Discussion on SGP4 vs. TrueData

Greetings Orekit Community,

I have learned a lot of concepts since I first joined this community, so I would like to thank you all for the valuable discussions we have made.

I’ve been playing around with Batch Least Square method and been feeding my code with Starlink “true data” gathered from space-track. There is, however, an interesting behavior. In total say we have 4321 position-velocity timestamped data.

Comparing the SGP4 propagator results with the true data and visualizing the difference between each positional axis, I recieve a graph as you see below:

But I can’t really understand why it just follows a somewhat straight path up until a specific point, and then error starts to escalate? Shouldn’t the error starts building up right away? What do you think the cause of this straight path?

There are other strange results that I want to shed light on. I may ask those as well, if we clarify this one first though.

Thanks for any ideas and discussions in advance,

Hi @Echulion,

Maybe the SGP4 (TLE?) you’re using was fit on a time span that covers the beginning of your test and then once you’re out of the fit span the data start diverging.
Or, more simply, if you plot the log of the difference you’ll see the error slowly building up with time.
What data are you using and what exactly are you doing with it ?