My own personal summary (which consolidates a lot of what people have said already)............
INS/IRS begin with a known position (the parking bay), and via very sensitive accelerometers compute movement from that initial position to calculate Ground Speed, Position etc. As the accelerometers are very sensitive, they provide
EXCELLENT instantaneous Ground Speed etc. data. The problem is (as
411A points out) that they do tend to slowly deviate from absolutely correct position, within the 3+3T allowance for a serviceable unit (That is, 3 miles + 3 miles per flight hour). On a 10 hour ocean crossing, a unit at it's allowed limits could be up to 33 miles in position error. This is perfectly safe as that would put the aircraft within normal Radio Nav coverage at the destination, but totally unacceptable for the modern RNAV approach. The earlier fix was to input accurate Radio Nav information (such as DME/DME fix) to pull the INS/IRS position back "into line".
Thus INS/IRS is an excellent short term navigation reference, but had it's limitations for long term.
GPS provides instantaneous position with
EXTREME accuracy. It operates by taking a series of fixes a few seconds apart, and computing the Ground Speed and tracking between these series of fixes, a bit like "Join the dots" in kid's games to get the full picture. "Instantaneous" data is merely an average of what happened over the last few seconds.
Thus GPS is an excellent long term navigation reference, but had it's limitations for short term.
Now, provide the INS/IRS with it's long term position problems, with the GPS with it's excellent short term position resolution, and you have close to the perfect system..... a system which is excellent for both short and long term navigation information. Both systems are "quite good" standing alone, but about as "good as it gets" in partnership.
vapilot2004, I'm sure that you knew, but for those who don't, Triple mixing is a bit of a misnomer. The 3 positions are not averaged (as the name would suggest), but the MEDIAN Latitude and MEDIAN Longitude of the 3 systems are used to eliminate possible "bad" data from one of the units (A dual system could not do this).
Yes,
seilfly, Vertical Gyros are a thing of the past with IRS aircraft. Attitude comes straight from the IRS. Some INS aircraft did use this, but INS came along as an "add-on" to many existing aircraft with VGs, and thus the INS' capability for attitude provision was not used on these aircraft (but could have been).
Now, for a bit of rest, I have a long ocean to cross in a few hours time (the big one) with GPS/IRS of course....... and I know that the FMC track line will be "bang on" with the Localiser at the destination, no more map shift for me!
I hope that this ramble has not muddied the waters.
Regards,
Old Smokey