Quote from underfire:
"The IRS needs to be updated or it does drift..."
Response from me:
"Really? Would you care to be more specific?"
Reply from underfire:
"IRU fast/regular ALIGN...."
underfire (again!),
You simply cannot realign an INS or IRS when the vehicle is in motion, which is what we are discussing here.
Quote from BOAC:
I still maintain that whatever 'updates' are available, only the derived positions are updated (via the Kalman filter) and not the platform alignment from which the root accuracy derives.
Agreed. Old hands will remember that, before the days of FMSs, we had dual or triple INS systems with an associated computer, into which you entered up to about 9 waypoints manually using the Lat/Long. IIRC, you could display track and distance between selected waypoints (or to the next one), the current TRK(T) and GS, the current HDG(T) and Drift, the current W/V (if you were TAS-equipped), or PPOS (present position).
If you were curious (or desperate), there was a facility to update the
displayed PPOS. Unless the INSs were all really in trouble, in those pre-GPS days you could never hope to get a
fix of position accurate enough in the air to improve on the (
dead-reckoning) position that an INS was giving you. (The least-poor method was when you were passing about 30 - 60 nm north or south of a VOR/DME station.)
Latest IRSs are much more accurate than the INSs we had in the 1970s, but
AFAIK they remain a DR (dead-reckoning) form of position estimation based on accelerations in azimuth from stationary at the start point. They are incapable of providing a FIX of position. If they get a bad start (mis-alignment) from the point of origin, particularly if the alignment latitude is slightly wrong, the track and GS values will be inaccurate, and therefore the position error builds up proportional to time. And they cannot be realigned when the vehicle is in motion.