Hi Marc.

I don't think that the INS drifts that much or that quickly for me. I found on a flight from SEA to HNL, in being over the Pacific for about four hours, I was about 1 mile off the track when I came within sight of the Hawaiian coast. You can align the INS once you get in range of VOR with DME in the usual way for the CIVA INS (or at least I believe you can with a single unit).

Mark