Last summer bdk6 posted a couple of links to navigation papers. Since the links to these links no longer work, I assume the links have been removed from LMR.
The main link is to a paper Using Rotations to Build Aerospace Coordinate Systems.
I also found Basic Inertial Navigation very useful (warning this link is to a pdf file).
bdk6 please come back to LMR.
I've found the NED coordinate system described in these papers very useful.