The ECI reference frame and tidal forces

MontBlanc2012

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Are you saying that checking a simulation with another simulation is better than checking a simulation against reality?

No, I'm not saying that. All that I'm saying is that Orbiter can provide a controlled test of model formulation.

As I understand it, you have two possibilities: either you have included the tidal terms in your equations of motion incorrectly; or there is some hidden problem with the way that you calculate the initial and/or final state vectors.

If you want to check the former, you can use a model like Orbiter. Alternatively, you can build another integrator that works in a global inertial coordinate system, and check that the ECI model with tidal forces yields exactly the same result as the one built for the global inertial coordinate system. If it doesn't then you probably have to revise the way that you model tidal forces. If it does, then you need to look more closely at the way you form the initial/final state vectors for the integration.
 

cristiapi

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My integrator already works with "global" inertial coordinate system (I use Solar System Baricenter as {0,0,0} point and Earth equator J2000 as reference plane).

My current implementation of the tidal accelerations matches your formula (the differences are on the last 2 or 3 significant digits).

The initial satellite state is obtained with a call to sgp4() (extensively checked in several ways) and with a transformation from TEME to J2000 equator (extensively checked in several ways). I also include the DeltaT (the simulation uses TDT).

It would be a good exercise to get your equation working in my simulation, but probably it's not worth to waste our time, also because the current result is very good.
 
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MontBlanc2012

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Just as a quick comment: if you are integrating in an inertial frame of reference centred on SSB then you don’t need the tidal terms. Using them would be ‘double counting’ and will lead to an error. It’s only if you are integrating in a non-inertial ECI coordinate system that you do need to add them.
 
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