Chuck
The problem with GNS is solar flare activity and bad fuselage mapping for proper antenna positioning. When we had them at the airline, it was sometimes impossible to fly Hono to the west coast. I remember having to do a lane resolution coasting out from AK on the way to Asia. We came really close to doing a turnback before the
thing resolved it's location. There were also issues with determining which one was the most accurate on the other side of the pond. Too easy to be tagged with a nav error on the coast-in.
I'll take INS/IRS with GPS update any day over GNS...or an astro compass and a cheap watch. Shoot, my iPad does better than a GNS...and I can scrape the ice off my windshield, too.
I still miss flying the Twin Otter.