Originally Posted by
flash8
IRS realignment in flight I was always under the impression is impossible...
Probably true for commercial aircraft. In theory it can be done. GPS can correct for IRS position errors, but not so much for attitude. There are GPS receivers that employ multiple antennas (on wingtips and tail) that can compute 'which way is up' based on comparing the phase of a received satellite signal at each. But I'm not sure how precise that will be.
Back in the 1960s, gyro IRS systems that were expected to remain precise over longer periods of time could use star spotter systems to provide in-flight calibration. Apollo had a manually operated system on board the command module. And the B-70 design was to incorporate an automated system that would work at high altitudes even in daylight.