Converting Orbital Elements

[unlimitedbacon]

Hi. I'm trying to make an Orbiter scenario for ISEE-3/ICE which will be making a pass by Earth in August of this year.

I dug up some TLEs from here and did the conversion. The orbit looks about right. Aphelion and Perihelion are correct and there is an intersection with Earth in August, but the spacecraft is in completely the wrong position. Its about 160 degrees from where it should be. My guess is that something is wrong with either the Mean Anomaly or Epoch, but I'm not sure what. Any ideas?

Here's the TLE in question. As I understand it, this is somewhat irregular because ISEE-3 is in a heliocentric orbit and TLEs are normally only used for Earth orbiting satellites.

1 11004U 78079A   89201.29324000 0.00000000 +00000-0 +00000-0 0    11
2 11004   0.0587  87.7713   0.0542062 100.0373   180.0000         0.0028228421
3 11004  2 UNK     Sun                            ELS93

Heres the SCN file I came up with.

BEGIN_DESC 
A long-lost pioneering spaceship, still functional thanks only to  chance and human error, is coming home for the first time in three  decades. It wants to explore new worlds. But we've forgotten how to talk  to it. 
END_DESC 
 
BEGIN_ENVIRONMENT 
  System Sol 
  Date MJD 56704 
END_ENVIRONMENT 
 
BEGIN_FOCUS 
  Ship ISEE-3 
END_FOCUS 
 
BEGIN_CAMERA 
  TARGET ISEE-3 
  MODE Cockpit 
  FOV 50.00 
END_CAMERA 
 
BEGIN_HUD 
  TYPE Orbit
  REF AUTO 
END_HUD 
 
BEGIN_MFD Left 
  TYPE Orbit 
  PROJ Frame
  FRAME Ecliptic 
  REF Sun
  TARGET Earth 
END_MFD 
 
BEGIN_MFD Right 
  TYPE Orbit
  PROJ Frame
  FRAME Ecliptic
  REF Earth
  TARGET Moon 
END_MFD 
 
BEGIN_SHIPS 
ISEE-3:DeltaGlider 
  STATUS Orbiting Sun 
  ELEMENTS 146577981500 0.0542062 0.0587 87.7713 100.0373 180.0000 47728.29324 
  AROT 90 0 0
  VROT 0 0 118.58 
  PRPLEVEL 0:1.0 1:1.0 
  NOSECONE 0 0.0000 
  GEAR 0 0.0000 
  AIRLOCK 0 0.0000 
END 
END_SHIPS

 

[BrianJ]

Hi,
possibly you have given the Mean Anomaly in your scenario elements (last field before Epoch) - I think it should be Mean Longitude.

[ame="http://en.wikipedia.org/wiki/Mean_longitude"]Mean longitude - Wikipedia, the free encyclopedia[/ame]

Any help?

edit--------------

Also, I think you need Longitude of Periapsis rather than Argument of Periapsis in the fifth field

[ame="http://en.wikipedia.org/wiki/Longitude_of_the_periapsis"]Longitude of the periapsis - Wikipedia, the free encyclopedia[/ame]

or am I barking up the wrong tree here?....

 

[SolarLiner]

I have a ready algorithm for that, in my AOSP module (for .NET apps) : http://sourceforge.net/projects/orbaosp/

If you're only interested in the actual function, it is here: http://sourceforge.net/p/orbaosp/code/HEAD/tree/trunk/OrbElements.cs (line 91)

EDIT: Here is a testing application I made for testing AOSP, might be useful.

Enter a http link or navigate to a local TLE file, then enter an output scenario or text file (if plain test it will only output the object names and converted elements). Then click Create and go see your file.
If you've output a scenario file, it will ask you the vessel class to be used for each object found.

EDIT2: By the highly eccentric orbit I can't really tell if the conversion will be accurate. Test it and see

 

[SolarLiner]

Yep, that's what i thought. TLE format is irregular, so app will not help.

 

[unlimitedbacon]

Thanks. I converted Argument of Periapsis to Longitude of Periapsis and Mean Anomaly to Mean Longitude. It looks much better now. There is a "close" approach with Earth on August 11, which is consistent with what this article says.

It only passes within 12,970,000 km, though. That doesn't seem very close to me at all. The article makes it sound like it should be passing near the moon's orbit. Anyways, heres the new scenario.

BEGIN_DESC 
A long-lost pioneering spaceship, still functional thanks only to chance and human error, is coming home for the first time in three decades. It wants to explore new worlds. But we've forgotten how to talk to it. 
END_DESC 
 
BEGIN_ENVIRONMENT 
  System Sol 
  Date MJD 56704 
END_ENVIRONMENT 
 
BEGIN_FOCUS 
  Ship ISEE-3 
END_FOCUS 
 
BEGIN_CAMERA 
  TARGET ISEE-3 
  MODE Cockpit 
  FOV 50.00 
END_CAMERA 
 
BEGIN_HUD 
  TYPE Orbit 
  REF AUTO 
END_HUD 
 
BEGIN_MFD Left 
  TYPE Orbit 
  PROJ Frame 
  FRAME Ecliptic 
  REF Sun 
  TARGET Earth 
END_MFD 
 
BEGIN_MFD Right 
  TYPE Orbit 
  PROJ Frame 
  FRAME Ecliptic 
  REF Earth 
  TARGET Moon 
END_MFD 
 
BEGIN_SHIPS 
ISEE-3:DeltaGlider 
  STATUS Orbiting Sun 
  ELEMENTS 146577981500 0.0542062 0.0587 87.7713 187.8086 367.8086 47728.29324 
  AROT 90 0 0 
  VROT 0 0 118.58 
  PRPLEVEL 0:0.0 1:0.0688790675 
  NOSECONE 0 0.0000 
  GEAR 0 0.0000 
  AIRLOCK 0 0.0000 
END 
END_SHIPS

 

[unlimitedbacon]

New question. I see that scenario files accept RPOS and RVEL for position and velocity relative to the body the ship is orbiting. Is there a way to set the absolute position and velocity (origin at solar system barycenter, J2000 ecliptic)?

 

[SolarLiner]

You can take the Sun as a reference, but not the barycenter. Also in Orbiter, the Solar System Barycenter is not simulated, so the Sun does not move. So I think you can take the absolute position and put it on the RPOS value.

 

[BrianJ]

Also in Orbiter, the Solar System Barycenter is not simulated, so the Sun does not move.

Are you sure that is true? I thought the Orbiter Sun does indeed orbit the barycentre.

@ unlimitedbacon:
The TLE you are using is decades old - orbital perturbations, solar radiation, etc. will have influenced the orbit since then. You could try running the sim from a date nearer to your TLE epoch, up to present day - you might get a better close-approach. Might be an interesting experiment. Depends how accurate Orbiter is over long timescales (and how accurate the original TLE is). Just a thought.

Cheers,
Brian

 

[martins]

Are you sure that is true? I thought the Orbiter Sun does indeed orbit the barycentre.

No, it's not true. The barycentre is computed, and the sun is orbiting it.

 

[SolarLiner]

We learn everyday. I didn't know that. Orbiter is even more accurate than what I thought!