How do you navigate a spacecraft?
For a spacecraft, determining the location and heading requires a radio link from the spacecraft to ground stations. This tells us the current distance because we can see the signal travel time. We also see the Doppler shift, a change in the received frequency depending on the speed at which the spacecraft travels towards or away from Earth.
We need a lot of measurements that cover a time span of days or weeks. Then a very complex, repetitive mathematical procedure on a computer finds out one set of orbital elements at a given time, which reproduces the set of measurements obtained. In theory, once the orbital state is known, the state can be computed at any later time. But in practice this does not work out, because the state is never known perfectly. Also, the orbit is perturbed by factors that we don’t know. Our prediction and the actual orbit will drift apart, and the above process must be repeated regularly. Getting to the target requires correction manoeuvres using small rocket engines on the spacecraft. These manoeuvres can be computed and applied every time the orbital state has
been updated.