Hi all,

I am trying for the first time the new SGP4 orbit determination feature. I could not find where to set the satellite mass.

Part of my code is the following:

```
matrix_decomposer = QRDecomposer(1e-11)
optimizer = GaussNewtonOptimizer(matrix_decomposer, False)
tle_prop_builder = TLEPropagatorBuilder(tle_initial, PositionAngle.MEAN, estimator_position_scale)
estimator = BatchLSEstimator(optimizer, tle_prop_builder)
estimator.setParametersConvergenceThreshold(estimator_convergence_thres)
estimator.setMaxIterations(estimator_max_iterations)
estimator.setMaxEvaluations(estimator_max_evaluations)
[...] # Add measurements
estimated_propagator_array = estimator.estimate()
```

When using a numerical propagator, I was setting up the mass in the numerical propagator builder:

```
propagator_builder = NumericalPropagatorBuilder(orbit_initial_cartesian,
integrator_builder, PositionAngle.MEAN, estimator_position_scale)
propagator_builder.setMass(mass)
propagator_builder.setAttitudeProvider(nadir_pointing)
```

Maybe the mass is not relevant in SGP4? Then in this case why is it required in the `SGP4`

propagator constructor?

Cheers

Clément