Feature/doppler measured frequency observation model

[simon-van-hulle]

Doppler Measured Frequency Model

This pull request implements an observation model for the Doppler Measured Frequency. The name is a temporary choice, but it is supposed to mean the received frequency in a Doppler observation.

Model

Here is some information about the implementation:

• The model requires that the receiver LinkEnd is associated with the observation time
• It requires ancillarySettings with FrequencyBands set so that the turnaround ratio can be calculated
• The model requires a TwoWayDopplerObservationModel (which it uses for the underlying LightTimeCalculators to make a MultiLegLightTimeCalculator
• A StationFrequencyInterpolator is necessary to determine the transmitted frequency
• The model relies on twoWayDopplerModel_->getMultiplicationTerm() so that it works both for averagedWithSpeedOfLight as well as the normal case.
• Finally the observable is as follows, where $h_{2wDoppler}$ is the 2-way Doppler observable and TAR is the turn-around-ratio as a function of the FrequencyBands:

$$h = f_T \cdot (1 + h_{2wDoppler} ) \cdot TAR $$

Note that it is currently only implemented for 3 LinkEnds (uplink + downlink) and I have only tested with transmitter and receiver being the same ground station.

Not ready to be merged

The following items need to be resolved:

• Change naming if necessary
• Implementation of the Partials
• Complete unit test. There currently is only a very preliminary test.
• Validation. I implemented a simple example (see tudatpy-examples), but this still shows a significant residual of which I don't know the origin.
• I think the current MultiLegLightTimeCalculator does not account for the changing transmitted frequency. We should check and fix this
• ... ?

Please let me know if anything is unclear or if more TODOs need to be added.