View Full Version : IRS nav

30th Jul 2002, 21:07
I'm trying to get my head around IRS nav. If IRS is self-contained how does it correct for inaccuracies in forecast enroute winds when an aircraft is using IRS nav only?

30th Jul 2002, 21:42
Not sure how far you are into IRS, 'kouldurepeat' (and I shall say this only once :cool:), but

IRS measures the acceleration forces on an aircraft, produced either by engines, brakes, bank, yaw, pitch or wind (plus some 'esoterics' you do not need to bother about). Therefore ANY change in wind is read by the IRS and computed. Forecast wind is only relevant to the FMC and has no bearing on 'IRS nav' which reacts to ACTUAL effects.

2nd Aug 2002, 02:07
Thank you BOAC, Iím an esoteric club wannabe. Bank, yaw, pitch feeding to the FMC
I understand but "change in wind" Iím stuck. How do IRS detect this? Is it the drift angle variations it requires to maintain a given magnetic heading?

2nd Aug 2002, 04:06
The IRS is essentially a big DR computer - it doesn't 'detect' wind as such. It continuously works out the aircraft's track and groundspeed using input from the accelerometers. It also receives heading input combined with TAS from the air data computer, and thus it can calculate the wind velocity at any time.

2nd Aug 2002, 16:51
Inertial Guidance Systems

The aircraft knows where it is at all times.
It knows this because it knows where it isn't. By subtracting where it is from where it isn't, or where it isn't from where it is (whichever is the greatest), it obtains a difference, or deviation.
The Inertial Guidance System uses deviations to generate error signal commands which instruct the aircraft to move from a position where it is to a position where it isn't, arriving at a position where it wasn't or now is. Consequently the position where it is, is the position where it wasn't: thus, it follows logically that the position where it was is the position where it isn't.
In the event that the position where the aircraft now is, is not the position where it wasn't, the Inertial Guidance System has acquired a variation. Variations are caused by external factors, the discussion of which are beyond the scope of this report.
A variation is the difference between where the aircraft is and where the aircraft wasn't. If the variation is considered to be a factor of significant magnitude, a correction may be applied by the use of the autopilot system.
However, use of this correction requires that the aircraft now knows where it is because the variation has modified some of the information which the aircraft has, so it is sure where it isn't.
Nevertheless, the aircraft is sure where it isn't (within reason) and knows where it was. It now subtracts where it should be from where it isn't, where it ought to be from where it wasn't (or vice versa) and integrates the difference with the product of where it shouldn't be and where it was; thus obtaining a difference between its deviation and its variation, which is a variable constant called "error".

Hope this sheds some light on the subject.

14th Aug 2002, 15:48
Thank u 4 your excellent and informative replies folks. Long live pprune;)