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Difference between IRS and AHRS?

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Difference between IRS and AHRS?

Old 19th Nov 2012, 00:00
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Difference between IRS and AHRS?

Hope this is not a dumb question. I am still working on my ratings and I might be getting ahead of myself but can anyone tell me the difference (in a nut shell) between the two systems? (IRS and AHRS)
Looked on-line but got even more confused than I'm already am.
IRS dependes on flux-valves and AHRS is a computer generated system?
regards
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Old 19th Nov 2012, 17:00
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Well, in a nutshell, an AHRS provides you with attitude / heading information while the IRS also gives you position information (and with that also data like GS, true track, Wind data, magnetic heading / track)

If you take a look at current jet aircraft, the IR / IRU systems have a ATT position. When switched to ATT, the IRS is basically just an AHRS system (but you'll have to provide a heading reference manually)

All solid state, [email protected] ring gyros stuff in these days.

Couple an AHRS equipped aircraft with GPS and you basically have the same information available as an IRS equipped aircraft.

Last edited by Ka8 Flyer; 19th Nov 2012 at 17:01.
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Old 19th Nov 2012, 17:04
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IRS is an inertial platform that can measure accelerations and displacement from a known position. I.e. it can be used for Navigation as well as provide attitude and heading reference.

AHARS will only give attitude and heading reference and needs another sensor to provide navigation such as a GPS system.

Most modern jet transports use IRS, most modern turboprops use AHARS.

There is no need for flux valves with an IRS system.
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Old 19th Nov 2012, 17:26
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A simple explanation of inertial navigation systems

The equipment, and hence the aircraft, knows where it is at all times. It knows this because it knows where it isn't. By subtracting where it is from where it isn't ( or where it isn't from where it is, depending on which is greater ), it obtains a difference or deviation. The inertial reference system uses deviations to generate corrective commands to fly the aircraft from a position where it is to a position where it isn't. The aircraft arrives at the position where it wasn't; consequently, the position where it was, is now the position where it isn't. In the event that the position where it is now, is not the same as the position where it originally wasn't, the system will acquire a variation. (Variations are caused by external factors, and discussion of these factors is beyond the scope of this simple explanation.)

The oldies are the best.
Happy flying.
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Old 19th Nov 2012, 22:19
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I never understood that 'definition' of the IRS.
Why not just say: it integrates acceleration (which can be measured directly due to F = m*a) once to obtain ground speed and once more to obtain a position offset. And to calculate the current absolute position, you need to have a reference position to which the offset can be applied.
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Old 20th Nov 2012, 00:32
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Thanks a bunch to all of you who took the time to answer my question. All the answers together helped understand the difference a lot better.
Things are getting clearer now!
Regards,

Pvtpilot
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Old 28th Nov 2012, 13:51
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Is it safe to say that AHRS is really just a replacement for the moving gyros that used to be in the heading indicators and attitude indicators. Remove these four gyros(Capt and copilots side instruments) and replace them with two [email protected] gyro AHRS units.
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Old 28th Nov 2012, 16:22
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Boeing is probably one of very few OEMs that mixes IRS and AHRS. On the fail operational equipped 737s three attitude sources are needed, however only two IRS are installed. The third source is the AHRS from the ISFD (aka standby instrument). Weird setup, but works like a charm.
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Old 28th Nov 2012, 16:27
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Airbus have 3 ADIRS: Air data inertial reference systems. They provide air data and position information, the latter using [email protected] ring gyros. Most also have GPS as well.

Jazz, old bean, no disrespect, but have you been smoking something? - you've lost me there totally.

U

Last edited by Uplinker; 29th Nov 2012 at 19:39.
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Old 29th Nov 2012, 16:12
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Interestingly in all the replies, nobody actually defined the acronyms:

AHRS - Attitude Heading Reference System

It gives you the attitude and heading of your aircraft, and nothing more.

IRS - Integrated Reference System

It gives you the attitude and heading of your aircraft, plus navigation information: integrating the reference systems.

Various manufacturers interpret these in their own ways, adding options and using different systems to provide the information as they see fit.

Anything with a compass and horizon ball has an AHRS, even a DC3:


Last edited by glum; 29th Nov 2012 at 16:17.
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Old 29th Nov 2012, 19:38
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Er, Glum old chap; check out post #9.

By the way, does anyone know why when we write [email protected], it always has the @ instead of the a?


U

Last edited by Uplinker; 29th Nov 2012 at 19:40.
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Old 29th Nov 2012, 20:08
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The AHRS gives you a little bit more than that...with the combination of gyros and accelerometers, it provides altitude, heading, and flight dynamics (roll, pitch, angle rate, and acceleration data).

AHRS is linked to the EFIS.

IRU is linked to the Flight Management Computer

(DC3 is my all time favorite ac...in SD there was a company that was retrofitting the interiors as corporate ac...some beautiful wood lined interiors...)

Last edited by FlightPathOBN; 29th Nov 2012 at 20:14.
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Old 30th Nov 2012, 11:00
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FlightPathOBN:

You're talking about a modern interpretation by a particular manufacturer of the AHRS, which has expanded to cover all those things.

I guess each aircraft manufacturer has gone down their own path as to what a modern AHRS is and does, and perhaps the acronym AHRS should be retired now that pretty much all flight data has been merged together and IRS seems more appropriate?

Although I guess IRS can also mean Inertial Ref System, so perhaps that's not a good one to use either!

Uplinker:

Not sure what you're getting at?
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Old 30th Nov 2012, 13:49
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So, are both these systems "deduced reckoning" ?
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Old 30th Nov 2012, 13:56
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By the way, does anyone know why when we write [email protected], it always has the @ instead of the a?
To prevent GoogleAds from displaying banners advertising hand-held [email protected] pointers. You know, the ones that are too often pointed at landing aircraft at night by brainless morons?
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Old 30th Nov 2012, 13:56
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Hi Glum,

You said nobody had defined any acronyms, whereas in the post immediately above you, I had done just that.

Regards,

U

Dg800
Ah, I see about [email protected], thank you. Yes, have been lasered myself a few times.

U

Last edited by Uplinker; 30th Nov 2012 at 13:58.
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Old 30th Nov 2012, 14:28
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glum,

with the original posters question, working on his ratings, it seemed the modern interpretation was best.

connecting directly to the electronic horizon, is much different than connecting to the FMC...

Last edited by FlightPathOBN; 30th Nov 2012 at 14:30.
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Old 30th Nov 2012, 16:37
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U - I was referring to the ones the OP had asked about in the thread title...
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Old 30th Nov 2012, 17:30
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Jazz, your post made perfect sense to me, thanks. It also made me grin.
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Old 4th Dec 2012, 08:45
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there is a audio version of it somewhere floating in this AHRSless internet
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