IRS and AHRS - What´s the difference?

[Robinson67]

Hey guys,

 

Searched on the net but seemed a bit confusing since many sources with different perspectives.

Would like to know yours;.

Thanks

 

[DaveCT2003]

Spot on:

 

http://www.pprune.org/tech-log/500802-difference-between-irs-ahrs.html

[OneWhoKnocks53]

Ahrs relies on magnetic flux, IRS relies on its own internal gyro (laser-ring) and accelerometers based on fixed reference position, however the term is becoming somewhat interchangeable.

[Robinson67]

What I understood is that AHRS coupled with a GPS can determine its position and therefore will work like an IRS.

DaveCT2003 and Knocks53 thank you.

[OneWhoKnocks53]

What I understood is that AHRS coupled with a GPS can determine its position and therefore will work like an IRS.

 

That's correct. In fact, Embraer dispensed with IRS altogether. Dual GPS and dual AHRS in its Legacy 500. Everyone at FlightSafety went "Really??", but apparently the new AHRS systems are ultra-reliable.

[Robinson67]

In fact I was reading the Phenom manual and I didn´t find any references to IRS, that`s the reason for my doubt.

So it`s clear now. Thanks again

[FormerSF3]

One of the planes I fly has both.  AHRS will align itself with attitude and heading after about a minute once the avionics are turned on.

 

IRS will only align itself after selecting ALIGN on the overhead panel, imputing a Lat/Long location in to the FMS, selecting NAV on the overhead panel, and depending on your Latitude could take 5-10 minutes.  However, without a GPS installed, IRS will know where you are -even pulling in to the gate at the end of the flight.  I realign at the beginning of the next flight and usually there is little error or none at all.

 

IRS was built to cross oceans before GPS came in to play.

[KevinAu]

That's correct. In fact, Embraer dispensed with IRS altogether. Dual GPS and dual AHRS in its Legacy 500. Everyone at FlightSafety went "Really??", but apparently the new AHRS systems are ultra-reliable.

I flew the E145 for thirteen years in real life, and as far as I know, the Legacy is pretty much the same thing with a plush interior and more gas. The AHRS provides no positional information and does not work like an IRS. Positional information comes from the fmc through gps, radio, and dead reckoning. There were two types of AHRS, the early one which used a magnetic flux valve situated out on the wingtip, and a later version which was ring laser. Both versions provided attitude and heading...only. The flux version provided its own heading at alignment. The later version required a position input during alignment in order to derive its initial heading.

 

For position, the fmc used the gps. If that was not available, it would use dme/dme or vor/dme. If that was not available, it would dead reckon. I lost both gps and radio for the fmc once and got to see just how good the DR mode was. After a two hour flight, the nav display showed the airport I had landed at, at over twenty miles away.

[OneWhoKnocks53]

I flew the E145 for thirteen years in real life, and as far as I know, the Legacy is pretty much the same thing with a plush interior and more gas. The AHRS provides no positional information and does not work like an IRS.

 

You are correct, the attitude and heading reference system does not provide position data. That comes from GPS or IRS. The Legacy 600, which is nothing more than an ERJ 135 with exec. interior does use IRS, however the 500/450 is a clean sheet design, looks a bit like a challenger 350. They opted for the AHRS/GPS system with the Proline Fusion cockpit. The Legacy 600 used an older Honeywell (Primus 2000?, not sure) system.

[ubersu]

As a side note, DOD is currently working on mini-inertial sensing units for the day when GPS goes tango uniform. My own opinion of Embraer's decision is that you really do need an IRS backup to GPS.

 

DJ

[michal]

Ahrs relies on magnetic flux,

AHRS relies on much more than magnetic flux, it is a tiny microelectromechanical (MEMS) package that contains gyros, accelerometers, etc.

If flux data is lost you will lose heading info but everything else remains.

[OneWhoKnocks53]

AHRS relies on much more than magnetic flux, it is a tiny microelectromechanical (MEMS) package that contains gyros, accelerometers, etc.

 

That I wasn't aware of, that would explain its standalone use w/o IRS in modern bizjets.