The `MeasurementBuilder` interface is used by Orekit measurements generation feature. It is specialized for each measurement type. Its `build` method returns a specific `ObservedMeasurement<T>` (i.e `Range` for `RangeBuilder`, `Phase` for `PhaseBuilder`…). Internally, all implementations work by creating first a dummy `ObservedMeasurement<T>`, then call its `estimateWithoutDerivatives` to build an `EstimatedMeasurementBase<T>`, retrieve from this intermediate object the true measurement value and finally build another `ObservedMeasurement<T>` with the correct value. This second `ObservedMeasurement<T>` is then returned through a bunch of classes (`AbstractScheduler`, then an internal step handler, then `Generator`). At the end, it is passed to a `GeneratedMeasurementSubscriber`). I have a use case where the information present in the `ObservedMeasurement<T>` is not sufficient; I would need to get the states of the spacecraft involved in the measurement, as they hold some additional states I need. I cannot just call again the propagators that are used by the `Generator` because doing this stalls the generation (probably some kind of dead lock or infinite recursion as the propagator calls the measurements generator which then would call the propagator back). Setting up another propagator also seems a waste of resources since the states have already been computed during the measurement generation; they were just thrown away when building the second `ObservedMeasurement<T>` from the `EstimatedMeasurementBase<T>`. This issue intends to change the API of `MeasurementBuilder` and all the intermediate classes so the measurements that are built and returned to users are `EstimatedMeasurementBase<T>` rather than `ObservedMeasurement<T>`. It targets Orekit 13.0 milestone as it changes several public signatures. Partial backward compatibility is ensured as `ObservedMeasurement<T>` can be retrieved from the `EstimatedMeasurementBase<T>` using its `getObservedMeasurement()` method. The returned `EstimatedMeasurementBase<T>` contains much more information than `ObservedMeasurement<T>` (states, participants...).