Thank you, I want to know how to achieve osculating to mean, how should BrouwerLyddanePropagator and computeMeanOrbit be used? What is the maximum order of the gravitational field defining parameter provider? 7*7?
Hi @newWL
Welcome to the Orekit forum!
They are several possibilities to convert osculating parameters to mean parameters. The three main ones are:

Using the method
computeMeanOrbit
ofBrouwerLyddanePropagator
. This method will compute a mean orbit based on the zonal harmonics from J2 to J5. Here an example presenting how to use it. 
Using the method
computeMeanOrbit
ofEcksteinHechlerPropagator
. This method will compute a mean orbit based on the zonal harmonics from J2 to J6. Here an example presenting how to use it. 
Using the method
computeMeanState
ofDSSTPropagator
. This method will compute a mean orbit based on a user defined model represented by theCollection<DSSTForceModel>
. Using DSST you can compute a mean orbit for big order and degree of Earthâ€™s potential. You can also take into account lunisolar attraction, solar radiation pressure and atmospheric drag. Here an example presenting how to do it.
I recommend you to start with the BrouwerLyddane or EcksteinHechler model. After that, you could try with the DSST which offer the possibility to have an accurate mean orbit compare to the two other ones.
Best regards,
Bryan
Very, thank you very, I learned about the information you provided.
Thanks for the reply.
I noticed in STK Astrogator module, there is an option which I can select from â€śBrouwerLyddane Mean Longâ€ť and â€śBrouwerLyddane Mean Shortâ€ť, shown in the image.
My question is whether there is a way to accomplish this in Orekit.
I donâ€™t understand what they mean by â€śMean Longâ€ť or â€śMean Shortâ€ť.
In Orekit, you can initialize the BrouwerLyddane propagator with a mean orbit or an osculating orbit. At the end, the propagated orbit will be osculating.
Something interesting for Orekit is to have the possibility to choose between a MEAN or OSCULATING propagation for the propagated orbit, like DSST do.
Best regards,
Bryan
What they mean by â€śshortâ€ť and â€ślongâ€ť is based on what angular variable is used for the averaging. For the â€śshortâ€ť case the angular variable is the anomaly, for the â€ślongâ€ť case the averaging is done by considering the rotation of the perigee.
There is a brief explanation here.
Best regards,
Emiliano