Finally tried the simple thing I shoulda started with. Propagate a basic ballistic #orbit and plot distance/speed (as opposed to pos/vel) vs t

Yep, there's a dip just in the speed. My non-smooth analysis results has nothing to do with numerical methods and is just a result of probably drag or a non-point earth #gravity model

My simplistic two-body difference method doesn't work near perigee and I think nothing can make it work without knowing the source data models (prob not #sgp4 either)

Thinking outside the box about how to reduce all effects, I do have another idea in my back pocket.

I could take the state vector at time t and *put it through the #sgp4 propagator* to see what it thinks t+1 should be. Then compare that to what I was given for t+1. Then the diff should be exactly the thrust acceleration.

I'm not sure I have a way to #sgp4 propagate a state vector so I'm leaving that alone for a bit. That'll be the next iteration if real #math doesn't work.

But I'm only subtracting Earth's #gravity. #sgp4 #orbit propagation also has other perturbations that might contribute to my noise.

If upping the order of my interpolation keeps the "noise" at the same level, I need to consider that what I'm looking at is the Moon's gravity or solar wind or whatever.

It's possible it'll be impossible to untangle my effect from all the other perturbations represented in the data. But at least I'm still #learning useful skills! #math #softwareengineering

@davidr @willmcgugan

Sounds to me #sgp4 is a crucial favorite for you. That does what you need and it is well maintained.

@tsvenson @willmcgugan I don't know about *favorite* but a *crucial* package I use is #sgp4. It's a standard algorithm ported to python.

Brando Rhodes (who also wrote the amazing #skyfield) maintains it now. He also vectorized it for an amazing speedup. AND he updated it for the end of the 60s era "Two Line Element set" format being phased out and "Orbit Mean-element Message" being phased in.

Sounds minor, but it is central to using #python for #space, which is, like, my bag, baby.