about heading calculation in polar regions

[khorton]

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”.

Attitude and Heading Reference System - AHRS (not AHARS) is not the same thing as  Inertial Naviation Systems INS (or Inertial Reference System - IRS).  An IRS is an INS that also outputs attitude and heading information in addition to the position info.   AHRS is somewhat similar technology to the attitude and heading reference from a Dynon EFIS. INS requires sensors with several orders of magnitude greater accuracy (and cost), and the higher sensor accuracy allows determination of heading during the initial alignment, and continuous integration of accelerations to determine position via dead reckoning once the initial position has been input to the unit.  Comparing an AHRS to an INS is like comparing a bicycle to a new Mercedes car.  For a good description of the state of the art in INS 9 years ago (and the state of the art has likely improved since then), see this article.  Note that it discusses gyro drifts (e.g. heading error) of a few thousandths of a degree per hour, and navigation errors of around 0.6 nm/hour, without any GPS or other (i.e. heading) aiding.

I've worked full time as an engineering test pilot since 1988.  I've done flight testing on over two dozen different models of aircraft fitted with INS (or INS).  I have a good understanding of the INS inputs and outputs for most of those aircraft.  I can assure you that most civil INS (or INS) achieve amazing navigation, attitude and heading accuracy without any input from flux valves, inclinometers or GPS.  Just because you don't understand how something works doesn't mean it doesn't work.  It just means you have some more learning to do.

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?

The systems descriptions in the aircrew documentation for most modern aircraft are very simplified, and crews are no longer expected to have an in-depth understanding of how things actually work.  They are simply expected to correctly apply the recommended procedures in response to various system indications.  Things were quite different back when aircraft were simpler - then crews were expected to have a very deep understand of what was actually happening, but modern aircraft are so complicated that it is no longer reasonable to expect the crew to understand all the layers of the onion.

 

[dynonsupport]

Little do you know that inside your EFIS is a glass of water and sand, magnified with a proprietary lens system. All of this sensor stuff is just fluff that we find on the internet. <grin>

 

[PilotKris]

Very interesting article, Thanks.

There are a few points from it that I'd like to quote as they strengthen my position:

"...aircraft I.N.s will almost invariably be integrated with GPS. or other sources"

"...most military I.N.s now include an embedded GPS receiver.

"No major evolutionary steps are foreseen for at least the next decade."

And lastly, I'd like to quote you:

 Just because you don't understand how something works doesn't mean it doesn't work.  It just means you have some more learning to do.

I very much doubt that the civil aviation industry as achieved an accuracy in their INS systems that so surpass the military that the designers have parted ways in terms of design (no embedded GPS or other source). That also would cause an unacceptable increase in cost of acquisition and maintenance. Yes, the INS would be capable of sand-alone operation but at a reduced level of accuracy.

By your own admission, just because you've operated a system (or couple dozen), it doesn't mean you really know how it works.

BTW, EVERY INS has embedded inclinometers (or their equivalent) except maybe the ones used in outer space.

I'll part with a final quote from your article:

"We can thus make a final forecast: Inertial Navigation (as a pure, stand-alone technique), may ultimately disappear - however, integrated (multi-source) navigation systems will continue to increase in sophistication, and will continue to require inertial data as an essential ingredient."

 

[PilotKris]

Little do you know that inside your EFIS is a glass of water and sand, magnified with a proprietary lens system. All of this sensor stuff is just fluff that we find on the internet. <grin>

Is that how they do it? Damn...

Now, if in floating in the water was a cork with a magnetized needle, I'd believe you.

 

[khorton]

Very interesting article, Thanks.

There are a few points from it that I'd like to quote as they strengthen my position:

"...aircraft I.N.s will almost invariably be integrated with GPS. or other sources"

"...most military I.N.s now include an embedded GPS receiver.

"No major evolutionary steps are foreseen for at least the next decade."

And lastly, I'd like to quote you:


I very much doubt that the civil aviation industry as achieved an accuracy in their INS systems that so surpass the military that the designers have parted ways in terms of design (no embedded GPS or other source). That also would cause an unacceptable increase in cost of acquisition and maintenance. Yes, the INS would be capable of sand-alone operation but at a reduced level of accuracy.

Keep in mind that inertial navigation systems have been around for decades, while the use of GPS in civil aviation is only a relatively recent development.  For example, FAA Advisory Circular 25-4 Inertial Navigation System was published in 1966.  

It is also worthwhile to review a classical definition of inertial navigation system, given in AC 25-4: "A self-contained navigation system which provides airplane position and other significant navigation information in response to signals resulting from inertial effects on components within the system".  Notice the words "self-contained". A classical INS does not require any outside inputs, other than an initial position (and electrical power).  No GPS input is required, nor a magnetic heading input.  In fact, INS works quite effectively in the vicinity of the magnetic poles, where any magnetic heading input would be completely useless.  While it is true that GPS and magnetic inputs could be used to enhance navigation accuracy, they are not required elements.

BTW, EVERY INS has embedded inclinometers (or their equivalent) except maybe the ones used in outer space.

I'm not really sure of the point you are trying to make.  An inclinometer is a form of linear accelerometer.  And yes, inertial navigation systems do have extremely sensitive linear accelerometers.  That is how they can measure the linear acceleration they are being subjected to.  The acceleration over time is integrated to obtain velocity, and the velocity is integrated to obtain change of position, which is added to the initial position to get the current  position.

By your own admission, just because you've operated a system (or couple dozen), it doesn't mean you really know how it works.

I am quite amused that while you have no idea which INS-equipped aircraft I have flown, you are certain that you know more than I about the details of the INS on those aircraft.

 

[PilotKris]

Ah Kevin…Now you’re contradicting yourself.

I'm not really sure of the point you are trying to make.  An inclinometer is a form of linear accelerometer.  And yes, inertial navigation systems do have extremely sensitive linear accelerometers.

I can assure you that most civil INS (or INS) achieve amazing navigation, attitude and heading accuracy without any input from flux valves, inclinometers or GPS.

My “point” was that your previous statement is incorrect.

I am quite amused that while you have no idea which INS-equipped aircraft I have flown, you are certain that you know more than I about the details of the INS on those aircraft.

That’s a little bit like saying “you don’t know what car I drive so how could you know it has wheels?”

Just because I can’t claim to be a test pilot, it doesn’t mean I don’t know a thing or two about INS theory (if not operation). While a lot of pilots are content to keep their knowledge of the internal workings of their aircraft to “it works with PFM (Pure F***ing Magic)”, I’m not. I really want to know what makes it “tick”.

All civil INS systems in operation today have some sort of heading input to increase the systems accuracy. In fact, most newer systems would be more properly called "multi-sensor" systems. The INS component might be able to function without heading input, but it would do so with significantly reduced accuracy.

My one and only point in this entire thread is that while the Airbus may not have a flux detector, it must have some sort of heading sensor (besides INS).

 

[khorton]

All civil INS systems in operation today have some sort of heading input to increase the systems accuracy. In fact, most newer systems would be more properly called "multi-sensor" systems. The INS component might be able to function without heading input, but it would do so with significantly reduced accuracy.

My one and only point in this entire thread is that while the Airbus may not have a flux detector, it must have some sort of heading sensor (besides INS).

This contradicts everything I have ever learned about INS.  Everything I have ever learned about INS indicates that they require no heading input at all, and all the information I have ever gleaned from flt testing supports this. INS systems have been used for decades in the far north, where magnetic heading inputs would be completely unreliable.

But, if I am willing to learn, if you can provide a credible source for your statement about all civil INS systems having a heading input.

 

[PilotKris]

Kevin,

I think that basically we have disagreement as to definition. I believe you're looking strictly at the INS/IRS component of the aircraft's navigation system. I'm looking at the entire aircraft's navigation systems and how they interact.

It is true that the INS/IRS component can figure out it's heading on its own if stationary (and not too close to the poles). However in modern aircraft, INS/IRS is not the only input to the aircraft's FMS (and PFD). The different sensors (such as GPS, VLF Omega, Loran, VOR, ILS, and maybe Flux) are talking to and updating each other constantly and what the pilot sees on his PFD/MFD is really a composite of all the different inputs.

INS/IRS is remarkably accurate on its own (.6nm per hour) but it's not accurate enough to be the sole source of navigation.

Civil INS/IRS are probably as accurate as they are ever going to get as manufactures can increase the accuracy much more cost effectively by integrating other sensors into the mix. A GPS module can increase overall system accuracy to a level higher than the best INS system ever built.