> a strictly IMU-only approach to lengthy navigation is, AFAIK, impossible This is true when using very-low-cost (i.e. consumer MEMS) sensors but as a general

> a strictly IMU-only approach to lengthy navigation is, AFAIK, impossible This is true when using very-low-cost (i.e. consumer MEMS) sensors but as a general statement it's false; many military and civilian space- and aircraft systems are solely or primarily inertial.. even ~50 year old passenger jets can make 12-13 hour flights with very high inertial accuracy - enough to find the dest airport runway without a radio beacon (though probably not enough to automatically land on it). For anyone interested in the technical history of inertial nav, I'd highly recommend "Inventing Accuracy" by Don MacKenzie.