Star Trackers
 

Star trackers are used in a number of applications for navigation, space craft, missiles, and the now ancient SR-71 which had a ten hour accuracy of .3NM with its 1960's technology.

Was wondering its effectiveness in the airline industry in as much as you need a clear sky, that is no cloud cover. Obviously no good if you are in IMC, so the question is, how much of your time at altitude is spent in IMC, a figure plucked out of the air I realise ie a wild guess. Not much, a lot, as a pax who has done a fair share of international and domestic travel cruise in IMC seemed an extreme rarity.

Question due to the GPS issues thread in R & N.

I'd say not a lot of time in the cruise is spent IMC, and it would be a pretty useful addition. Obviously, the people who actually account for it would probably disagree.
Less useful for shorter, lower flights in Europe, perhaps.

The IRSs are reasonably accurate for cruise flying though, so if you turn off the GPS inputs before you get to an area of known spoofing you'll generally be OK. The 76 happily flew across the pond with no GPS and few issues, other than an occasion jink to get back to where it should be when radio updating kicked in again.

IIRC, there was a US military proof of concept flight done recently using magnetic field maps and sensors as the means of navigation. Pretty incredible stuff!

You are right that IRS only means your not really lost without GPS but I think (open to correction) one big problem in these days of Performance Based Nav is that a loss of GPS may mean a loss of ability to guarantee whatever Nav Performance is needed for the block of airspace you are in.

OK for RNAV 1, 2, & 5 airspace with no GPS, as long as you have a DME receiver, so depends where you actually want to fly.