True north and IRS

[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

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]

Originally Posted by rrat

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
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

[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

[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

[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.....)