ins/irs 'schuler effect'
 

Years ago ,with the DC8's,and the intro' of INS,word was spread regarding the Schuler/ effect whereas the INS's would 'wander',as was evident via the X/Track readout of the 2nd unit,a certain distance then return.information was dissemenated re'this phenomenon by LITTON..Has anybody still have the reference for thei Info'???Thanks and Cheers:confused:

It was a theory discovered by a German of that name early last century, and it's an 88 minute period. That's the period that a pendulum that's the length of from the Earth's surface to the centre of the Earth.
I'm not sure how that relates to an INS, but he was ahead of his time as it was about 50 years before the technology was good enough to build something that needed his equations to correct themselves.

I may be wrong in all this, but it's what I read or heard some time ago.

I don't have the data to hand, but from memory it's about 78 minutes and 150 metres or so at the surface (or the surface of SW England anyhow) - basically the earth wobbles!

To measure it, take a good INS, and a GPS system tied to it, then feed the position outputs from both into a computer. When you've data for 4 hours (more the better), plug the position data from both into a spreadsheet (or any other analysis package) and in a third column / data set calculate the linear distance between the two position reports. You should get a nice neat sine-wave of distance against time with a 78 minute period.

I did do this once as part of a student exercise at ETPS (comparing INS and GPS position reports on the school's BAC 1-11) but damned if I can find the graph I'm afraid, otherwise I'd post it.

Another way to do it would be with only the INS system, strap it down, and keep comparing position by distande from the first position report - should give you the same result.

Didn't somebody once use the frequency of the Schuler cycle to calculate the mass of the earth?

G

Its a fairly simple thing, and something a pilot does not need to worry about as they have absolutly no control over it.

Small correction to 18's reply, its an 84 minutes on the surface of the earth.

How this error effects an INS I will try any outline as simply as possible.

The basis for an INS is the inertial frame of referance, for an INS to "show" no motion it must be "falling" towards earth accelerating with gravity at 9.81 m/s or 32.2 ft/s however you want to look at it so that in its frame of referance it is not accelerating, a stationary INS is accelerating towards earth at a rate of gravity.

If the INS is not "falling" then to get a "zero" change in the inertial referance frame the INS must deduct the "gravity" out.
Accelerometers measure acceleration, some of the acceleration the accelerometer "sees" is gravity.

The magnitude of gravity changes with height and position on earth, and even the composition of the ground below. (if you remember newtons law of gravity, gravity reduces with height from the centre of mass "earth" by an inverse square, and earth is not a perfect sphere, it has a flattening factor, the more dense the material, the higher the mass, the higher the gravity attraction)

http://www.jpl.nasa.gov/images/earth...652-browse.jpg

This image shows the variation of gravity across earth.

Now we have established that the INS need to know what the gravity is at its present position to, but where am I, and what is my gravity now ?

So the poor old INS needs to go away and do loop using the acceleration->velocity (vector)->position-> calculate gravity loop back at the new position, this is a feedback loop which will oscillate, at the Schuler frequency, and is dependant on the accuracy of the calculated gravity at the position.

How much will it vary ...
Altitude : for commercial aircraft less than 1%
Latitude : 0.0001->0.001 %

What I also find amazing is that the weather can effect the earths actual gravity field, if you look at this GEOS page you will see evidence of a "gravity wave" moving at 20-30 knots as a result of a tropical depression.

From the published conclusions of a Simulation study of INS navigation you can see some typical errors, they state
NB If you are a CAA/JAR examiner this is not the sort of thing a pilot will need to be examined on, from what I hear there is already enough useless crap you guys write questions on.

Gents,thanks for the info (I did 37 years Airline),recent enquiry as to this by a ETOPS facillitator.I remember Litton put something out after a few years of INS ops.....
I've an enquiry in with them also..Cheers:O

I'm not sure that I agree with Gengis here. I always thought that an aircraft INU was "schuler tuned", so that the tendancy to drift or oscillate about its position was cancelled out. If this is the case then surely a serviceable INU would not exhibit this trait and schuler effect would not be measureable.

I certainly remember that a crew report of oscillation with a period of about 80-90 minutes was cause for an INU removal due to failure of the internal schuler circuits.

i agree with mono. I'm sure the schuler period is accounted for in an INS.

mono & wobbly

Newer INS systems tend not have this problem esp your laser rinig type setups they are more advanced, ones that were more mechanical did, M7V started off by talking DC8, not 747-400.

swh,

I got my instrument licence 20 years ago when, although LRG IRU's were about, the mainstay were the Litton and Carousel INU's which used standard rate gyros (the Carousel INU platform was actually mounted on a rotating "carousel" to reduce any steady state errors. If you opened it up to look inside it was a joy to behold!).

I digress, When I went on courses we were told of Schuler effect and informed that it was accounted for in the system. Hence my reference to unit removal if a 80-90 minute drift oscillation occured.

I don't deny that INU's may exist which can have their Schuler circuits disabled so that the Schuler period and amplitude can be determined (that is why I clarified my comment with "AIRCRAFT INU") I just have no experience of them.

Thinking back, I don't think I've ever worked on an INS whose basic design was later than about 1975 so that may explain the difference between my theory and anybody else's. Having said that, I'm not sure the amplitude (a few hundred metres at worst I believe) of the Schuler cycle is such as to cause unacceptable errors in any INS - it's not used as an instrument landing system is it ! I wonder (and need to get some old course notes out when I've time) whether the issue of Schuler errors, that may need something doing about, in a working INS may be cumulative rather than absolute? In other words that the cycles can lead to a "divergent position error" that could steadily increase to peaks big enough to be a problem.

G

I'm sorry, but none of you has quite got it right - you've all picked up on certain points of detail , but you haven't understood the problem and why it occurs.This is not an easy subject to explain in a couple of paragraphs on a website like this but, in broad terms, Schuler tuning works like this:

You have to keep the INS accelerometers level with respect to Earth gravity because, if you don't, they will pick up a small component of gravity and misinterpret this as lateral acceleration. You can't use gravity to reference the vertical and keep the platform level because you're trying to use it to measure accelerations - so you can't use as a datum the thing that you are trying to measure.

So you have to keep the accelerometers level to Earth gravity by using gyros. The trouble is, gyros are rigid with respect to inertial space, not to an Earth-based frame of reference. As you travel over the Earth, the direction between you and the centre of the Earth (ie, the gravity vector) changes. So you have to tell the gyro this. It's called the vertical component of Transport Wander(or Profile Rate, in some books). The way you correct the platform/gyro is to have a feedback circuit from the velocity integrators which keeps the gyros level to Earth. It's called the V/R loop (for latitude) or the U/R loop (for longitude).

Incorporating a V/R feedback loop into the design of an INS/IRS to keep the platform level is called 'Schuler tuning' - a fancy buzzword which, in my opinion, tends to confuse.

However, if the initial conditions leading to the calculation of the velocity are in any way in error (which they are on nearly every flight, to a very small extent), then the V/R loop oscillates - at a period of 84.4 minutes, giving you a position error (usually up to a couple of miles or less) which will never increase to beyond its maximum amplitude, and then repeats itself in a perfectly sinusoidal pattern every 84.4 minutes.

Schuler Tuning is used in both the INS and in the IRS and so you get Schuler period oscillations in both the INS and IRS.

It is not possible to remove it within the INS/IRS, but if the output of your IRS goes to an FMS, the error is mathematically modelled and some of it is removed within the FMC.

Hope this helps.

84 minutes !
 

There's an interesting explanation and history of the basic physicshere , as well as a useful explanation from Marconi in this slightly lengthy pdf file

P

Oxford Blue.
Very good old chap. You could have copied that directly from my honeywell notes.

Even the latest state of the art IRUs (Ring lazer with Align in Motion) still schuler loop.

Forgive me for intervening, for I am not a world expert on INS.

It seems to me that we are talking here of two different classes of errors linked to the Schuler characteristics of the system, those caused by internal system faults and those caused by external problems.

As Oxford Blue says, any INS system has to determine during set-up and maintain during flight a value of earth gravity in order to establish first the vertical and then the horizontal navigational axes, N/S and E/W. To maintain these datums once set up and free to navigate it has to respond to movement about the earth as though it were a Schuler pendulum, rotating the horizontal axes by one degree for every 60nm moved. This means that for any error in the earth gravity loop, that is alignment or function of the accelerometers or of the first stage integrators, there will result bounded errors of velocity and position oscillating at the Schuler frequency. Errors in the gyros or in the second stage integrators will produce steadily increasing errors, sometime with a Schuler oscillation superimposed.

However, a basic INS may assume it is working over a uniform spherical earth, and the earth is neither uniform nor spherical. This will mean that the calculated position of the vertical and horizontal axes will diverge from the actual earth axes with movement around the earth, and that this will introduce Schuler errors
even in an INS that is internally error free. There is also the problem of errors due to vertical aircraft acceleration and the navigation errors that result from flying at different heights above the earth’s surface

An INS that contains a model of the earth that is a geometrically correct oblate spheroid will be more accurate, and one that contains a map of the earth’s gravitational fields even more accurate. This data is available, but only the latest INS systems have the capacity to use it.

It is comforting to know that errors due to external factors are small, much smaller than those that routinely appear from internal INS defects.

I thought the Marconi summary was very clear and good.

Dick W

Interesting info, thanks gents.
(I like to get my story right)