Secondly, since we use tracking data to help determine our trajectory, we need to know what phenomena will affect the measuring and collecting of that data. This includes the locations of the stations on the surface of the Earth, the orientation of the Earth at the time the measurement is made, the effect of the atmosphere and the ionosphere on the signal from the spacecraft to the station, and finally the electronic delays in both the spacecraft hardware and the station components that collect the data. That's quite a lot of stuff to keep track of, but is necessary in order to obtain the best possible trajectory solution. Finally, we have to be careful to check the tracking data before using it, because sometimes we get delivered some bad data that would mess up our solution process. So the art of orbit determination sometimes involves editing out bad data, adjusting slightly flawed data, and reporting these errors to the Deep Space Network, who needs to know how they are doing.
Orbit determination is also experimental in nature.
The NAV team will try different strategies in orbit determination, always looking for the right strategy. Sometimes we'll use only Doppler data and ignore the range, or maybe we'll try the opposite. Sometimes we'll try estimating a particular effect and see what the result is, or we might choose to leave it unmodelled. Experience with orbit determination is very important to know if you've got a good solution, an okay solution, or a bad solution. After a number of experiments have been tried, we'll select a few of the better experiment results for consideration in selecting the 'right' solution.