As I understand it, to do navigation effectively in space, you need to measure (or be able to calculate) at least 10 things:

- Your orientation in space (3 dimensions, easily calculated from the angles to distant stars)

- Your position in space (3 dimensions)

- Your velocity in space (3 dimensions)

- Time/Duration (1 dimension, measured with a local clock)

Unfortunately, measuring round-trip time only gives you the *radial* component of distance from your base.

- the two transverse directions could be determined from very-long baseline interferometry (VLBI) - essentially a group of satellites forming a space-based telescope capable of measuring very fine angles.

- You can calculate your postion by dead reckoning if you accurately know your velocity and time, plus your initial position (mathematical integration)

- Or you could measure angles to nearby stars

And measuring Doppler shift only gives you the radial component of speed

- Again, VLBI can help measure the transverse components of velocity

- You can calculate your velocity by dead reckoning if you accurately know your orientation, acceleration and time, plus your initial velocity (mathematical integration)

- Alternatively, high-resolution spectroscopy can precisely measure your velocity relative to bright stars, in 3 dimensions

During the course of an interstellar flight, the positions of nearby stars will change, since they have their own independent velocities through the galactic disk. So you need to know the positions and velocities of these stars, too. That is a lot of unknowns, and you would want to take repeated measurements of a lot of stars to navigate accurately.

Fortunately, the Gaia satellite is collecting some of this information as we converse. You need the ability to refine these estimates as you are travelling, observing these stars over extended periods, from multiple angles.

But for a journey to another star, the main thing you need to know is your distance and velocity relative to your destination, so that simplifies the problem.

The space between the stars is mostly empty, but not *entirely* empty. A colision with a dust grain when traveling at interstellar speeds is likely to prove fatal.

Many a ship has foundered on uncharted shoals.