# Modelling ISS orbit in GMAT

I am a complete newbie in all things space related. I am trying to work out how I can model the orbit of the ISS in the NASA open source tool GMAT.

I can get the TLE for ISS (ZAYRA) from space-track.org

1 25544U 98067A 22201.02342443 .00008269 00000-0 15250-3 0 9990 2 25544 51.6415 173.9494 0004948 29.0959 82.3667 15.50036788350272

I found a nice python library called TLE-tools that will read in the TLE and provides a to_orbit() method.

Here is a gist for my code:

https://gist.github.com/kieranshanley/00f80997e4d07501187461d9fc702e7a

That code gives me these keplarian orbit parameters:

SMA = 6794.755557613242 km ECC 0.0004948 INC 51.6415 deg RAAN 173.9494 deg AOP 29.0959 deg TA = 82.42290206591902 deg

I created a new mission in GMAT and used those keplarian parameters - the only other parameters I changed where mass - 420,000 kg and the Epoch to 19 Jul 2022 07:45:43.000

I compared the output to the the ground track currently being shown for the ISS

The plotted ground track plotted in GMAT seems very different - is this error caused by reverse engineering the keplarian orbit for ISS - I know you are only meant to use the TLE as an input to the SPG4 algorithm to allow you to predict the current position but I was hoping the error would not be as apparent as it seems.

Is there a better way to create a model of the ISS in GMAT that I can then use to compare with its actual location and get better results ?

Cheers,

Kieran

UPDATE

notovny pointed out that if I changed the Epoch I would also need to calculate a new TA. I used the Epoch from the TLE and found a site that allowed you to enter in the date and time and where the ISS would be at that point in time. When I did that I got the results shown below - everything matches up pretty nicely. I looked at the track drawn for the ISS in GMAT and compared it with that shown from the ISS tracker and it seems to visit all the same points on the ground - yay - thanks for the help !

• If you change the Epoch, you're going to have to also change True Anomaly: The Epoch is when the true anomaly of the orbit is valid for your set of parameters. This will result in both position discrepancies and ground track discrepancies. Commented Jul 20, 2022 at 16:29
• Thats a good point - i will change the epoch to match the time of the TLE and see if i can find where the ISs was at that point in time. I will post the results back here. Thanks for your help Commented Jul 20, 2022 at 23:09
• Nice! Since you've solved your problem with help from @notovny, consider posting an answer. Commented Jul 21, 2022 at 0:07
• Hey @OrganicMarble - yes notovny definitely helped me out - How do I post an answer ? Sorry retarded question I know. Commented Jul 21, 2022 at 0:14
• No, it's fine, we all were new once. Just type the answer into the box below the label Your Answer and when you're done, hit the Post Your Answer button. After that, you can edit it using the Edit field below it if you need to (I always need to). Commented Jul 21, 2022 at 0:23