Greetings. I am currently learning Orekit as an astronautical engineering student by following the given tutorials and examples. I am new in both java and orekit library. The library is awesome. Before asking my question I would like to thank to the community and the developers of the library especially Luc Maisonbe for the didactic examples.

My question is about the Phasing example in version 10.0. As far as I understand from this tutorial, we are trying to design sun synchronous and repeating ground track orbit. Somehow I understood the initialGuess and improveEarthPhasing methods, yet I have got stuck on improveSunSynchronization and improveFrozenEccentricity methods. In those methods, SecularAndHarmonic class has called and hence there are terms such as `secular degrees`

and `pulsations`

. For instance, in improveSunSynchronization code the SecularAndHarmonic class is used as follows;

// find all other latitude crossings from regular schedule

SecularAndHarmonic mstModel = new SecularAndHarmonic(2,

2.0 * FastMath.PI / Constants.JULIAN_YEAR,

4.0 * FastMath.PI / Constants.JULIAN_YEAR,

2.0 * FastMath.PI / Constants.JULIAN_DAY,

4.0 * FastMath.PI / Constants.JULIAN_DAY);

So, what should I understand from these evaluations? What are the 2 and 4 pi over julian day and year terms? (namely the pulsations.)

Also in this phasing example there is a comment like this.

// fit the mean solar time to a parabolic plus medium periods model

// we will only use the linear part for the correction

mstModel.fit();

What is the meaning of parabolic plus medium periods model?

In ApiDocs, SecularAndHarmonic class is defined as fitting evaluation of osculating orbital parameters. So, in these examples are we fitting osculating elements to mean or somethingelse ?

In addition, if you guys have any theoratical document about this example or the concept of the example, may I ask the references kindly?

Again, Thank you very much for this spectacular library.

Kind Regards,

Uzay