NavList:

My final thought is that GPS has not replaced one aspect of CN: compass error checks.  Sure you can use various methods on the coast, but out at sea, the ONLY way to check error is by using celestial navigation.  GNSS is great for telling you COG, but not so good with true heading.

GNSS can determine heading completely statically, but you need two antennas separated by a reasonable baseline on the ship (say 2 meters or more).  The high-precision carrier-phase observables are used.  Effectively, the vector between the two antennas is estimated, good to a centimeter or so, and the azimuth of that vector projected onto the horizontal plane is the desired heading.

I guess at some point celestial is going to give you better precision, if the ship is steady enough and you can acquire the angle between a mark on the ship and the celestial object with, say, a theodolite or sextant.  But if 0.1 degree is good enough, GNSS can do it.

I've never used the commercial products (which are likely to be pricey), but here's one from Hemisphere:

Cheers,

Peter