True north and IRS

[Lotetu]

Good morning,

I would like to know how the true north is updated in the IRS during flight.

Thanks,

Lotetu

[BOAC]

I'm not sure you understand INS? Why would True North need updating? The north pole does not move. Do you mean how does the platform know where true north 'is'?

Answer - it is a 'giro'. Unlike blondes, you tell it once and it remembers:)

[Spooky 2]

True north is a fixed position while magnetic north varies over a period of time. I don't believe there is any updating of true north as it is the definitive icon in inertial nav. Magnetic north on the otherhand is constantly moving and in a period of 960 years supposedly makes a complete 360. Haven't had a chance to check this out yet! :rolleyes:

[Captain Smithy]

A Gyro is always referenced to a fixed point in space, hence the reason you have to adjust a DI WRT the Mag Compass every so often in a non-slaved system like in a light aircraft, since the Earth always moves... however in the case of an INS, it is referenced to True North (as it never moves).

I believe most INs use ring-laser gyros as opposed to mechanical gyroscopes.

Smithy

[Lotetu]

I mean that the IRS determines the true north during alignment on ground.
But how it can keep it in flight? Off course a gyro can keep it for a while, but without update we would see the deviation after a long flight.
I checked in airplane and after 16 hours operation there was only 0.1 dergee diference between the three IRS.
So somehow it is updated.

Lotetu

[Intruder]

That's what the IRS is designed to do! The Ring Laser Gyros today have very little inherent drift, so the accelerometers that are the system keep track of True North very well without external updates. One key is a very accurate initial position, which GPS provides.

[Slasher]

I think what Lotetu is getting at is precession and apparent drift.

Precession needs mass and laser gyros have none, so then
theres no precession. But Id be interested too in how it
updates TN on its own while crossing many latitudes (without GPS).

[Lotetu]

Inruder is not fully right. It is not possible that the gyro is just keeping the north during fllight.
On ground it is determined from the turn of earth, that's why we have to provide the system accurate initial position and the airplane must be steady during alignment.

Laser gyro could be very accure, but it is impossible that it is not collecting any error in several hours.

If we put an IRS into "ATTITUDE" mode than the heading must be entered manually. This heading has to be updated manually in every 10-15 minutes, like on a mechanical gyro. But the gyro is the same than in normal IRS operation.....

[BOAC]

Guys - providing you are prepared to accept that a modern inertial platform has VERY FEW errors, please accept that True North is NOT 'updated' at any time (without an alignment) but merely 'maintained. If you ran a platform for several days without an alignment then the errors would obviously would accumulate. We have moved on from gyros spun up with string sitting on top of a miniature Eiffel Tower:)

Also bear in mind that a difference of 0.1 degrees would not have any noticeable effect on your displayed heading, attitude, nor on navigation, especially since position is constantly updated whenever in range of the required signals. You just would not notice 0.1 degrees UNLESS you look at the displayed heading data.

NB Vastly simplified for simplicity!!

The effect of a 0.1 degree error in true north would simply mean that all the electronic 'fudges' applied to the platform 'attitude' to compensate for earth rotation would be in error by 0.1 degrees - not a lot! Looking at a platform at the equator with the platform 'pointing' north, the electronically 'applied' platform roll rate, for example, instead of 15 degrees/hour would be in error by 15*sine 0.1degrees. Not a lot! I make that 0.02618 degrees per hour. Thus in simple terms, after 10 hours, the indicated bank angle would be out by 0.2618 degrees. Even I cannot fly to that accuracy. I don't think you would fail your IR renewal on that.

[Lotetu]

Thanks BOAC,

but in this case why we have to reset the heading frequently in "ATTITUDE" mode? I already tried it, and really after about 10 minutes the deviation was 3-4 degrees. The gyro is the same.

[BOAC]

As far as I know none of the corrections are applied by the system in Attitude since it has no idea where it is on the earth's surface, so it is using the earth's gravity to produce an attitude and requires a heading input at frequent intervals. I have never used it, so I do not know what would happen if you failed to input heading.

This is a guess.

I think you would still have a usable attitude display, but bear in mind the heading requires a variation input which is constantly changing with position (which it does not know) and the derivation of heading would not be corrected for earth's rotation or aircraft motion and thus would become unreliable fairly quickly.

Edited to add an afterthought:)

[rudderrudderrat]

Hi,

Almost 100 years ago, Sperry and others used the fact that the axis of a spinning gyro, with gravity causing precession, will align parallel with earth's spinning axis and hence they had something which will point to True North. They didn't need GPS to find True N. The Sperry Gyrocompass will point at True N. for ever even when at sea. It doesn't just "remember it" - it constantly finds it.

See the appendix in Sperry Gyrocompass Mark 14 (http://www.hnsa.org/doc/gyromk14/index.htm)

An IRS Laser gyro does a similar job. It can work out the direction of True North during the align process. It measure the rate of apparent wander and it's direction and then calculates it's Latitude (N or S) without any input from GPS or gate position. (For a gyro aligned parallel with earth's surface, Apparent wander is zero at the equator and 360 degs in 24 hours at the poles.) We have to put in present position because it can't sense Longitude (and it's a cross check that we know where we think we are against the Lat it accepts.)

The IRS constantly measures the direction of plumb vertical using accelerometers and hence can sense again the direction of True North. Once it's Navigating and moving over the earth's surface, it needs to know ground speed and position to allow for sensed errors (used to be called steaming errors on Ships with Sperry's gyroscope).

When in ATT, there is no navigation data so "steaming errors" are quite large, hence we have to update the direction frequently.

[Lotetu]

Yes BOAC, this could be the answer.
Maybe in "Attitude" there is no correction for the earth's rotation as the system doesn't know the lateral position. Off course change of variation also plays, but normally it is less significant.

If I have enogh time next in an airplane on ground (long slot etc.), I'll put the stby. system into "attitude" and I will check the deviation for given time. Then I can calculate the the degrees/hours deviation, and I can compare it to the theoretical value.

Just I need a table or formula for this.

Anyway I think that I got the answer for my original question.

Thanks for all of you,

Lotetu

[rudderrudderrat]

Hi Lotetu,

If you get a chance to sit with ATT on stand for an hour or so, I'd be interested to know what drift rate you get.

[BOAC]

If I have enogh time next in an airplane on ground (long slot etc.), - remember you will have no error from aircraft motion!

[rudderrudderrat]

Just I need a table or formula for this

I think you should see a stationary drift rate of (360/24) * Sin Lat .

[M2dude]

Guys, my own slant on this is that with any INS or IRS, ATT mode gives just basic attitude without corrections for earth rate or transport rate errors. It's a basic mode to keep you flying if things go very wrong with your INS or IRS.

Dude :O

[Mr Optimistic]

Global positioning systems, inertial ... - Google Books (http://books.google.co.uk/books?id=aPQxnMg9QG8C&pg=PA367&lpg=PA367&dq=ins+errors&source=bl&ots=F_kzESdkpR&sig=9SEka7ng0iUzaJWjRo6FviOG9DA&hl=en&ei=FnGLTJqhJIPGlQfDpsFg&sa=X&oi=book_result&ct=result&resnum=10&ved=0CDkQ6AEwCTgK#v=onepage&q=ins%20errors&f=false)

Don't know what limited 'ATTITUDE' mode is but suspect its as stated above. INS senses true north by sensing earth's rotation when platform is at rest: direction of 'g' gives local horizontal. With schuler tuned system transport rate couples error effects to limit divergence.

[Gulfcapt]

Rudderrat got a great explanation, well said.

If I could add just a couple of points:

Modern IRS's sense the earth's rotational velocity during alignment; therefore, it knows what latitude it is at. Inserting present position during alignment tells the unit whether it is in the northern hemisphere or southern hemisphere.

Once the unit is in nav, it switches to "position keeping" rather than "position finding"- it knew where it was when it was aligned and keeps track of where its going.

How the IRS is mounted in its rack can have a tremendous impact on drift rate. Even the slightest tilt makes a huge difference. A unit that has more than average drift prior to blocking out may need to be re-racked.
Best,
GC

[M2dude]

If you go back to the good old Delco Carousel 4 INS, true north was established as follows. During alignment the X & Y axis gyros would sense the error generated as the earth spin was detected and tilt the stabilised platform in order to keep their position in inertial space. Due to the earth’s spin axis, the platform would always dip in the east, making the establishment of true north automatic, the cardinal points were now locked in, until the system was shut down after flight. Once alignment was complete and NAV was either pre-selected or manually selected when alignment was complete the platform was then aligned to perfect horizontal, the gyros being torqued to retain their true horizontal also. As the aircraft moved along, the platform was Schuler tuned with aircraft acceleration in order to guarantee exact horizontality. An INS (or strap down IRS) can calculate latitude during alignment, by measuring the degree of platform tilt (or in the case of a modern IRS, the total component of all of the three RLG outputs). Longitude however can not in any way be calculated, and so the present position co-ordinates must be manually input.
As far as racking problems go, yes it is vital, repeat VITAL that the INS/IRS is fitted securely to the rack, but all such units have a central locking pin to ensure positive location with the rack, you should never be able to get this wrong. If someone says to you that they've fixed inertial attitude errors by re-racking the unit, then it was never located correctly in the first place. (The rack alignment is set up during aircraft manufacture and this alignment is totally critical, and must never be altered).

Dude :O

[Gulfcapt]

As far as racking problems go, yes it is vital, repeat VITAL that the INS/IRS is fitted securely to the rack, but all such units have a central locking pin to ensure positive location with the rack, you should never be able to get this wrong. If someone says to you that they've fixed inertial attitude errors by re-racking the unit, then it was never located correctly in the first place. (The rack alignment is set up during aircraft manufacture and this alignment is totally critical, and must never be altered).

Dude http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/embarass.gif


Thanks Dude, I stand corrected on my terminology. Our two jets both came from the OEM with one of the three IRS racks aboard out of alignment. It was no small task to have this corrected by them. IIRC, they referred to the process as "re-racking." Musta just been an in-house term.
Best,
GC

[rudderrudderrat]

Hi Dude,

How did Concorde's INS keep sense of True North when you were flying with a Westward ground speed which matched Earth's rotation rate? In that case the aircraft would have no apparent rotation around Earth's Axis in space, so no apparent wander.

I'm guessing that the flight time was so short, that the INS just had to "remember" where True North lay.

[BOAC]

the INS just had to "remember" where True North lay - this whole thread tells you that the INS ALWAYS 'remembers' where T N is, regardless of profile rate (now it seems called 'transport rate') and earth rate. It matters not what speed you fly.

[simflea404]

Good morning,

I would like to know how the true north is updated in the IRS during flight.

Thanks,

Lotetu

As far as I know it isn't! The Inertial Nav needs to be aligned on the ground and is not updated until you land. The alignment procedure aligns the IRS/INS to TN using the Coriolis effect (and this alignment is made faster by inputting the PP...Latitude being the important parameter). There will always be a drift during flight but the laser IN has made this a lot less than earlier designs...the drift now is due mainly to noise rather than mechanical friction. When you land and arrive at the gate, the IRS should give you the errors when you put in a fix with your current position.

When using attitude mode, I believe the frame of reference is not to TN,,,but wherever the gyro aligns and although the pitch, roll and heading information is okay in the short term will be much more likely to drift (because there is no compensation for Coriolis). I imagine that since the only other source of information available in these axes is heading, then updating this will reduce the errors in the other two.

Checking out the attitude mode when stationary may not give you the sort of drift errors you are looking for because you will have no accelerations. Without these, the errors will not be anywhere near the same.

I'm not sure what formulae you are after (and how advanced)...ones for Coriolis or ones for drift rate depending on system used?

[rudderrudderrat]

Hi BOAC,

Don't you use this system?

"Method for determining the heading of an aircraft.
United States Patent 4930085.

A method is disclosed for determining the heading of an aircraft which is equipped with both an inertial navigation system and a device capable of receiving navigation satellite signals and computing the aircraft's position, speed and acceleration therefrom. While the aircraft is still on the ground and before flight operations begin, an initial alignment of the inertial navigation system with an earth-fixed coordinate system is performed, and during this alignment, data corresponding to the initial attitude of an aircraft-fixed coordinate system relative to the earth-fixed coordinate system is also determined and stored. Thereafter, during flight operations, the acceleration vector of the aircraft relative to the aircraft-fixed coordinate system is measured by the inertial navigation system and then transposed into a coordinate system which has been horizontally aligned with the earth-fixed system using the initial attitude data. second acceleration vector, corresponding to the acceleration of the aircraft in the earth-fixed system, is computed using the navigation satellite system. The aircraft's heading is then determined by computing the difference, i.e., the angle between, the two acceleration vectors. The method is amenable to Kalman filtering techniques and, by eliminating the need for highly accurate gyroscopes or magnetic compasses, permits rapid and highly accurate heading determinations to be made at much less expense."

[bjornhall]

Checking out the attitude mode when stationary may not give you the sort of drift errors you are looking for because you will have no accelerations. Without these, the errors will not be anywhere near the same.

But the main reason for the 'north' direction to change is not system errors but simply the rotation of the earth, and that will be the same. So you should see roughly the same effect in the air as on the ground, and the formula has already been given: 360/24 * sin(Lat). Just like a perfect, i.e. precession free, directional gyro.

[BOAC]

err.............yes? Maybe no.

Isn't it remarkable that by the time we get to page 2 of a thread there is always an enormous echo in this empty hall.

[M2dude]

Gulfcapt
Our two jets both came from the OEM with one of the three IRS racks aboard out of alignment. It was no small task to have this corrected by them. IIRC, they referred to the process as "re-racking." Musta just been an in-house term.

ahh now it makes sense (really a worry how these racks got misaligned during installation, normally such a lot of care is rightfully taken over this). I can imagine the difficulty involved in getting it right, I've seen alignment during aircraft manufacure and it needs a lot of care and precision.
Thanks very much for the feedback GC :ok:
Rudderrudderrat
How did Concorde's INS keep sense of True North when you were flying with a Westward ground speed which matched Earth's rotation rate? In that case the aircraft would have no apparent rotation around Earth's Axis in space, so no apparent wander. I'm guessing that the flight time was so short, that the INS just had to "remember" where True North lay.
Hi again (in a different thread for a change :)). The 'remembering' true north is a case of the Z axis gyro taking up a position in inertial space during alignment, the angle between itself and true north will be a constant; the gyro remainig fixed on this position for the entire time the INS is operating. Sector lengths are not really an issue as far as the 'true north' bit goes, in fact it's quite irrelevant. The only time issue are just the normal time growth errors associated with any inertial system. (The fundamental limit for an INS being 3 + 3T NM; T being the time that the INS was in NAV mode).
Time growth errors were of course helped by our short sector lengths, but we still used aided navigation, where within range of a VOR, the co-sited DME clant range was used to greatly deruce errors. While oceanic we would mix the inertial positions of all 3 INSs to produce a mean computed position.

Dude :O

[Lotetu]

Yesterday I measured the drift on ground in "ATTIUDE".

It was a brand new Airbus and on this there is no display on the IRS mode selector panel, but only on FMGS MCDU. On MCDU it is indicated "True heading" and "M heading". After changing to "ATT" the "M heading" indication is changing to "heading".

The lateral was 40 27.9

The duration of the observation was 20 minutes.

On MCDU the "True heading" was not changed but "heading" decreased 3.3 degrees. Means 9.9 degrees per hour.

360/24 X sin 40 27.9 is 9.7347

Maybe a longer period would give more accurate result.

I still can't undersand that on MCDU the "true heading" was not changed, maybe just a bloody French idea again.

Next I'll check it on airplane with conventional IRS panel.

Lotetu

[Dick Whittingham]

2dude

When you say the IRS is "aligned" during aircraft manufactur, what is being aligned and to what datum. Just curious

Dick

[M2dude]

We are not talking about the IRS itself Dick, but the RACK that the intertial system (INS, INS, ADIRU etc.) will 'sit' on has to be PERFECLY aligned to the aircraf local horizontal, using highly precise measurements. (I guess that automation helps tremendously these days though).

Dude :O

[Blacksheep]

The aircraft is accurately aligned E-W in the earth frame and the IRS on its mounting is checked against this alignment to ensure the accuracy of the system/platform interface. The mounting rack is then shimmed to trim out any errors.

It is worth keeping in mind that for current Inertial Systems the aircraft IS the intertial platform (sometimes referred to as a "strap down" system). So, refering to the original question, the IRS is initially aligned to true north and the laser gyros detect rotation rates around the three platform axes. At the same time, accelerometers aligned to the platform axes are detecting rates of change in movement along these axes. All these rates are then integrated in the IRS's digital computer to resolve the changes in roll, pitch and yaw attitude together with the velocity along the axes into a change in position and true heading. True North is not detected once the aircraft is in motion. Current true heading is determined by the detected changes from the initial true heading.

[Idle Thrust]

I cannot resist posting this old chestnut, just to lighten up an interesting discussion:

Airline 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 greater), 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 now 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 discussions 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 was 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 it 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 the difference between its deviation and its variation, which is variable constant called "error".

[BOAC]

Thanks IT - best explanation to date:ok:

[forget]

Speaking of Ring L*ser Gyros, I was in a Buffalo NY bar one night when I got chatting to the guy next to me. Turns out he's with Litton INS. I casually asked him when his electro-mechanical marvels would be superseded by Ring L*sers. 'Long time yet' he said, the opposition (who I knew held the Patents) can't get the right quality optics'.

As it happened I'd been thinking about this and suggested that zero gravity optics production aboard the Space Shuttle would solve that. Well, the guy leapt off his stool, turned bright red and hollered (US expression) 'Who the **** told you about that?'.

He wouldn't let it go and I had to find somewhere else to drink. I never did find out if I'd hit the nail on the head, and a raw nerve, or whether he actually knew as little as I did.

[Dick Whittingham]

g2dude, Blacksheep,

As I understand it you accept a fixed alignment of IRS to its box and the box to the rack so you can move the rack and thus align the IRS derived E/W axis to the aircraft E/W axis

But why is this critical? And why align in azimuth for choice?

Dick

[BOAC]

But why is this critical? - I may be missing the point of your question, Dick, and forgive me if I am being over-simplistic, but the Azimuth AND heading that the electronic wizardy spits out has to represent the attitude and azimuth of the aircraft, ie the 'pitch' must be referenced to whichever datum the designers decide is the pitch datum (presumably the roll axis), and roll likewise (to wings level). Heading has to be aligned with the aircraft roll axis. If these are not set accurately then all derived acceleration vectors and attitude references will be out by 'whatever' which will degrade the overall nav performance of the kit.

[monkey_wrench]

A modern ring Laser Reference Unit is fairly complex, and it is extremely sensitive to movement in all axes - as you would expect. It does not suffer so badly from the mechnical degradation of the gyro systems. It will always give an output relative to True North - it also has a database (which needs updating by the manufacturer from time to time) which stores the Magnetic Variation around the world. Note a unit that does not have the Magnetic Database is referred to as an AHRS unit since it provides Attitude, Heading Reference only. So it is the application of the Database to True North which provides the output to the aircraft.

When in the "ATT" position, the unit is completely immune to wind or any other influence since it senses raw data and is effectively an AHRS unit. Once in "NAV" then the unit will start to be affected by external influences - hopefully at a rate not worse that 3+3T where T is Time and 3 is NM. so after 6 hours an individual unit should not be more than (6x3)+3=21 NM from the actual position.

Civilian units can only be aligned (wind, etc zeroed) on the ground but Military units can be aligned in flight.... and are more accurate.

I have installed several Honeywell Laser reference units in aircraft in the UK.
To do this on modern digital aircraft, the aircraft is jacked (in the hangar) and the units are individually levelled in pitch and roll axis.... normally using a digital spirit level at reference points on the aircraft. By taking the 429 output to the Bus with an ARINC 429 reader, you can achieve accuracy of 3 decimal places if you have the time and effort....... It should be noted that just tightening the rack down on the Avionics shelf is enough to change the reading, such is the sensitivity....

Once aligned in Pitch and Roll the aircraft is wheeled out of the hangar and placed on a surveyed line of True North. A plumb bob is dropped from the nose and tail and a measurement is taken and the aircraft difference from the nose to the survey line and the tail to the survey line is calculated..... back to the classroom and use a little pythagoras and you come up with the adjustment needed to read True North....

Now for a funny story.... a number of years ago, I asked the Ordanance Survey to survey a line for me so that I could perform the above.... having tried to align the aircraft which has only +/- 3 degrees of adjustment, I was disturbed to find I needed much, much more. This was on both Avionics racks - one located on the left and one located on the right of the aircraft with each of the 3 units installed indicating a similar large adjustment...... The man from the Ordanace Survey was called, who in no uncertain terms clearly was unhappy that some spotty individual was questioning the accuracy of his work.... "Don't you know who I am" he railed..... Having checked his line, he embarrassingly pulled up his marker and moved it a bit..... he had perfectly surveyed the reference line to GRID North which of course all UK Ordinance Survey Maps are aligned to but are a few degrees from True North.....

[Dick Whittingham]

BOAC et al.

I can see that for the attitude displays the alignment must be reasonably accurate but surely the nav solution is internal to the IRS. Or put it this way, if you flew with your flight axes all wrong - bit of rudder and opposite aileron - surely this will not destroy the nav solution?

Sorry if my questions seem a bit dim. This bit is new to me.

Dick

[BOAC]

Internal to the box, yes, but tied to the aircraft. In simple terms Nav depends on the integration of acceleration of the c of g along each axis to establish velocity and then the time at that velocity to establish where the INS isn't (as Idle Thrust says:)), so sideslip would not affect, since the motion would still be correctly sensed, just as movement of wind in any direction accelerates the c of g. In your sideslip case the platform would detect motion along the roll AND pitch axes and wiggly amps and straight volts would decide where the c of g had really gone (or hadn't gone.....)

[rudderrudderrat]

Hi Dick,

Your PFD gets it's info from the IRS. Unless it's mounted correctly, you'd have the wrong pitch attitude showing or one wing low indication when you were straight & level etc.

I agree - it would make no difference to the IRS with it's ability to do the Nav.

[FE Hoppy]

Honeywell micro IRU in the Ejets have align in motion capability. GPS required and takes between 15 and 30 minutes. On ground stationary align like most is between 6 and 17 minutes depending on Latitude.

L@ser gyros drift just as spinning gyros did. In the case of the old spinning gyros the major source of drift was the bearings. With l@sers its the imperfections in the mirrors that causes noise.

A strap-down system has to compensate for gravity, earth rotation and the earths spherical shape to work effectively.

Vertical velocity and altitude are calculated using the acceleration that is measured perpendicular to the earths surface. However, an inertial accelerometer cannot distinguish between gravitational force and actual aircraft acceleration. So any accelerometer that isn't perfectly parallel to the earth's surface will measure a component of gravity in addition the the aircrafts true acceleration so the irs has to subtract local gravity from the measured vertical acceleration.

As the gyros measure accelerations in inertial space we need to correct for the earths rotation in inertial space too. Thats once per 24hrs and once per year (people forget about that one) that works out at 15.04 degrees per hour. This correction is subtracted from the measured eastward acceleration. This is known as earth rate compensation. Without it for example, if you were on the equator and flew for 12 hours the gyro would think it was upside down.

Transport rate is the correction for navigating in curved paths over the earths surface as remember the gyros work in inertial space and so the accelerations need correcting or again the gyros would think we were upside down if we flew half way around the earth's sphere.

Now for the trivia:
It takes 3000 volts across the anodes to the cathode to start the lasing action and Honeywell ring laser gyros are triangular. The glass is made from Cervit due to its stability over wide temperature ranges.
The laser does a nice quantum trick of changing frequency to exactly match the path length as the gyro is rotated and its the change in frequency that is used to calculate the rotation rate.

[rudderrudderrat]

Hi FE Hoppy,

The l@ser does a nice quantum trick of changing frequency to exactly match the path length
I thought that at rest, a stable interference pattern was formed by the recombined laser light. The pattern changes when it is accelerated and the shifting interference pattern is measured.

[FE Hoppy]

I thought that at rest, a stable interference pattern was formed by the recombined laser light. The pattern changes when it is accelerated and the shifting interference pattern is measured.

Yep, I dumbed it down a bit. The laser travels in opposite directions around the triangular path so when the gyro is rotated the light wave in one direction gets a increase in frequency shift where as the opposite path gets a reduction in frequency. The two light waves are combined on a photo detecter and cause a fringe interference pattern. The number of fringes is proportional to the frequency difference and the fringes are detected as pulses on the photocell so appear at the output in digital form ready for processing.

[Lotetu]

I checked today the heading drifting in "ATTITUDE" on airplane with conventional IRS panel. The heading decreased on the panel and on ND as it was expected. I still can't understand why the true heading was not changed on MCDU. Next I will switch an IRS from OFF directly to ATTITUDE. Then we will se...

[M2dude]

One thing that I recently discovered, is that Ring Laser Gyros actually WEAR OUT with time, within about 15-20 years. It's not a joke, being fundimentally a gas 'discharge' tube, apparently eventually the lasing surfaces degrade with time.
I was gobsmacked when I discovered that one. (It seems that fibre optic LGs do not suffer the same fate, at least so far ;)).

Dude :O

[rudderrudderrat]

Hi Lotetu,

Once you've selected ATT, then you've stopped the IRS from processing data. Did the MCDU have the message, "IRS Frozen at"?

See FCOM 4.3.25. IRS monitor page.

Displays the selected IRS in large white font.
When data is frozen, IRS is replaced by "IRS FROZEN AT", followed by the time at which the pilot has frozen the display.

[renard]

Regarding re alignment on the EJet.

I did one two weeks ago in flight and it took 33minutes.

We tried flying straight and level but eventually had to turn and descend to our destinations , and was impressed that it could cope with turns, descents and increases in speed.

[Obi Offiah]

We tried flying straight and level but eventually had to turn and descend to our destinations , and was impressed that it could cope with turns, descents and increases in speed.
Was this during or after the re-alignment?

[renard]

During re alignment.

[FE Hoppy]

Faster re-alignment with large heading changes I'm afraid. If you have to do it again go for a couple of 1 minute legs at 90 degrees to each other and it will speed up the alignment significantly. It's to do with the algorithm used to separate the total accelerations felt by the IRS. I'm assuming the GPS signal was good throughout.

This is straight from my friends at Honeywell but I'll try and find the references in the books.

[Lotetu]

Thanks Rudderrudderrat,

it was an option to freez the IRS. I didn't do that, but maybe it did itself. There was no message. The coordinates were lost (dashes).
There are still things to be checked.

[rudderrudderrat]

Hi,

It looks like I won't need to bring my 1944 brass binnacle with Sperry Gyrocompass (to find True North) on the flight deck any more.

This Honeywell link (see page 1.9) (http://www51.honeywell.com/aero/common/documents/Laseref_V_Micro_IRS_SM.pdf)

"Rapid Dispatch Option
If extremely rapid dispatch is required, the operator may also elect to use the Align-in- Motion function to complete the alignment in flight. When the IRS is powered-up, the attitudes, accelerations, and rates are available within 5 seconds. If ARINC label 043 (Set Mag Heading) is received from the FMS once at power-up, then all TSO outputs will be available for dispatch. When the IRS completes the align-in-motion, all parameters will be available at Full Performance as specified in section 6.0."