NavList:

Ed - I work on satillites for a living, and the one that we are currently building uses both star trackers and sun sensors to determine where we are pointing.  Normally we just rely on the star trackers.  We use two of them pointed at about 50 or 60 degrees from each other.  In theory you would only need one, but we find that having two of therm pointed at some large angle away from each other is much more precise.  We aren't trying to determine our position, just our "attitude" - i.e., where we are pointed.

The sun sensors are a backup.  The problem with star trackers is that if you are rotating rapidly, they don't work fast enough.  So if you are in a tumble you can't rely on them. Consequently we also position about a dozen sun sensors about the spacecraft, and use them to get us more or less stabalized before we can switch on the star trackers for more precise alignment.