Really cheap INS?
I was thinking the other day about how cheap you could make an INS - I then realised you could probably do it with the later smartphones on the market.
I've got an HTC Desire HD and inside it's little case is -
Three-axis accelerometers
Three-axis gyros
I don't know how accurate they are or what the resolution is, but could a cheap & cheerful INS be obtained from a suitable app? (Yes I've looked, couldn't find one)
And yes GPS has made a lot of this sort of thing redundant, but it'd be interesting to see how your mobile phone works as an INS in-flight .... in flight mode of course.