I am trying to find if the computeMeanOrbit function of the BrouwerLyddanePropagator returns short or long mean orbital elements. This question was also asked here. Short term averaging refers to averaging the J2-J5 perturbation effects over one orbital period (e.g., 96 minutes for satellites in LEO) and long term averaging refers to averaging the J2-J5 perturbation effects over the rotation period of the perigee (e.g., 105 days for satellites in LEO).
A simple way to check this would have been to check the mean argument of perigee (AOP) returned by the computeMeanOrbit method over several days. If the mean AOP changes linearly with time over e.g., 100 days, it’s the long averaging. If the change is non-linear, then it uses the short averaging. Unfortunately, I could not test this myself becase I always run into the negative eccentricity issue as is discussed here.
For example, here is a figure which shows the difference between short and long Brouwer-Lyddane mean AOP over about 106 days, for satellite at 560 km altitude.
Being able to get different kind of mean elements would be interesting. It’s not available yet. You can open an issue for this feature request. Contributions welcome.
As I mentioned on another thread today, @alberto-ferrero is working hard to fix the negative eccentricity issue.
I’m not sure but I’m tempted to say: neither… If you look at the code of BLModel.propagateParameters it seems that what we get are the mean elements before even the application of the secular rates.
Not yet, unfortunately
That would be a nice addition, as @Serrof already said it.
Having this for the Eckstein-Hechler and SGP4/SDP4 propagators would be very nice too.
I’m coming back on my previous erroneous comment. I’ve tried your test on 105 days with a slightly quasi-polar eccentric orbit (0.1) at 1000km altitude and I get a nice linear evolution of the argument of perigee.
So I guess the returned mean elements are the long mean orbital elements.
Please note that the difference in effects of BL Short or Long AOP will be visible more at lower altitudes (~500 km) and lower eccentricities (e.g., 0.01 or less). Also, I keep the gravity field parameters (degree, order) as (5, 0) for these checks to only account for J2-J5 perturbations, as Orekit’s Brouwer Lyddane considers only these parameters for averaging as well.
Your results from 1000 km altitude at 0.1 eccentricity will be similar for both short and long averaging. For example, here are some results from GMAT for 1000 km altitude and 0.1 eccentricity, with J2-J5:
At this stage, I am not sure myself if the method I suggested is the best way to find out about the averaging technique, but I will keep checking, go through the Orekit code you posted and update here if I find something.
Thanks a lot for checking and verifying. I can also confirm your results. The ouput from Orekit looks similar to the Brouwer-Lyddane Long AOP from GMAT for similar simulation parameters, and also changes linearly with time for a gravity model of (5, 0).
