GMAT targeting - why targeting RMAG and DEC works while RadApo and INC doesn't?

The GMAT examples demonstrate targeting using this method for targeting a specific Apoapsis increase maneuver:

2. Apply Maneuver
3. Propagate to the Apoapsis
4. Achieve "the distance when at apoapsis = X"

This first runs the simulation until the Apoapsis point and then looks for the distance at this point.

The second way this could be done in theory is:

2. Apply Maneuver
3. Achieve "Apoapsis = X"

This, instead, applies the maneuver and looks at the calculated apoapsis of the orbit, without doing any propagation.

In theory those should be the same and both methods should work in the target command. However, the second version (and INC instead of DEC for an inclination change) often doesn't converge, even if it comes really close to the answer. Why is this?

• Shouldn't you need a Propagate statement after applying the maneuver? Otherwise the differential corrector has a correction which is effectively nil to apply. Nov 23 '20 at 18:26
• I'm not sure, should you? It does work in some cases, but I think it depends on whether the calculated parameters make sense. Especially in the case of inc changes, it can often be a hit or miss, even using propagation statements etc. I'd like to understand if this is because of GMATs internals or some fundamental misunderstanding of mine when it comes to targeting/executing these maneuvers. Dec 3 '20 at 12:27

• GMAT calculates the RadApo property from the keplerian elements, which are obtained directly from the cartesian state ($$\vec{r}$$ and $$\vec{v}$$) so any change in the spacecraft state (a maneuver which applies a $$\Delta v$$) is reflected at once in RadApo. The same happens with INC, which is a keplerian element. You can find the exact formulas in the GMAT documentation (in particular in the GMATMathSpec file), in its installation folder.

About if it is necessary to propagate:

• For these two parameters it isn't necessary to propagate to check if you are achieving the desired change in those properties. However, although the spacecraft will have the desired properties right after the maneuver, be ware that they may change slightly during the afterwards propagation if perturbations are being taken into account.

If you are performing the maneuver in the right place it should work, at least for simple cases of maneuvers as raising the apogee or inclination change (I haven't been able to reproduce your issue).