Celestia Forums

Hi.
I'm a beginner of Celestia and I'm very impressed of it.

I want to know how precise the constellations of planets and other objects like satelites are.

I assume that a forecast of visible passes of ISS is not (yet) possible ( )
At http://www.heavens-above.com you can find out when and where you can see ISS but the result from there doesn't mach the approx. result I get from Celestia: When I set Celestia to the date and time where I expect ISS to be seen from my location ISS has been displayed very far from my location.

Who knows more about it ?

Celestia's ISS orbit is fairly old at this stage- and it doesn't update itself with time. You can get new orbital elements every day from http://www.celestrak.com , but they're in a TLE format that needs to be converted over to ssc format. Grant Hutchinson made an M$ Excel converter for TLE - ssc, i'm not sure what the download link though.

There's no auto update function, so for accurate elements you'd have to update the ssc every day...

JackHiggins wrote:Grant Hutchinson made an M$ Excel converter for TLE - ssc, i'm not sure what the download link though.

It's here:
http://www.lns.cornell.edu/~seb/celestia/hutchison/spreadsheets.html#2

Grant

thank you guys.
it would be great if updating orbital data for sattelites to celestia can be done by a click in the future.
is this already a topic for the TODO list ?

moreover it would be nice to know how precise the data is.

Even with current elements, pass predictions with Celestia will never be as accurate as with SatTrak or some other specialized program. It's because the NORAD TLEs are really designed for and meant to be used with a specific algorithm, the 'SGP4 model', that most satellite prediction programs use but that Celestia doesn't. When you convert TLEs to Celestia format, you basically ignore the first entire line of information, which has data about the drag and perturbations that the satellite experiences. Celestia just uses the 'mean elements', which means that it only models the 'average' elliptical orbit, without the alterations to that orbit caused by the sun, moon, non-sphericity of the earth, atmosphere, etc.

Because of this, Celestia will be most accurate for mid-altitude, highly circular orbits. LEO, GEO, and highly elliptical orbits will be signifigantly (though I dunno how much) less accurate.

For each new set of TLEs that NORAD generates, though, the perturbations are averaged out, and thus Celestia's model should be pretty accurate for the actual orbit of the prediction, and decreasingly accurate in either direction.

It would be extremely cool if Celestia allowed something like...in an SSC.

FYI, I'm currently in the process of making a /large/ addon SSC that includes all 9500+ objects in orbit currently tracked by NORAD. It'll be several megs, but I'm setting it up so I'll be able to regenerate it occasionally with updated TLEs. My current version is pretty rough, but it shows the 'geosynchronous belt' quite nicely.

Unfortunately, no one location on the web that I can locate provides a unified listing of all 'official' TLEs anymore (apparently NORAD complained), so I'm having to use Orbitel to concatenate a large number of shorter lists. It's kindof a pain.

It would be extremely cool if Celestia allowed something like...

Indeed it would... Feature request?!!

FYI, I'm currently in the process of making a /large/ addon SSC that includes all 9500+ objects in orbit currently tracked by NORAD

Cool! Can't wait to see that- although my PC cringes at the thought of having to load & render them all...