NavList:

"It would then start searching the sky looking for a
star of a specified color and intensity.  When it found a star with the
right color and intensity, it would place that star in the center of the
visible star field and then rotate around until it found a second star
of specified color and intensity.  With two, it would then look for a
third.  This would enable the onboard systems to determine its orbital
position and orientation.  From there, the IUS would look at the end
state of the mission profile (the desired high altitude orbit, inner
planetary mission, etc) as well as other parameters and "decide" the
length of each rocket engine burn as well as turning points, and places
where it should seek stars to confirm it's position."

 

Notice that this process doea not yield a positional fix in the sense of ordinary celestial navigation. Instead, it's checking the spacecraft's orientation. Knowing which way you're pointing is much harder in space than in other applications.