NavList:

Thank you Frank.  I think I’m getting there.  Do you have to wait until an asteroid is dead in line with a star in your star list, or was that just as a simple example?  Could you say “I’m so many degrees off on a particular bearing from being in line, or my asteroid is lined up on a point 2/3 way between Betelgeuse and Bellatrix, and up a touch? 

Presumably, this is where the pixel counting comes in; the point behind the asteroid from which the position line originates can be defined by the transformation of its pixel coordinates to astronomical coordinates.
Also, with GNSS, or reverse GNSS, you’re dealing with three, ideally four plus, ranges and you’re hoping for a position defined by a small tetrahedron (although I appreciate that in GNSS this is also used to correct your receiver clock).  How do you deal with three, or three plus, straight position lines in 3D space, which might not quite touch?  Is there some sort 'least squares of perpendiculars solution'?  

DaveP