position change in ECEF EoM

Hi

I might have some rust on my mechanics, but I couldn't understand why both the ECEF and the LLA position changes with time under a zero body forces and moments.

It is easy to test, an ECEF quaternion EoM block with zero inputs and arbitrary initial position (not at a pole) and zero initial body translational and rotational velocities and a WGS84 planet.

I know that the position change is due to the transport term (specifically the centripetal acceleration) emerging from solving eom in a rotating frame. However, these transport terms will affect the inertial position (ECI position) not the ECEF and LLA positions. Thus a stationary point on a rotating planet will have a constant ECEF and LLA position, but its ECI position will be changing with time.

Would anyone kindly explain what am I missing? or is the block doing something it shouldn't?

Kind regards,

Mohamad

Answer 1

[Saif Awwad]

I faced a somehow similar problem some time ago, which I could not answer until today. Please let me know if you find an answer.

Thanks.

[MTA]

OK Saif, I'll let you know if I had a progress, you too please.

[Saif Awwad]

Hi Mohamad. Thanks for the reply.

Frankly, I doubt I will have any progress. This is an old problem on which I got stuck some time ago, so I decided to leave it aside until a brighter idea comes up. And now you helped me bring it back to the surface again.

I am not sure whether your time allows, but I am kind of hoping to get some help on a part of my work. Is it okay if I send you a file or two to check and confirm?

[MTA]

Hi Saif

Sure, you can personal message it if it is not contributing to the thread directly.

[Saif Awwad]

With a body of zero forces and moments, is the change in both of ECEF and LLA positions analogical? As you said, only ECI should be changing with time. So I am doubting some sort of a bug in the code converting ECEF to LLA or vise versa.

I will be messaging about further topics outside the scope of this thread.