Latitude limitations for the use of IRS

[DJ Flyboy]

Hello, during one of my interviews I was asked about the N and S latitude limitations for the use of IRS. Does any one know the exact latitude N and S values? Apparently they aren't the same latitudes and South latitude is lesser than the north. What is the reason for this? What is the reason for the limitation in the first place? I cant seem to find the answer in any of the books I have.

[hoss183]

I cant answer the question directly, but knowing how IRS works, i cant see why there would be any latitude limits. A gyro based IRS would track movements in 3 axes wherever it is on the planet surely?

[FlyingStone]

The exact latitude limitations will depend on the aircraft type. Most manufacturers offer "polar" option, which removes the latitude limitations and I believe the limitation is more related to the FMC and autoflight rather than the IRS, as the FMC has to transit into the "polar" mode and use true headings/tracks/courses for navigation rather than magnetic ones. Same goes for the autoflight system.

This Boeing article (http://www.boeing.com/commercial/aeromagazine/aero_16/polar_nav_by_model.html) on navigation in polar regions might be useful.

[DJ Flyboy]

I cant answer the question directly, but knowing how IRS works, i cant see why there would be any latitude limits. A gyro based IRS would track movements in 3 axes wherever it is on the planet surely?

Thanks, that is what I thought too but apparently there is a limit, at least according to the person interviewing me.
I remember there being a limit for INS. Approximately 70 degrees N/S because the INS cant figure out E/W from N/S due to high latitudes. Any idea about that. Cant seem to find the reference anywhere though.

[DJ Flyboy]

The exact latitude limitations will depend on the aircraft type. Most manufacturers offer "polar" option, which removes the latitude limitations and I believe the limitation is more related to the FMC and autoflight rather than the IRS, as the FMC has to transit into the "polar" mode and use true headings/tracks/courses for navigation rather than magnetic ones. Same goes for the autoflight system.

This Boeing article (http://www.boeing.com/commercial/aeromagazine/aero_16/polar_nav_by_model.html) on navigation in polar regions might be useful.

I agree. Correct me if I'm wrong but doesn't the IRS always give true track/GS/ True heading?

[FlightDetent]

DJF, not really. The machine does calculate with TRUE track inside, however, the information supplied out to the various other aeroplane systems is converted to MAGNETIC heading for compatibility. There is a magnetic variation table (hopefully well updated) inside the IRS boxes.

What FlyingStone means is that some aeroplanes have a switch to change all navigational instruments / computers into TRUE track, which enables polar area navigation. The IRS do not care as long as they had been built to operate at high latitudes.

At least on the CEO Airbus generation.

[DJ Flyboy]

The machine does calculate with TRUE track inside, however, the information supplied out to the various other aeroplane systems is converted to MAGNETIC heading for compatibility. There is a magnetic variation table (hopefully well updated) inside the IRS boxes.
Thanks, I was trying to imply exactly that.

So basically there are no limitation then apart from the switching to true navigation in polar regions? As in no machine (accelerometer) restrictions?

[wiggy]

Thanks, that is what I thought too but apparently there is a limit, at least according to the person interviewing me.
I remember there being a limit for INS. Approximately 70 degrees N/S because the INS cant figure out E/W from N/S due to high latitudes. Any idea about that. Cant seem to find the reference anywhere though.

As has been said by FlyingStone the limit is going to be equipment specific..FWIW a 70N limit would be very limiting for a lot of Long Haul operators since it would rule out many Polar/Arctic routes taken by many IRS equipped aircraft, and we certainly operate well north of 70N on some of our routes.

[FlyingStone]

So basically there are no limitation then apart from the switching to true navigation in polar regions? As in no machine (accelerometer) restrictions?

Probably not, but nevertheless limitation prevents you from operating outside of that area.

It could also be the case that certain aircraft are fully able of polar flight from the factory, but require an AFM supplement or modification enabling operation over the poles, which obviously costs a lot of money.

[compressor stall]

DJ Flyboy Don’t confuse the two limitations.

1. Alignment. Most IRSs can’t align above the low 80s N/S as there is not enough change in velocity to align the gyro. Think a gyro at the pole is just rotating like a record player. These limitations are invariant of hemisphere.

2. Operational areas that give accurate mag track for the ADIRUs. This is dependant on the magnetic variation tables which are no symmetrical in each hemisphere.

[DJ Flyboy]

2. Operational areas that give accurate mag track for the ADIRUs. This is dependant on the magnetic variation tables which are no symmetrical in each hemisphere.

Ahh, got you. Any idea of the latitude limitations for the second point? Thanks.

Thanks everyone for the answers. Much appreciated

[hoss183]

DJ Flyboy Don’t confuse the two limitations.

1. Alignment. Most IRSs can’t align above the low 80s N/S as there is not enough change in velocity to align the gyro. Think a gyro at the pole is just rotating like a record player. These limitations are invariant of hemisphere.
Thats complete rubbish, you clearly dont understand how IRS works with that statement.

[fox niner]

Well, the Boeing answer is in the FCOM:
ADIRU
ADIRU alignment must not be attempted at latitudes greater than 78 degrees, 14.75 minutes.

This is the latitude of Svalbard/Longyear ENSB.

[FlightDetent]

hoss183 The higher the LAT, the longer the align time still, not? Spill the beans or at least a link for a nice read.

[wiggy]

I'm hazarding a guess at Gyrocompassing being possibly more problematic at high latitudes but that's possibly ancient history and since a long time since I heard a specialist navigator moaning about the dark arts of Inertial alignments so I'll stand by to be corrected and getting the full story.

[Sidestick_n_Rudder]

IIRC A320 has limitations both for alignment and operation at higher latitudes - don’t have the FCOM at hand to give specific numbers. Anyhow, not possible to operate in Polar regions and use True REF

OTOH A330 has only alignment limitations and has to be used in true ref above certain LAT.

[FE Hoppy]

Every AFM Iv'e read has alignment limits for IRS. Often pan handled around the North Pole.
Some also include operational regions for older units.

Would love to hear Hoss183's secret info about how a gyro (laser or otherwise) works. I've only read Honeywells description.

[Smythe]

Boeing Polar Route navigation by model (http://www.boeing.com/commercial/aeromagazine/aero_16/polar_nav_by_model.html)

Other information: High Latitude Operations (http://code7700.com/high_latitude.htm)

Jepp

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/600x521/arctic_radio_vhf_jeppesen_ap_71b96f07dc274c963fcf2a4450cb6ce e1f841cbb.png

[Alpine Flyer]

The Fokker 70/100 had a limit, something like 78N. No TRUE/POLAR mode, which is probably the reason.

[compressor stall]

A320F has a latitude limit for alignment of 82 N/S with or without the polar nav mod. And between 60 and 82 it takes significantly extra time.

One of the advantages of the polar nav mod is that it is selected before the boundary of the Mag database area to avoid tripping the AP and ATHR off as the ADIRUs swap from Mag to True automatically as usually they don’t swap in exact sync. Additionally there is a Grid reference on the screen above 65°.

Ahh, got you. Any idea of the latitude limitations for the second point? Thanks.



From the Generic A330 FCOM:
The ADIRU gives true heading instead of Magnetic Heading
Above 82°N
Above 73°N between 90°W and 120°W
Above 60°S (make sure you understand that above in the Southern Hemisphere is higher latitudes, ie below on the map!)


IIRC A320 has limitations both for alignment and operation at higher latitudes - don’t have the FCOM at hand to give specific numbers. Anyhow, not possible to operate in Polar regions and use True REF

OTOH A330 has only alignment limitations and has to be used in true ref above certain LAT.


The limitation on the A320 and the A330 are the same if the Polar Nav mod is fitted. Obviously most narrow bodies don't go into the polar region so it's not fitted as standard. A while ago I was told that there was only one narrow body Airbus in existence with the Polar Nav Mod, although with the growth of the LR and CJ narrow body fleet, I imagine that would no longer be true. The operational limits are the same as I quoted above.

Semantics are an issue here- don't mix up the geographic term Polar Regions with the polar regions area as determined by Honeywell et al. If the system is capable to navigate in True up to 82N, then the same is true for up to 82S, but for the IRS manufacturers the Polar Region definition starts at 60S due to magnetic database absence so (legalities aside) you could practically operate in True in the polar region at say 65S. The same logic for that 90W-120W sector.

[Smythe]

[14 CFR 135 (http://code7700.com/high_latitude.htm#references), §135.98 Operations in the North Polar Area.] After February 15, 2008, no certificate holder may operate an aircraft in the region north of 78° N latitude (“North Polar Area”), other than intrastate operations wholly within the state of Alaska, unless authorized by the FAA

[giggitygiggity]

Magnetic Variation varies SIGNIFICANTLY near the North and South Poles within a short distance. The Variation Table is a data set containing variation information for geoid positions. The limited range of that table basically applies variation to compass headings to produce true headings and therefore reliable NAV data. Due to minor variation differences nearer the equator and sub-tropics, it can be pretty vague and therefore small in size so takes up less RAM in the archaic A320 FMGC. The A320 wasn't really designed to operate trans-polar, so this wasn't an important feature. In the A330, which came a few years later, RAM had come down in price and due to the nature of long-haul, it would be assumed that trans-polar routes would be planned, therefore it has a better variation table.

I didn't realise that it would cause the AP/FD to trip off without the mod, as compressor stall said, but obviously that makes sense as the FMGC is basically saying the NAV data is unreliable. I assume then the 'Polar Mod' is either a RAM upgrade, or simply a better optimisation of the FMGC data, eg less airports/routes/waypoints filling up the RAM to enable a larger VAR Table? Additionally, I assume this is why autoland is prohibited in certain airports with specific FMGCs. On my fleet, autoland is prohibited in odd places like Johannesburg or Keflavik as the autoland system would presumably align itself in the flare with whatever was tuned as the ILS CRS (a rapidly varying magnetic heading). I haven't tried it but i'm told it aligns itself with the CRS during FLARE so if you were to put a wrong figure into the RAD NAV page, you'd end up with a pretty awful landing, hence the reason we check the CRS during the LAND FMA.

[khorton]

I heard this story many, many years back when I was flying S-2s in the Canadian military. This was back in the days before GPS, and when IRS had spining gyros and gimbles - no fancy ring laser gryo stuff.

There was a CP-140 Aurora on a northern patrol, way up north, close to the North Pole. Some bright guy suggested it might be neat to fly right over the North Pole. The crew commander agrees this is right fine idea, so one of the navigators gives the pilots vectors to try to get right over the NP. First pass, and the head nav stated that they had missed by a tiny bit. Second pass, same story. Third pass, and they nailed it, and all the gimbles line up and they toppled all three IRS. All the nav systems and heading references gone. Every direction is south, but some of the souths are a better choice than the others.

They climb, hoping to get on top of the cloud to spy the sun. The cloud goes up higher than they can climb. No radio navaids in range. Crew crapping bricks. Finally one of the navs, running the radar in ground mapping mode thinks he recognizes some of the Artic Islands, and they roll those dice and start heading towards what they hope is Canada. Finally they get far enough south that the ADF picks up an NDB in northern Canada and all ends well. The beer tasted especially good that night.

[Gauges and Dials]

Every direction is south, but some of the souths are a better choice than the others.

This is a key point. Neither the concept of heading nor the concept of longitude make any mathematical sense at the north pole. That's not an engineering limitation of any compass, gyro, INS, or GPS system: it's a limitation in the underlying mathematical geometry. To navigate at the pole you need to use a coordinate system which is something other than latitude/longitude, with courses and headings defined relative to that coordinate system.

[Smythe]

I always thought it was set at 78 degrees because of the wandering.

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/800x1196/800px_magnetic_north_pole_positions_2015_svg_a1e97afaa5507ab 939b2c8f1e1f52490667f0736.png

[hoss183]

Every AFM Iv'e read has alignment limits for IRS. Often pan handled around the North Pole.
Some also include operational regions for older units.

Would love to hear Hoss183's secret info about how a gyro (laser or otherwise) works. I've only read Honeywells description.
I'm not talking about the align accuracy or time, that is clearly effected by the magnetic / true variation.
​​​​​CS claimed that the gyro itself wouldn't work at or near the pole because of velocity, that bit is rubbish, a gyro works anywhere including in space.

[wiggy]

I'm not talking about the align accuracy or time, that is clearly effected by the magnetic / true variation.
​

Genuine question - Is that actually the case? As far as I'm aware (emphasis on that!!) that certainly wasn't the situation with the old steam driven INSs.

The old systems levelled the platform to local horizontal and then ( and I'm very very much simplifying here) "found" and torqued the platform to align to True North by sensing the effect of the Earth's rotation on the platform.. mag var didn't come into the align process, am I wrong?/is it done differently now?

(Very belated edit to add that I'm assuming strap down systems must align in a different manner to that used in gimballed platforms - but the underlying question still stands - do they use variation in their alignment process?)

[compressor stall]

I'm not talking about the align accuracy or time, that is clearly effected by the magnetic / true variation.
​​​​​CS claimed that the gyro itself wouldn't work at or near the pole because of velocity, that bit is rubbish, a gyro works anywhere including in space.

You might not have been talking about alignment, but I was. That’s why I even put a “1. Alignment “ at the start of my paragraph and you even quoted it. :ugh:

And I’m eagerly awaiting your answer to Wiggy.

[Bergerie1]

wiggy,

You are right about the old INS - nothing whatever to do with magnetic anything, everything to do with sensing the rotation of the Earth. I am long out of date, but I would guess it is the same today with modern systems.

[aterpster]

[[url=http://code7700.com/high_latitude.htm#references]14 CFR 135, §135.98 Operations in the North Polar Area.] After February 15, 2008, no certificate holder may operate an aircraft in the region north of 78° N latitude (“North Polar Area”), other than intrastate operations wholly within the state of Alaska, unless authorized by the FAA
Most U.S. Operators in the region are Part 121, not 135.

[eckhard]

As far as I remember, the 747-400 had an operational limitation which prohibited flight at latitudes exceeding 89 degrees. In other words, one had to miss the pole by at least 60nm. My guess is that this was to avoid the problems experienced by the unfortunate CAF Aurora crew.

Cessna Citation and CJ models with which I am familiar have a “keyhole” shape around the North Pole and Hudson Bay within which flight is prohibited. As they have AHRS but not INS/IRS, they rely on magnetic sensing to feed the compasses and I think that the large magnetic dip values in this “keyhole” reduce the magnitude of the horizontal component below a value that is required by the flux sensors.

I did once accept a direct routing from YFB towards YYQ which just nibbled inside the edge of this keyhole, but the wings didn’t fall off.

There is a “DG mode” which removes the magnetic sensing input and then requires the pilot to manually align the displays with the wet compass, just like a Cessna 150.

[FE Hoppy]

I'm not talking about the align accuracy or time, that is clearly effected by the magnetic / true variation.
​​​​​CS claimed that the gyro itself wouldn't work at or near the pole because of velocity, that bit is rubbish, a gyro works anywhere including in space.

Alignment has nothing to do with variation.
Next?

[Smythe]

As far as I remember, the 747-400 had an operational limitation which prohibited flight at latitudes exceeding 89 degrees.

Boeing reference in Post #18

[wiggy]

Boeing reference in Post #18

Thanks for that..like Bergerie I recall the 89N limitation being stated in our (company produced) FCOM for the 744.

[Smythe]

Not sure when the charts and procedures show grid vs mag. For SCC we use grid.

[krismiler]

Australia operates an A319 from Tasmania to its Antarctic research station which is outside the normal latitude limits for the type.

[megan]

Australia operates an A319 from Tasmania to its Antarctic research station which is outside the normal latitude limits for the typeAnd we have the Captain of said aircraft commenting in this thread, should he want to put his hand up.

[judge.oversteer]

How on earth (sorry for the pun) did the USS Nautilus manage to do it so accurately all those years ago?
Remember, they were underneath the polar icecap.
Just asking.
JO.

[Gauges and Dials]

How on earth (sorry for the pun) did the USS Nautilus manage to do it so accurately all those years ago?
Remember, they were underneath the polar icecap.
Just asking.
JO.

Getting to the pole is easy, just keep steering 000 True. ;-)
There's a tiny bit on the navigation here: https://en.wikipedia.org/wiki/USS_Nautilus_(SSN-571)#Operation_Sunshine_%E2%80%93_under_the_North_Pole
More detail here: https://www.ion.org/publications/abstract.cfm?articleID=101690

[judge.oversteer]

GnD
Many thanks for that, especially the second ref.
Cheers JO.

[Smythe]

Remember, they were underneath the polar icecap.

GPS not invented yet, and underwater, no update anyways. Same as today.

All IRU.

[krismiler]

Submarine navigation uses/used Very Low Frequency radio waves which can penetrate under water.

[compressor stall]

And I believe they can update position referencing known underwater topography.

Which is probably one of the many reasons why why the Chinese were doing this. https://www.abc.net.au/news/2019-04-21/china-increases-surveillance-near-png/11028192

[Smythe]

Submarine navigation uses/used Very Low Frequency radio waves which can penetrate under water.

No, not correct. The Omega system was the precursor to GPS. It does not work underwater.

When GPS was initiated 2 years later, the Omega Towers and system were abandoned.

radio waves nor GPS penetrate water. The systems use sound navigation (SONAR) for navigation. Certain systems place pingers, which provide location underwater, but this is very, very new.

Its all up to the inertial system..

[krismiler]

https://fas.org/nuke/guide/usa/c3i/vlf.htm

​​​​​​ (https://fas.org/nuke/guide/usa/c3i/vlf.htm)VLF can penetrate seawater to around 20m, getting close to the surface and raising an antenna improves things considerably. Inertial navigation drifts over time and needs periodic updates to maintain accuracy.

[Smythe]

But look at when the Omega system went live, 1971....but did not have worldwide coverage. (Woodside in AUS was not opened until 1982.)

It took a long time to get a position from the Omega system and 20m depth is basically on the surface for a sub.

And I believe they can update position referencing known underwater topography.


Yes, there are hyper accurate maps that are used in navigation, it just requires the sub to ping with SONAR, giving away its position. There are pingers set up that broadcast all the time, and those are effective (unless the sub shadows a ping and gives itself away!)
On a side note, we were using the Woodside Omega tower for a project, before the idiot killed himself jumping off of it, and it was demolished...too bad.

For a bit of history, inside an Omega bunker:

Signal generator 100mm copper coils, all wood frames and bolts were MSR plywood..quite the engineering. (there were 10 of these, 10m tall in the building)

https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/1200x1600/otcoils_copy_3612c01bec39eedd9453f58b48d126ea755b11f1.jpg


https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1200x1600/img_5851_copy_3ab18553496150e37444fe0b59e79816a0f0ac80.jpg


https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/1200x1600/img_5855_copy_2cdea257a18af8de8d672a6e7a2ef5676ae9bad7.jpg

[fizz57]

It took a long time to get a position from the Omega system and 20m depth is basically on the surface for a sub.

Omega was indeed used by US Navy submarines, as this (https://apps.dtic.mil/dtic/tr/fulltext/u2/a285948.pdf)(caution: large document) makes clear. There was no need for the submarine to rise to 20m, a long-wire antenna (also used for communications) could be floated up towards the surface. Ironically Omega was made redundant by higher-accuracy IRS's in the 70's and 80's and several Omega stations were converted for communications use.

Yes, there are hyper accurate maps that are used in navigation, it just requires the sub to ping with SONAR, giving away its position.

....which is why many navies operate "geophysical research vessels" to produce detailed gravimetric and magnetometric maps, not as accurate as bathymetry perhaps but completely passive. Of course a lot of important scientific results were also generated.

The gear required to measure the acceleration due to gravity to a fraction of a percent in a surface vessel subject to wave motion has to be seen to be believed.

[dook]

Perhaps hoss183 really doesn't understand that the value or Cosine Latitude diminishes towards the true poles.

In fact I don't think he understands as much as he would like us to think.

Our inertial platform would work at the poles, but alignment north of about 78-80N took eons.

[Vinnie Boombatz]

A few more tidbits on the submarine voyages --

A 4 page article in Popular Science in 1959, "How the Nautilus Hit the Bull's-Eye at the Pole" (https://books.google.com/books?id=wyUDAAAAMBAJ&pg=PA161):

North American Aviation retirees' newsletter (https://static1.squarespace.com/static/56c78acd0442626b2590f5ea/t/59058f64579fb3c5cc557537/1493536630816/2012-1_Spring.pdf) (9 MB file). Article starting on pg. 4 with a technician's log of his trip under the pole aboard the USS Skate, taking place at the same time as the Nautilus voyage.

AIAA paper (https://arc.aiaa.org/doi/abs/10.2514/6.2001-4292) on N6A, free page 1, all others pay cash:

[blind pew]

I had the "honour" of doing our first flight over the pole.
it would have been in the extended range version as fuel was tight and we often planned Fairbanks with technical onroute diversion to our destination of Anchorage.
Dispatch was excited and the over the top route was only a couple of minutes longer than minimum time.
The INS took 12 minutes to initialize including loading the route from a betamax size cassette.
I do not remember any lattitude restriction wrt initialization.
Above 65? we had to go to polar navigation which split the two fms displays, the computers worked on individually calculated position rather than median and we were not allowed to take out the nav mode on the autopilot.
All went well until we crossed the pole with my display showing a normal indication to the next waypoint (328?nm) whilst the captains showed a track which deviated approximately 30 degrees.
I tried "direct to" on both key pads but was still left with a large track split. Fortunately we had been watching a large moon in the night sky so we had some sort of heading reference.
I checked the triple lat long displays which all were fairly similar and decided with the skipper and engineers blessing that I would ignore SOP, take out nav and manually intercept the meridian. When we reached the next waypoint everything went back to normal.
I wrote it up very clearly but a month later I got a huge bollxxking for ignoring SOP which would be entered on my file.
This was not entirely unexpected as far to many mangement pilots in all of my three legacy carriers didnt have the where with all of line pilots, the exception being technical and base trainer pilots.
A month later I was rostered to assist on a sim check which was being carried out by the chief technical pilot whom I didnt know but I thought that it was a serious problem that should be properly investigated.
It was and I recieved a note that Douglas had discovered a programming error which had been rectified.

[compressor stall]

Great story BP - thanks for posting.

It reminds me of one from one of my colleagues back in the late 70s or early 80s. He was flying a biz jet to the N Pole with a load of some half a dozen nav engineers and their latest nav equipment in the back to test. Approaching the pole, they were counting the number of (latitude) minutes from 89°N. In rough unison each engineer called out 56, 57, 58, as the latitudes clocked over, then 59 , 00 then 59 again, except one voice that piped up 61...

[Intruder]

Well, the Boeing answer is in the FCOM:
ADIRU
ADIRU alignment must not be attempted at latitudes greater than 78 degrees, 14.75 minutes.

This is the latitude of Svalbard/Longyear ENSB.
Which Boeing? The 744 FCOM says:IRS HIGH LATITUDE ALIGNMENT
A high latitude alignment must be accomplished when the latitude of the origin airport is greater than 70°12.0’ and less than 78°15.0’.
IRS Mode selectors ............................................................ ...... OFF, then ALIGN
Note: The IRS Mode selectors must remain in ALIGN for a minimum of 17 minutes.
CDU ............................................................ ................................................... Set
Enter present position on SET IRS POSITION line of position initialization page.
IRS mode selectors ............................................................ ...........................NAV
There is no limitation on operating the IMUs. FAA OpSpec B050 requires all 3 INS systems to be fully operational for flights through the Area of Magnetic Unreliability ("system" is not defined explicitly, but FMCs may be included in that definition). Regardless, common sense would dictate that GPS and FMCs are operating properly for Polar operations.

[Mr Optimistic]

Well as I understood it an INU in gyrocompass mode is using its rate sensing gyros to detect earth rotation. If the X axis is fore aft, z upwards, then it rotates about the z axis until no earth rate is detected on the y axis, so the earth rotation vector is in the x z plane. At the pole the rotation vector is straight up z no matter how you rotate to swing the X axis around. So the component of the earth rate that matters is in the x y plane. This reduces as cosine latitude.