Scenario ISS state vectors

[DaveS]

You're using the wrong TLE. June 17 for non-leap years is Day of Year (DOY) 168. So you want the TLE that has the date of 15168 for June 17. So this is the TLE you want to use:

ISS
    1 25544U 98067A   15168.52366114  .00016717  00000-0  10270-3 0  9028
    2 25544  51.6448  82.3523 0004357  52.0913 308.0634 15.55450505 28090

Note how it say 15168.*********** in the third column of row 1? That is the epoch year (15 for 2015) and epoch day (168 for June 17). The numbers after the dot in the date is the fractional portion of the day(time of day). So when looking at historical TLEs, knowing how to read TLEs are important: http://celestrak.com/columns/v04n03/#FAQ01

 

[boogabooga]

I just did it for DaveS' TLE posted above. Here is my resulting scn:

BEGIN_DESC
Contains the latest simulation state.
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 57190.7589409954
END_ENVIRONMENT

BEGIN_FOCUS
  Ship GL-02
END_FOCUS

BEGIN_CAMERA
  TARGET GL-02
  MODE Cockpit
  FOV 60.00
END_CAMERA

BEGIN_HUD
  TYPE Docking
  NAV 0
END_HUD

BEGIN_MFD Left
  TYPE Docking
  NAV 0
END_MFD

BEGIN_MFD Right
  TYPE Orbit
  PROJ Ship
  FRAME Equator
  ALT
  REF Earth
  TARGET ISS
END_MFD

BEGIN_SHIPS
ISS:ProjectAlpha_ISS
  STATUS Orbiting Earth
  RPOS 2988085.81 -2553801.77 -5531910.38
  RVEL 3467.619 -5292.605 4314.996
  AROT -110.00 -15.00 -84.00
  AFCMODE 7
  DOCKINFO 1:0,GL-02
  IDS 0:588 10 1:586 10 2:584 10 3:582 10 4:580 10
  NAVFREQ 0 0
  XPDR 466
END
Mir:Mir
  STATUS Orbiting Earth
  RPOS 1212068.51 400376.42 6547029.94
  RVEL -7601.166 85.803 1406.315
  AROT 0.00 -45.01 90.00
  AFCMODE 7
  IDS 0:540 10 1:542 10 2:544 10
  XPDR 482
END
Luna-OB1:Wheel
  STATUS Orbiting Moon
  RPOS -1193007.66 1893509.40 0.20
  RVEL -1252.273 -788.996 -0.000
  AROT -0.00 0.00 -58.72
  VROT 0.00 0.00 10.00
  AFCMODE 7
  IDS 0:560 10 1:564 10
  XPDR 494
END
GL-01:DeltaGlider
  STATUS Landed Earth
  BASE Habana:2
  POS -82.3988276 22.9994604
  HEADING 174.13
  AFCMODE 7
  PRPLEVEL 0:1.000000 1:1.000000
  NAVFREQ 0 0 0 0
  XPDR 0
  GEAR 1 1.0000
  AAP 0:0 0:0 0:0
END
GL-02:DeltaGlider
  STATUS Orbiting Earth
  RPOS 2988111.84 -2553791.02 -5531909.48
  RVEL 3467.619 -5292.605 4314.996
  AROT -87.91 73.87 111.30
  AFCMODE 7
  PRPLEVEL 0:1.000000 1:1.000000
  DOCKINFO 0:1,ISS
  NAVFREQ 2 466 0 0
  XPDR 0
  NOSECONE 1 1.0000
  SKIN BLUE
  AAP 0:0 0:0 0:0
END
SH-01:ShuttleA
  STATUS Landed Moon
  BASE Brighton Beach:1
  POS -33.4375000 41.1184067
  HEADING 0.00
  AFCMODE 7
  PRPLEVEL 0:1.000000 1:1.000000
  NAVFREQ 0 0
  XPDR 0
  PODANGLE 0.0000 0.0000
  DOCKSTATE 0 0.0000
  AIRLOCK 0 0.0000
  GEAR 0 0.0000
  PAYLOAD MASS 0.0 0
END
END_SHIPS

BEGIN_SpaceNetwork
END

BEGIN_ExtMFD
END

First of all, don't pay too much attention to ArP for near circular orbits. That value is changing on the timescale of seconds.

Second of all, if I understand correctly, what the add-on is doing is using SGP4/SDP4 to propagate up to the current time, then telling Orbiter what the state vector is. The Orbiter engine handles the propagation from there. The TLEs are meant to be used by the SGP4/SDP4 and the elements may differ somewhat from what the Orbiter engine says they should be.

See "How does one convert two-line elements to some other format?"
https://celestrak.com/columns/v04n05/index.asp#FAQ05

 

[Wolf]

You're using the wrong TLE. June 17 for non-leap years is Day of Year (DOY) 168. So you want the TLE that has the date of 15168 for June 17. So this is the TLE you want to use:

ISS
    1 25544U 98067A   15168.52366114  .00016717  00000-0  10270-3 0  9028
    2 25544  51.6448  82.3523 0004357  52.0913 308.0634 15.55450505 28090

Note how it say 15168.*********** in the third column of row 1? That is the epoch year (15 for 2015) and epoch day (168 for June 17). The numbers after the dot in the date is the fractional portion of the day(time of day). So when looking at historical TLEs, knowing how to read TLEs are important: http://celestrak.com/columns/v04n03/#FAQ01

This is exactly the TLE I have used (it refers to Orbit 2808 on day 168 at 12:00 UTC)

Before every TLE there is a note:

"The mean element set is posted at the UTC for which position is
just north of the next ascending node relative to the above
vector time"

does that mean the TLE do not refer to the same time of the related vectors?
seems like the TLE refers to a different time (where the ISS has just passed the Ascending Node) hence the mismatching in the values would make sense....

Actually if I've done the math correctly the TLE time (0.52366114) corresponds to 12:34:03 UTC. That, compared to the vector time, differs by 34' and 03": if I import the TLE in Orbiter and my scenario is at 12:00:00 then the ISS position will of course be wrong...

---------- Post added at 09:49 PM ---------- Previous post was at 09:31 PM ----------

  Coasting Arc #2 (Orbit 2823)
    ---------------------------------------
 
    Vector Time (GMT): 2015/169/11:03:07.929
    Vector Time (MET): N/A
    Weight (LBS)     : 877564.0
 
              M50 Cartesian                         M50 Keplerian  
    -----------------------------------       --------------------------------
     X    =         684269.05                 A    =         6783317.25  meter
     Y    =       -6207248.30  meter          E    =           .0007333
     Z    =       -2652265.88                 I    =           52.01036
     XDOT =       4850.787839                 Wp   =          103.10024
     YDOT =       2770.773168  meter/sec      RA   =           76.91885  deg
     ZDOT =      -5246.900773                 TA   =          106.63614
                                              MA   =          106.55561
                                              Ha   =            220.535  n.mi
                                              Hp   =            211.905
 
              M50 Cartesian                         J2K Cartesian  
    -----------------------------------       --------------------------------
     X    =        2244977.19                 X    =          766512.78
     Y    =      -20364987.85  feet           Y    =        -6199136.13  meter
     Z    =       -8701659.72                 Z    =        -2648742.97
     XDOT =      15914.658266                 XDOT =        4844.931229
     YDOT =       9090.463149  feet/sec       YDOT =        2824.985846  meter/sec
     ZDOT =     -17214.241380                 ZDOT =       -5223.350522
 
              TDR Cartesian                         TDR Cartesian  
    -----------------------------------       --------------------------------
     X    =      -18555721.26                 X    =        -5655783.84
     Y    =       -8703026.53  feet           Y    =        -2652682.49  meter
     Z    =       -8685279.05                 Z    =        -2647273.05
     XDOT =      13108.318642                 XDOT =        3995.415522
     YDOT =     -10914.848454  feet/sec       YDOT =       -3326.845809  meter/sec
     ZDOT =     -17113.489887                 ZDOT =       -5216.191718
  
  
    The mean element set is posted at the UTC for which position is
    just north of the next ascending node relative to the above
    vector time
 
 
    TWO LINE MEAN ELEMENT SET
  
    ISS
    1 25544U 98067A   15169.48735715  .00016717  00000-0  10270-3 0  9010
    2 25544  51.6439  77.5405 0003346  68.0866 292.0643 15.55172845 28243

In this other example the vectors are taken at 11:03 UTC but the TLE time is 11:41:47 UTC. Again they don't seem to match.

maybe I am doing something wrong and confusing myself...

 

[boogabooga]

I think that there is a difference between the TLE time and the time for the "Coasting Arc #2" that is being displayed.

Don't go by all the coasting arc stuff at the top. I see now that it isn't even always using the correct reference frame for Orbiter- Keplerian elements are given in M50, not J2K.

The TLE is what you need. The add-on is using SGP4/SDP4 so it should be correct.

 

[Wolf]

So what is the correct UTC time I should use in the scenario? Is it the TLE one or the one reported at the beginning of the vectors section?

 

[boogabooga]

Good question, because now I see what you misunderstand.

AFAIK, you can use ANY UTC time in your scenario. For maximum accuracy, have your UTC Time be within a few days of the TLE time.

You think that you have to load the TLE at the exact right simulation time for it to be correct. That is not how TLEs work or what they are for. What the TLEs do is feed information to the SGP4/SDP4 model. This model can propagate the state vectors in time, the same way that Orbiter can propagate state vectors in time. SGP4/SDP4 will arguably do a better job than the Orbiter engine itself.

When you use the TLE scenario editor add-on, it will propagate the ISS up to the current sim time, so don't worry about it too much.

BTW, I know the add-on is spot on because I have used it to track the ISS and various space debris that can be seen with the naked eye. The "real-world" observations lined up with the SGP4/SDP4/Orbiter prediction very well.

 

[Wolf]

Thank you boogabooga, now I think I understand better.

Following your tips I have generated a launch scenario for the upcoming SpaceX CRS-7 mission.
Launch is scheduled on 26 June 2015 at 15:09:00 UTC
I have pasted the closest (timewise) ISS TLE

 ISS
  1 25544U 98067A   15177.51915134  .00016717  00000-0  10270-3 0  9096
 2 25544  51.6448  37.4298 0002876  90.3713 269.7769 15.55378511 29490

and then projected those by shifting the sim time to T-9' (15:00:00 UTC) but I was surprised to see that the ISS position is totally out of plane at launch time.

This is a screenshot taken after MECO (where RInc is still way too high)

View attachment 13757

something is obviously wrong here...

 

[boogabooga]

The ISS itself has been maneuvering lately:
https://blogs.nasa.gov/spacestation/2015/06/18/first-of-two-station-boosts-complete/

But now you have a testable prediction- either they will have to maneuver the ISS again or they will have to change the launch time of CRS-7. :hmm:

Try loading a new TLE every day from now until launch. And please post the result like you just did. It would be an interesting experiment.

 

[Wolf]

The ISS itself has been maneuvering lately:
https://blogs.nasa.gov/spacestation/2015/06/18/first-of-two-station-boosts-complete/

But now you have a testable prediction- either they will have to maneuver the ISS again or they will have to change the launch time of CRS-7. :hmm:

Try loading a new TLE every day from now until launch. And please post the result like you just did. It would be an interesting experiment.

Looks like you can see the future boogabooga! :speakcool:


https://blogs.nasa.gov/spacex/2015/06/19/crs-7-launch-date-june-28-1021-a-m-edt/

And at this revised time the ISS seems to be in-plane now! will test it and see what happens