@Npars01 @randahl @kleisli Thank you so much for this superb and pertinent set of references (except the last which was someone singing a song). VERY useful and I'm saving them for future reference. It's a long time since I used inertial nav and a lot seems to have changed since. Recently I chatted with a group of children flying model planes on a nearby field. They were simulating approaches to a virtual runway using inertial nav. I was amazed that IN now only needs a cheap chip, no gyros.