Fascinating thread, you guys, with loads of good gen. Glad that the initial confusion between on-the-hoof INS/IRS position updating (impossible on any system I came across) and FMS position updating (happening all the time) was corrected by kijangmin and AMCS.
In the absence of today’s FMSs, with divers forms of position updating, it was important to align the INSs with the most accurate position possible. This is why, on non-FMS aeroplanes with early INS nav computers, we used to use the actual gate position. This was obtained from our Aerad/Jeppesen and/or the gate marker board at many airports. [In the latter case, beware of confusing seconds with decimals of minutes.] The more accurate the latitude, the less the INS drifted during the journey.
As kijangnim has pointed out:
An error in position input during initial alignment, apart from obvious bias, will give the wrong "G" initial value and induce wrong vertical acceleration (delta between the sensed real one and the erroneous one)
[Unquote]
And FE Hoppy:
The IRU has to align itself in-order to accurately diferrentiate between aircraft accelerations and earth accelerations.
[Unquote]
We could update the INS nav computer position en-route, but BelArgUSA’s method of doing it as you flew over a VOR was not an accurate one, because of the cone of confusion (for want of the correct term) over the station. Provided the VOR had a co-located DME, the best simple way was (and is) to take the DME distance as you fly due-south or due-north of the station (remembering to allow for the magnetic variation at the station). You know the longitude of the VOR, and simply apply the DME distance to the latitude. Try to do it when you are far enough away to minimise the DME slant-range error, but close enough to avoid a big error with the VOR bearing. I think about 30-90nm was considered a good compromise. [In the absence of DME, you can try a 45-degree or 26-degree “running fix”, assuming you know your track and GS; but you need some sort of chart for that, and you wouldn't use it for an update unless desperate.]
That earth-versus-aircraft acceleration business brings to mind the recent thread:
“A320 pitch-up at AP disconnect”