possible, and I'm sure someone must have done it: a lookup of "tle to ssc celestia" came up with this old thread where there seems to be a Perl script to do it. You could even automatize it so every time you launch Celestia, you could have the right orbit already set up.

I do not understand the "It's .ssc file is not accepting the usual edit" part of your post. Every ssc file can be edited with a text editor.

Cheers,

Guillermo

[scroll]

how to sync a real-time orbit with that awesome ISS model that's available, woud you? (viewtopic.php?f=6&t=13360&start=120#p122203)
It's .ssc file is not accepting the usual edit...

If you mean making the simulated movement of the model ISS reflect the actual position of the station, you can't do it just within Celestia. The orbit of the ISS is very variable and you can't have a fixed set of parameters represent it reliably. The programs that do it (Cartes du Ciel, for example) need to update the data for the orbit frequently to do it succesfully.

Of course, you could do it in principle: you need to donwload these data (in a form usually called "two-lines elements" (easily available) and convert them to a keplerian orbit for use in Celestia. That's perfectly possible, and I'm sure someone must have done it: a lookup of "tle to ssc celestia" came up with this old thread where there seems to be a Perl script to do it. You could even automatize it so every time you launch Celestia, you could have the right orbit already set up.

I do not understand the "It's .ssc file is not accepting the usual edit" part of your post. Every ssc file can be edited with a text editor.

Cheers,

Guillermo

Sorry Guillermo I didn't explain it well at all. I can update the ISS's position using the procedure from this post (viewtopic.php?f=2&t=12582&hilit=iss+position#p105240). In comparing with the ISS tracking site Heavens Above it appears to be an accurate procedure, although I appreciate updating fairly regularly is required as you mention. The ISS.ssc file that I modify (edit) is the one that comes with Celestia 1.6.0.

However if I try the same procedure with the relevant ISS.ssc file for the new ISS model avaiable from Linuxman then it doesn't work, Celestia doesn't even display the ISS let alone have it in the correct orbit. So I have to revert back to using the Celestia default ISS.

As you know your way around the Program I thought I'd take the opportunity to ask. A bit off topic for this thread, hopefully Linuxman will reply over on his thread.

[scroll]

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