True north and IRS
Good morning,
I would like to know how the true north is updated in the IRS during flight. Thanks, Lotetu |
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:) |
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:
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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 |
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 |
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.
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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). |
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..... |
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. |
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. |
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:) |
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 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. |
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 |
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. |
Originally Posted by Lotetu
If I have enogh time next in an airplane on ground (long slot etc.),
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Just I need a table or formula for this |
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 |
think this reference is 'open' to read on internet
Global positioning systems, inertial ... - Google Books
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. |
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 |
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 |
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/sr...s/embarass.gif Best, GC |
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. |
Originally Posted by rrat
the INS just had to "remember" where True North lay
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Good morning, I would like to know how the true north is updated in the IRS during flight. Thanks, Lotetu 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? |
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." |
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. |
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. |
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. 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. 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 |
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 |
2dude
When you say the IRS is "aligned" during aircraft manufactur, what is being aligned and to what datum. Just curious Dick |
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 |
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. |
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". |
Thanks IT - best explanation to date:ok:
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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. |
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 |
But why is this critical? |
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..... |
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 |
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.....)
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