Hello all,
I'm in the process of doing a V&V suite of the Orekit numerical integration and force modeling system. I'm comparing this against a data set we generated for validating one of my company's products, SDT. In that process I've run into a subtly that comes up periodically but that is not directly addressed by Orekit right now. Orekit has coded up the EGM96 reader to assume the EGM96 constants for ae and mu as defined by this site:
However a second version of this, the so-called WGS84 EGM96 model, has a slight different ae and mu. The rest of the coefficients are the same. You would get that model from this site:
Because the geopotential model is the same the big difference are these two slight differences in ae and mu, but that's enough to throw things off if you are using a different model. In the EGM96FormatReader code we have a note that reads, "both EGM96 and EGM2008 use the same values for ae and mu. if a new EGM model changes them, we should have some selection logic. based on file name (a better way would be to have the data in the file...)" For my tests I simply added an argument that let me override the ae and mu with the WGS84 versions. However that would mean that in order to use the WGS84 coefficients the user has to manually override that behavior. Would it maybe be better for us to include an "egm96" and a "wgs84egm96" file, both identical, and add the logic to search for the override the coefficients with the WGS84 coefficients when that occurs?
Hank