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Re: [Orekit Users] Problem when doing propagation from different point of views

Quentin Nénon <q.nenon@gmail.com> a écrit :


Hi,


Hi Quentin,



I would like to add some comments in order to make the discussion progress.

You will find enclosed another main class using the previously posted ones
and it is basically doing the same thing as before but it uses the Venus
point of view (and adds the perturbation due to Venus to avoid to use a GM
constant of 0., that throws an exception). You can therefore see that the
problem is not only due to the definition of the Sun inertially oriented
frame. The problem still can be in the definition of all the inertially
oriented frames.

Another interesting aspect is when you change the force models you add to
the propagators. If for instance you remove the moon or sun perturbation,
you have errors on the position and velocity that change. This makes me
think that the problem might be in the definition of the perturbative
forces ?

If any of the users have an idea of what might be the problem, I would be
very happy to discuss it here,


I finally found some time to look at your problem. I'm sorry for the delay.
It seems to me the problem is due to an oversimplified model. In your code, you propagate a trajectory using Keplerian attraction and third body perturbations only in all cases, using Sun, Earth and Moon in various combinations. The combinations are logical, i.e. when you consider the Sun as the center, Earth and Moon are perturbing bodies, and when you consider Earth as the center, Sun and Moon are the perturbing bodies. Then you compare the two trajectories and notice a large position error (more than 2000 kilometers. This error happens after 100 days in an escape trajectory, where the position of the probe starts at about 14000 km from Earth center and reaches about 7.2 million kilometers 100 days later.
Assuming the two trajectories should be equals is similar to assuming the Earth trajectory with respect to Sun is also only dependent on Sun and Moon, which is not true. I think that what you observe is partly due to the non-Keplerian part of the Earth trajectory which is included in the JPL ephemeris used to compute the various frames, and which is not included in your simplified force model.
In order to check this, I have tried to compare a trajectory that would in fact always remain at the center of the Earth. The first part of the trajectory (as seen from Earth) is simple to compute with perfect accuracy: the position is always exactly at the center of the EME2000 frame ;-) The second part (as seen for Sun) is computed using your simplified model, i.e. using only Sun as the main attraction body and Moon as a perturbing body. This second trajectory, once converted to EME2000 fram does not stay at the origin, it moves approximately in the same motion as the error you see between your two trajectories. It does not really represent the error you observe, but goes in the same direction, the curves have the same shape, and when you compute the ration between your error and this trajectory for all coordinates x, y, z, the ratio remains roughly between 1.0 and 1.8. As the accelerations I compute in this case are computed from the center of Earth, which is always muche closer to Moon than your probe which escapes from the system, it is not surprising that I don't rebuilt the exact same error you see. However, since the dynamics of the curves are similar and the order of magnitude is similar, I guess this could be an explanation.
As a summary, Earth motion *is* a complex motion, which is fully taken into account in the JPL ephemerides, and which cannot be accurately represented with only Sun and Moon gravity fields. If you need to do computation far from Earth, you would probably have to take other planets into account (I would say at least Venus and Jupiter, perhaps Mars too).
Also note that even if the two trajectories are not equal, they are very close and your propagation duration is long. Don't forget your probe is 7 millions kilometers away from Earth after 100 days. Given the simplistic assumption you do on Earth motion, I would say between 2000 and 3000 kilometers is not that bad. Earth has moved about 250 millions kilometers during this duration. 

Do you agree with this argument?

best regards, and sorry again for the delay
Luc



Thank you,

Quentin


2014-03-20 9:57 GMT+01:00 Luc Maisonobe <Luc.Maisonobe@c-s.fr>:


Le 20/03/2014 09:53, Quentin Nénon a écrit :
> Hi,
>
> I would like to complete my answer of yesterday. What I called
> "differential accelerations" is just taking into the relative
> acceleration of the two frames.
>
> You will find enclosed a new main class, step handler and a "MyChart"
> class for the plots. I modified a little the step handler and the main
> and you now have the plots of the difference of position and speed
> between the two propagators over the time.

Hi Quentin,

I will only be able to look at this in a few days, I am very busy with
some urgent matters right now.

Sorry for the delay
Luc

>
> I am using the jfreechart-1.0.17 and jcommon-1.0.21 libraries for the
> plots. You will find enclosed the jar files that should be linked to the
> project to have the plots.
>
> Hope this can help.
>
> Thanks again,
>
> Quentin
>
>
> 2014-03-19 16:10 GMT+01:00 Quentin Nénon <q.nenon@gmail.com
> <mailto:q.nenon@gmail.com>>:
>
>     Thanks again for the fast answer.
>
>     You will find enclosed a MyStepHandler class and a new main class
>     that is using this step handler to keep the positions with an output
>     step of 1000 secondes (you can modify it in the main class if you
>     want). I then write the results in a txt file that I am then used to
>     visualize using Excel. I can create charts if you want.
>
>     I already tried to verify that the problem was not due to the fact
>     the the Earth EME2000 Frame is not inertial. It seems to already be
>     taken into account by using differential accelerations for the
>     ThirdBodyAttraction rather than the actual acceleration created by
>     the perturbative body. It means that the acceleration due to the Sun
>     perturbation in the Earth Frame is equal to the acceleration of the
>     satellite due to the Sun in the Sun frame MINUS the acceleration of
>     the Earth due to the Sun in the Sun Frame. If the Earth Frame was
>     inertial, the acceleration of the Earth in the Sun Frame due to the
>     Sun would be 0. and the acceleration of the satellite would be the
>     same in the Earth and Sun Frame.
>
>     Moreover, if the Earth motion around the Sun was not taken into
>     account, much bigger errors would be expected and all the
>     propagations in the EME2000 would be (I think very) wrong.
>
>     Thank you,
>
>     Quentin
>
>
>     2014-03-19 15:35 GMT+01:00 MAISONOBE Luc <luc.maisonobe@c-s.fr
>     <mailto:luc.maisonobe@c-s.fr>>:
>
>         Hi Quentin,
>
>         Quentin Nénon <q.nenon@gmail.com <mailto:q.nenon@gmail.com>> a
>         écrit :
>
>
>             Hi Orekit users,
>
>             First, let me thank Luc very much for the very fast and
>             effective answer he
>             gave to my last topic. It is very nice and enjoyable to have
>             support and
>             suggestions from the Orekit developer team and users.
>
>
>         Thanks a lot.
>
>
>
>             I have another issue I would like to submit to the Orekit
>             users. I am
>             trying to use Orekit to propagate  an interplanetary
>             trajectory and in
>             order to have the best possible precision, I am creating a
>             manager of
>             sphere of influence. The goal is therefore to be able to
>             propagate the
>             motion of the spacecraft in different frames (first, in
>             inertial frames).
>
>             You will find enclosed a main class that is doing the
>             propagation of the
>             same motion but from two different point of views :
>
>             -The first one is to consider that the spacecraft is turning
>             around the
>             Earth central body and has newtonian perturbations coming
>             from the Sun and
>             from the Moon
>             -The second one is to consider that the spacecraft is
>             turning around the
>             Sun central body and has newtonian perturbations coming from
>             the Earth and
>             from the Moon.
>
>             I am using the Orekit physical data available on the Orekit
>             website. I have
>             Orekit 6.1 and commons math 3.2 as dependances.
>
>             At the end of the two propagations, I have a difference in
>             the position of
>             about 2800 kilometers, that is not very good ... I verified
>             with my own
>             patch that it is not a problem due to the distances between
>             the celestial
>             bodies (see the EarthMoonBarycenter topic).
>
>             Does anyone has an idea of why I have this result ? Am I
>             doing a mistake
>             when I am adding the force models to the propagators ?
>
>
>         I just skimmed over the code and did not see any obvious error.
>         Could you store not only the final position but a few hundreds
>         intermediate points at fixed date (you can use an
>         OrekitFixedStepHandler for that) and create a plot showing the
>         error evolution throughout the propagation?
>
>         I wonder if the problem could not be related to the fact Earth
>         frame is not really inertial (due to motion around Sun) and in
>         this case it shows up.
>
>         best regards,
>         Luc
>
>
>             Thanks again,
>
>             Quentin
>
>
>
>
>
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