Quote, "The one inertial system that I am aware of that could be position updated in flight was the Carousel INS. I remember the start of the technique was to select position and hold over a known position like a VOR overhead, memory fades beyond that point." Alex, I believe it was the "DME Update" We used it Oceanic over the Pacific back in the early 80's. Fly to an en route VOR/DME (Marshall Islands IIRC in our case) and press DME Update when overhead,(holding was not required). Max allowable cumulative displacement error at trip end was 3 + 3T NM, where T = Time in Nav mode. Cheers.