NavList:

Brad,

I agree, the most likely cause of systemic GNSS failure would be war.  I Would add North Korea to the list however.  Firing off a high-altitude skyburst with a nuclear device would destroy a large number of satellites.  In this case GNSS and comms would be drastically affected.  However, I still think the overall likelyhood of this event is quite low.

Selective availability is something I considered (I remember those days), but Frank is correct, the use of other constellations would probably still provide enough precision for sea-navigation.  As an interesting aside, I was was anchored off of Israel a few years ago and EVERY GPS receiver on the ship failed due to position loss, simultaneously.  I pulled out my battery GNSS that used alternative constellations, and I had a position.

Since my navigation isn't really for fun, I take to heart the adage "never rely on a single source for fixing position."  In coastal waters I take bearings and constantly watch radar ranges (via Parallel Index) to ensure I am where the GNSS/ECDIS says I am.  Even now as I write, my mates use radar range and bearings, logged every hour, at anchor, to confirm the ECDIS position.  I still teach and personally shoot stars periodically from the bridge underway to let me sleep easier at night.

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.  This point was recently driven home for me as I recently bought a doppler RDF array at home, and it has a plug in GNSS.  It claims to be able to plot bearings on Google Earth, but only when you're moving...

Jeremy