Hello,

I am currently investigating Gravity force model for my propagation study and I was wondering how is the raw Cosine and Sine coefficients coming from a EGM data is getting converted into time-dependant coefficients. How are the coefficients related to the time?

For example,

Lets consider EGM 96,

The raw values are:

`C_{2, 1} = -0.186987635955e-09`

and

`S_{2, 1} = 0.119528012031e-08`

and when I query the C and S parameters I get different values:

```
gravityProvider = GravityFieldFactory.getNormalizedProvider(2, 1)
gravityProvider.onDate(initialOrbit.getDate()).getNormalizedCnm(2, 1)
gravityProvider.onDate(initialOrbit.getDate()).getNormalizedSnm(2, 1)
```

the above gives

`C_{2, 1} = -5.164244223389185e-10`

and

`S_{2, 1} = 1.642505567284939e-09`

and when I add a shift to the date,

```
gravityProvider.onDate(initialOrbit.getDate().shiftedBy(100.0)).getNormalizedCnm(2, 1)
gravityProvider.onDate(initialOrbit.getDate().shiftedBy(100.0)).getNormalizedSnm(2, 1)
```

`C_{2, 1} = -5.164245325805116e-10`

and

`S_{2, 1} = 1.6425059324084946e-09`

What could I be missing?