| | [No title] (Site not responding. Last check: 2007-11-07) |
 | | -- Visibility of the spacecraft's trajectory comes from radiometrics (doppler, embedded timing/ranging signals, triangulation techniques, etc.), which are reliant to some extent on knowledge of the condition of the telecommunication system aboard the spacecraft as well as reliability of DSN (Deep Space Network) stations on Earth. |
 | | Until/unless a spacecraft is very close to a planet such that the planet's gravitational influence dominates all other factors, radiometrics are the sole visibility into trajectory (pointing is too crude at such distances to provide trajectory information). |
 | | In short, radiometrics are your window into your trajectory, and working radiometrics is also an artform. |
| quest.arc.nasa.gov /mars/ask/new/Trajectory_to_Mars.txt (361 words) |