about heading calculation in polar regions
- Thread starter mhmenon
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Our heading measurement comes from a 3 axis magnetometer which takes the downward component of Earth's magnetic field into account to produce heading information that far exceeds conventional "wet" compass measurements.
As you get close to the poles (and we're not sure how close because frankly we haven't tested in the region), the dip (downward) component of Earth's magnetic field will become great enough that it will become harder to resolve magnetic North. Essentially, Earth's magnetic field points very sharply towards the ground in the polar region, and this makes it difficult to measure heading magnetically.
Note that any virtually all magnetic sensors will suffer from similar inaccuracies near the poles.
As you get close to the poles (and we're not sure how close because frankly we haven't tested in the region), the dip (downward) component of Earth's magnetic field will become great enough that it will become harder to resolve magnetic North. Essentially, Earth's magnetic field points very sharply towards the ground in the polar region, and this makes it difficult to measure heading magnetically.
Note that any virtually all magnetic sensors will suffer from similar inaccuracies near the poles.
Canadian_JOY
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Check my earlier reply concerning "grid" functionality. This effectively answers the question you've asked here.
And yes, I still find it hard to believe that pioneering arctic aviators managed to survive flying over such inhospitable lands with so little in terms of navigation equipment. Of course, some of them were able to use celestial navigation, but that's awfully hard to do when flying a single-pilot aircraft!
And yes, I still find it hard to believe that pioneering arctic aviators managed to survive flying over such inhospitable lands with so little in terms of navigation equipment. Of course, some of them were able to use celestial navigation, but that's awfully hard to do when flying a single-pilot aircraft!
Just stumbled over this old thread, and living above the polar circle- that's arctic region eh?
It's simple really, you can use a wrist watch as a compass 24 hrs a day. The sun is at 0000 at 000, at 0600 at 090 at 1200 at 180, at 1800 at 270 and always low enough in the sky to give a good reading.
In winter you just don't fly
Kind Regards,
Jos Okhuijsen
It's simple really, you can use a wrist watch as a compass 24 hrs a day. The sun is at 0000 at 000, at 0600 at 090 at 1200 at 180, at 1800 at 270 and always low enough in the sky to give a good reading.
In winter you just don't fly
Kind Regards,
Jos Okhuijsen
its not like that jos, when they fly over polar region or (grid region - generally know as) they dont follow magnetic heading values, they will take the inertial values that are calculated from the gyroscope and use those heading values for navigating, this value referes to the true north value irrespective of the region you r in, they will never follow the wrist watch north pointer for flying an aircraft
There are 2 seperate issues here. Firstly, in high latitudes in the region of the magnetic pole, the angle of dip and large (and rapidly changing) variations make a magnetic compass unusable. The second is that close to the true N pole, you can't even fly a true heading. Consider flying north along the zero meridian. As you transit the pole, your true heading switches from 360 to 180. Now take a more realistic view as you transit close to the pole on a heading other than north. On a great circle route, your true heading is continually changing, as the pole swings past.
I have flown such a route on a few occasions in a commercial aircraft from New York to Hong Kong. We still fly inertial and GPS great circle track but we display grid north and have a chart with a grid overlay referenced to the Greenwich meridian. So in this case, flying north to the pole along the Greenwich meridian, your true heading is 360 and your grid is 360. As you transit the pole, your grid heading is still 360 but your true is 180!
In days of yore, you synched your DG to the magnetic compass (corrected for variation) on North. After that you plotted each leg on the grid and used grid heading to fly hoping the drift rate was acceptable and you found something you recognized. Much easier with a triple ring-laser gyro inertial and twin GPS! - but no easier to visualize and get your head around........
By the way, the latest Airbuses don't even have a magnetic sensor. They use inertial to get true heading and have a world-wide data-base of the local variation which is applied to display magnetic. Tail wagging the dog, I think. One of my favourite line-check questions - "where is the flux detector on this aircraft?" - gets them every time......
I have flown such a route on a few occasions in a commercial aircraft from New York to Hong Kong. We still fly inertial and GPS great circle track but we display grid north and have a chart with a grid overlay referenced to the Greenwich meridian. So in this case, flying north to the pole along the Greenwich meridian, your true heading is 360 and your grid is 360. As you transit the pole, your grid heading is still 360 but your true is 180!
In days of yore, you synched your DG to the magnetic compass (corrected for variation) on North. After that you plotted each leg on the grid and used grid heading to fly hoping the drift rate was acceptable and you found something you recognized. Much easier with a triple ring-laser gyro inertial and twin GPS! - but no easier to visualize and get your head around........
By the way, the latest Airbuses don't even have a magnetic sensor. They use inertial to get true heading and have a world-wide data-base of the local variation which is applied to display magnetic. Tail wagging the dog, I think. One of my favourite line-check questions - "where is the flux detector on this aircraft?" - gets them every time......
It's a ring laser rather than mechanical gyro so I don't think it has the same alignment issues. If I remember back to my theory days, it works out north by assessing the local earth rotation rate which is dependant on latitude or something (waffle, waffle.....).
The IRs do drift. The triple mix position is just one input to the Flight Management computer. It then updates that position, normally with GPS or with radio aids as secondary. In the event that updating is lost, it applies the last known error vector to the IRS position. The accuracy of this position will degrade over time without further updates but unless one IR is wildly out, the triple mix is extremely accurate. The FM has an algorithm for calculating the rate of degredation depending on the quality of the last update and time elapsed and lets you know if you cannot comply with the area RNP. Fiendishly clever.........
The IRs do drift. The triple mix position is just one input to the Flight Management computer. It then updates that position, normally with GPS or with radio aids as secondary. In the event that updating is lost, it applies the last known error vector to the IRS position. The accuracy of this position will degrade over time without further updates but unless one IR is wildly out, the triple mix is extremely accurate. The FM has an algorithm for calculating the rate of degredation depending on the quality of the last update and time elapsed and lets you know if you cannot comply with the area RNP. Fiendishly clever.........
Ok... I'll grant that if you know your posistion (say via GPS) and you're stationary, you might be able calculate your heading based on the earth's rotation (do you also have to figure motion of the orbit of the earth around the sun too?). But how the heck can you do that while you're airborne? Yes, you can get your POSTION many different ways but not heading.
Still can't get my head around not having ANY heading input, they must have some sort of magnetic sensor or other proxy. Maybe DF off of a GPS satalite?, DF off of a VLF Omega groundstation? or even ADF?.
Wait... I think I've got it! GPS antenna on the nose of the plane and another one on the tail (or each wingtip). That could give you a very accurate heading input. You could even get pitch and roll if you had 3 antennas... That MUST be how they do it.
Earth's rotation... right!
You had be going there for awhile but there's no way the French are that clever!
khorton
New Member
The IRS can sense the earth's rotation, and is able to determine the 3 dimensional direction of the earth's axis of rotation. Once you know the exact orientation of the axis of rotation, you know where north and south are, and you can determine the heading.
I don't know if the have to account for the rotation of the earth around the sun or not. Maybe the effect is so small that it can be ignored, or maybe it is significant. I don't know. That is a very interesting question.
I don't know if the have to account for the rotation of the earth around the sun or not. Maybe the effect is so small that it can be ignored, or maybe it is significant. I don't know. That is a very interesting question.
I agree, but only if you're on the ground.
INS can only sense motion. In the air, it would have no way of knowing if the motion was the result of movement of the aircraft or movement of the airmass (it could null out the earth's rotation if it knew it's position) so it would have no way of calculating heading via INS alone. There still must be some sort of heading input other than INS while airborne.
The 21 century solution would be GPS calculated heading/attitude vs the 20 century solution of fluxgate and gravity.
PilotKris
The IRS only senses earth rotation in align mode. You tell it where is is by lat/long input. Once it is put into NAV mode, it senses movement by 3 accelerometers. Since it knows it's initial conditions (speed=0, position as entered), it can integrate the accelerations to give velocity and position. Since velocity includes a direction element, it knows which way it is moving and since it is bolted to the aircraft, it can deduce the aircraft heading.
It is a self contained system requiring no further input. The clever updating and mixing thing is done by the FMS which takes inputs from IRS, GPS and radio aids and uses it's in built algorithms to give you a position.
It is a self contained system requiring no further input. The clever updating and mixing thing is done by the FMS which takes inputs from IRS, GPS and radio aids and uses it's in built algorithms to give you a position.
Position (location) yes. BUT NOT HEADING!
Direction of movement does not equal heading beacuse of wind, earths rotation and/or uncoordinated flight (slip or skid).
No INS can function totally independent of ongoing attitude/heading input because of drift. The traditional approach would incorporate inclinometers and a flux detector. It is impossible for the Airbus system (or any INS system) to operate independently of any heading input. While it might not have a magnetic compass, the system MUST have some form of heading input (such as differential GPS) while airborne.
khorton
New Member
The IRS knows the intial heading that it has calculated during the alignment. Then, while the aircraft is moving, it senses any changes in heading, as the mechanical or laser-ring gyros inside the IRS sense rotations about any axis. The rotations sensed about the yaw axis are integrated to calculate the current heading. The IRS uses true heading internally, but it knows what the position is, and it has a look up table of variation at various locations, and it uses this to convert true heading to magnetic heading.
I've flown many different aircraft types that use IRS to produce the heading that is displayed to the pilot, and I can assure you that it works very well.
I assure you, sir, it works like that!! How do you think they got to the moon - or do you believe that was all a hoax!
Yes, they got to the moon with INS and no compass. They used celestial sightings to update the INS to compensate for gyro drift.
In aircraft we can use inclinometers and a magnetic sensor (flux detector) to provide compensation. My guess is that the Airbus system (assuming it has no magnetic sensor) uses differential GPS to up date the INS. There just is no way the French could make a absolute zero-drift INS system that could be dead on accurate for a 14 hour, 8,000 mile trip.
Even laser gyros drift (some) over time. Just as the Astronauts needed to update their INS using celestial sightings, the Airbus system gets heading (and attitude) updates from somewhere (most likely differential GPS).
And I assure you that every single one of those systems had some sort of attitude input (inclinometer) and heading input (flux dectector or differential GPS etc.) to help the system compensate for gyro drift. None were 100% gyro and accelerometer based.
I'm not saying what was displayed on the Heading Indicator wasn't from the INS, I'm just saying that the INS couldn't operate dead-on for 14 hours with absolutely zero drift. It has to be updated from somewhere (even if you didn't know it).
khorton
New Member
This sounds an awful lot like "my mind is made up, don't confuse me with the facts". There is obviously no point in continuing this conversation, and it isn't at all relevant to Dynon's in amateur-built aircraft, so I'll sign off now.
I did some research on the subject. There were a lot of chat room/bulletin board posts on the subject of the Airbus INS. I was amazed at the number of Airbus Drivers who had no idea how the data displayed on their screens was calculated. They knew how to use the system, they knew that the system could sense the earth's rotation but really had no a clue how the Magic Box figured it all out. They did know that the system did not have a magnetic sensor (as I agree is possible).
http://www.pprune.org/forums/archive/index.php/t-9344.html
Without consulting an Airbus Avionics Engineer, the best empirical information I could locate was from "down-under" in an Australian Airworthiness Advisory Circular.
http://casa.gov.au/ame/download/aac9-95.pdf
In this document is mentioned Group 20 aircraft (which includes Airbus’) who's “AHARS systems typically have rate gyros, magnetic compass or GPS inputs feeding into a computer which calculates heading for display”.
This was the best I could come up with on short notice but it is not based on the opinions of Airbus Drivers (who appear to be pretty clueless how their “Magic Bus” actually works). Kinda scarry don't you think?
http://www.pprune.org/forums/archive/index.php/t-9344.html
Without consulting an Airbus Avionics Engineer, the best empirical information I could locate was from "down-under" in an Australian Airworthiness Advisory Circular.
http://casa.gov.au/ame/download/aac9-95.pdf
In this document is mentioned Group 20 aircraft (which includes Airbus’) who's “AHARS systems typically have rate gyros, magnetic compass or GPS inputs feeding into a computer which calculates heading for display”.
This was the best I could come up with on short notice but it is not based on the opinions of Airbus Drivers (who appear to be pretty clueless how their “Magic Bus” actually works). Kinda scarry don't you think?