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:

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.

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.

- remember you will have no error from aircraft motion!

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

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