IRS update in flight.

[Blacksheep]

It is hard to generalise when there are so many different systems in use. Just to try and keep things simple but clear - In any Inertial Navigation System [INS] the inertial navigation is done by an INU or an IRU that is fed with present position on start up. Initially this present position and the position displayed on the control panel are the same, but as the flight proceeds the displayed position is the inertial position with updates added to remove errors introduced by platform drift. The inertial position remains in the computer as the basis for all calculations. Between the platform and the display a navigation computer continually adjusts the displayed position to include position updates entered from sources external to the platform. The external source may be a pilot input as described by Alex Whittingham or automatic updates within the navigation system using DME/DME cross checks as explained by oxford blue, but in either case the inertial position remains in the computer until the INS is finally shut down at the conclusion of the flight.

Manual in-flight update by overflying beacons cannot be accurately judged due to the width of the 'cone of confusion' at altitude. I think back to our 707's using, say Shannon VOR, to update a Carousel INS after crossing the Atlantic, when they would be up at 30,000 feet where the "cone" over the beacon may be up to ten nautical miles in radius - which was of course, the most commonly reported error distance reported by the crew! At the end of the flight the difference was written up as a defect and, with the 'Carousel' system, the first step is "Update Flushing" to remove any in-flight updates from the system so that the actual Inertial Position appears in the display. This is usually found to be accurate and the defect lies in an inaccurate update.

To return to lonerider's question, in the case of Honeywell systems at least, all navigation calculations are performed in the Inertial Reference System's own computer. The IRS calculates where the aircraft actually is from the inertial position as modified by fixes, the FMS calculates where the flight plan requires the aircraft to be and, if there is a difference, the FMS produces outputs to the AFCS or Flight Director to enable the aircraft to acquire and maintain the desired flight path.

This arrangement combines the benefit of the short term accuracy of the inertial sensors with the long term accuracy of multiple position fixes from ground based navigation aids or from GPS.

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Through difficulties to the cinema