IRS question

[Lester]

Hi,
Here's my question to the panel.... The good FCOM tells me "an IRS removes erroneous groundspeeds on the ground after it's been stationary for 3 mins". OK, but.....how does it know it's stationary if it's suffering with erroneous ground speeds???
Now, on a previous type I flew (small and pointy), with and old gimbled INS ,the system use to consider anything less than 4 kts was an erroneous ground speed and would start to correct for it regardless. Problem? Yep, slow taxy would screw it up.
Any ideas how the IRS does it?
Many thanks in anticipation
Lester

[BOAC]

how does it know it's stationary if it's suffering with erroneous ground speeds? You tell it. Therefore ANY GS is erroneous and is electronically nulled. 'Excessive movement' during alignment will negate alignment.
Of course - it isn't ACTUALLY 'stationary'.....................:)

[Golden Rivet]

Park brake discrete ?

[Lester]

You tell it. )
Sure, during align it knows it should be stationary. But my understanding is that if you stop during taxy for 3 mins+ it starts to resolve those ground speeds again - how?
Ref - park brake; Been there:
P1 "OK park brake off"
P2 "NO WAIT...the INS is still in align!!"
P1 "It's OK we didnt move, put it in NAV now"
P2 "Yeah, should be OK it's in NAV now"
The rest was history with 800 kt ground speeds - doh!

[BOAC]

Sorry Lester - thought we were still in 'align'. 800kts? - risking a 'fast taxy speed' thread here:)

I know not the answer - presumably this is A/bus (FCOM?). I'm not aware of the 737 doing it.

I assume that with GPS input it would be possible?

[ICT_SLB]

There are two sources, or more correctly, sets of sources for ground speed within the IRU itself. The primary is the set of 3 laser gyros themselves and the other is an equivalent set of 3 accelerometers. The software can thus differentiate when the IRU is stopped without an external (GPS or air data) source although most IRUs take in air data for windspeed & direction calcs.

Some modern IRUs (e.g. Honeywell Laseref V) can now realign in flight using GPS-aiding - something that unheard of outside some military applications a few years ago.

[cribble]

ICT SLB
After a good deal of thought, I am still unable to grasp how rotation sensors(the ring laser gyros) can do an output for translation and thus velocity?

[ICT_SLB]

ICT SLB
After a good deal of thought, I am still unable to grasp how rotation sensors(the ring laser gyros) can do an output for translation and thus velocity?

The Laser gyros are not true gyros. In fact they are, in fact, optical devices (glass block or coils of fiber optic cable) with a fixed path. The laser is shone along the main axis in both directions and, after going around the path, interferes with itself to give rise to fringes (areas of brighter & darker light) which is detected to give a measurement of movement in the direction of the light path. The light paths differ by the actual movement of the platform in the time it takes to go around the path. No movement, no fringes as there is now no difference in the effective light path. There are normally 3 laser gyros at mutual right angles to each other so that the actual resolved accelerations & angles can be calculated & read out.

This is a VERY simplified version of what's inside an IRU and doesn't even go into the calculations - my defence is that I only install & test inertial systems not build them and it's been longer than I care to think since I sat in the Singer-Kearfott lecture at Boring....

[cribble]

(possibly off thread)
ICT
Correct up to a point. No rotation, no moving fringes would, IIRC, be the correct statement. A Sagnac Interferometer (general case of Ring Laser Gyro) won't output if there is no movement of the interference pattern. There will, AFAIAA, be no change in the intereference pattern without an apparent path length difference, and there will be no path length difference without rotation. Unless there are some quantam physics properties that override simple mechanical maths, then there would appear to be no way that this rotational sensor could also sense a linear translation. I stand, as always, to be corrected.

edited to insert "moving" into the phrase 'no fringes'

[ICT_SLB]

Cribble,
If there is movement, there will be a difference in actual light path equivalent to the distance moved by the inertial platform in the linear direction of the path (rotation doesn't come into it). As I said, I left out a lot of other considerations. If you want more, can I suggest going to the Honeywell or Northrop-Grumman (makers of Litton IRS) websites.