Difference between IRS and INS
 

Hey!

I was wondering if the IRS and INS are one and the same thing. Wikipedia hasn't been of much help on this one. I understand that INS employs gyros and accelerometer and given an initial condition (location) it can compute the current location after factoring in all the changes in velocities and direction since initialization. What about the IRS?

Somebody told me that the INS employs mechanical gyros whereas IRS uses laser gyros. Is that right?

Generally, yes.
The original Litton INS units demonstrated a maximum guaranteed radial error limit of 2 NM per hour of operation.

Basically the IRS is an upgrade of the INS, instead of using the gyros and accelerometers to align the platform that they are based on, it basically calculates how far out of alignment said platform is and inputs it into the database. The advantages of the ring laser gyro allow the IRS to have a shorter "spin up" time and a longer life. You still have to input a latitude for the system to work out the earth rate of the gyros.

Yes that is correct the original INS which were carousels computed position by the means of mechanical gyros and accelerometer where as the later IRS Litton 92s IIRC determined postion by laser gyros and were much more accurate but can not remember the exact spec.

hmmm..

it is all in the acronym.

IRS = Inertial Reference system
INS= Inertial Navigation system

IRS is part of the INS - simple

whether spinning wheel gyros or ring lasers are used.

INS navigates (tell it where you want to go and it will tell you how to get there - usually via the flight director and almost always can be coupled to the autopilot. The heading and attitude outputs are a useful by product of the stable platform (either virtual or physical) used to supply the navigation circuits with groundspeed and direction information) The system is stand alone.

IRS only supplies reference information (attitude, heading, speed, acceleration, etc,etc,etc. This is then fed to other systems which calculate a navigation solution (FMGS) or display the output as a compass rose or attitude ball.) The system cannot navigate, it can only provide present position information.

The details of the internal mechanisms are irrelevant(IRS not always solid state). There are INS's that use RLG's and IRS's that use standard rate gyros.

This is what we're being taught in flight school right now:

IRS / INS / IRU are essentially different names for stable platforms which provide
1.) pitch, bank and yaw information by using whatever fancy gyro / laser technology suits the budget
2.) position information by mathematically integrating data which was received from accelerometers strapped to the platform
In modern airlines there are ADIRU units which couple IRUs with an air data computer to feed the complete flight data package directly to the EFIS.

Then there's AHRS (Attitude Heading Reference System), which only provides pitch, bank and yaw information. There are no accelerometers, so no position info can be derived. Can be found in light bizjets such as the CJ1 we've just received :ok: Also featured in the G1000 and electronic standby instruments.

Hope that's all correct so far :}

From what I seem to remember, the basic principle of IRS is a laser beam of known polarisation and frequency, which is subsequently bounced off a number of mirrors (3 or 5) before intersecting with itself at a point before the first bounce. The resulting beam is then scanned and momentary acceleration derived from difference in polarisation and perhaps other properties of the beam.

And then Mr Riemann comes along.

What is up with everyone almost consistently writing "l@ser", when the spelling is "laser"? L-at-ser? Latser? What is that? Some secret email address I don't know about?

Edit: test: laser
Edit2: That is stupid! How come if you write "laser" without the quotation marks, it appears as l@aser...?

Speedbird715
This is what we're being taught in flight school right now:

IRS / INS / IRU are essentially different names for stable platforms which provide
1.) pitch, bank and yaw information by using whatever fancy gyro / l@ser technology suits the budget
2.) position information by mathematically integrating data which was received from accelerometers strapped to the platform
In modern airlines there are ADIRU units which couple IRUs with an air data computer to feed the complete flight data package directly to the EFIS.


I would have to aruge it's close but not quite right. As has already been pointed out, a INS will navigate while a IRS won't. The IRS/ADIRU provides information to the FMC and the FMC then used the "refined" position it calculates to navigate. That information is then used in the ND.

411A

The original Litton INS units demonstrated a maximum guaranteed radial error limit of 2 NM per hour of operation.

I never heard that figure. I always remembered 3 + 3T was the limit. With T being the time the INS has been in NAV.

For most units, correct.
However, the Litton units fitted to the L1011, especially for SVA, were as I suggested.

Well I would say the ADIRU sends information to the FMC which ignores it in favour of the GPS for navigation. But the AD part sends air data direct to the PFD.

And I would like to agree with 411A. The original Litton was better than the Carousel. It was whirring gyros but more tech involved, and could couple to the FMC, and be programmed by it (in 1976). But the carousel lasted longer! If you didn't punch its buttons too hard!

Have flown the 747 classic with the Delco, and Litton 72 and 92 and the tolerance on all three was 3+3T.

As far as the ADIRU, I was just trying to address the specific nav function and was not going to get into the PFD display since the original question was regarding the IRS/IRU functions.

I have also flown the Litton 92 with the FMS interface and again the tolerance we used was 3+3T. IIRC, there was even a GPS interface with the FMC.

SpannersatCX has nailed it.
Forget about the type of gyro's and whether it has built in GPS and/or Air Data capabilities. These are irrelevant to the question.
INS computes a navigation solution whereas IRS only supplies present position to a separate navigation system (FMS, Area Nav, Mission Computer, etc).
There is nothing more to it.
Some L1011's and DC10's originally had Litton 58 mechanical gyro IRS units feeding an Area Nav system. Others had INS's without the Area Nav. Purely a customer option.

I remember reading an engineering history book (sorry, title forgotten--something like _The Invention of Precision_) which was basically about gyroscopes. As I recollect, the author claimed that strapped-down laser systems had many advantages over whirly gyroscopes, but increased accuracy was NOT one of them. True or not?

Yes, I think you and our Tristar pilot are talking about different things.
I think that 3 + 3T is always used as the criteria for removing an IRS for maint.(on the second occurance) We still use it today on modern laser systems. But nowadays it is really u/s if it gets to that limit, whereas in the 70s it quite often got near there.

Edit I didn't write @ I wrote a, but can't edit it!

bfisk
 

bfisk

It's the lemming syndrome - emulation is the best form of flattery.
( not shorthand for flat battery)

Thanks everyone! That clears a lot of my doubts.

PS: I didn't intend to write laser, I wrote "laser" and it transformed into laser on its own! Spooky!

Spooky me too!

see if it happens again : Laser


It blinking well did happen ...again !

As I Remember It
 

INU = Inertial Navigator Unit, i.e. LTN-58, as used in triple to feed PP, HDG, GS, etc., to dual Collins AINS-70 RNAV (area navigation system) on National and KSSU varieties of DC10, and dual Ambac RNAV on L-1011. It also provides attitude to the instruments and WX radar.

INS = Inertial Navigation System, containing an INU plus Nav computation, i.e. LTN-51, 72, 92, Delco Carousel IV, and Collins INS-61B. It did the navigation, and fed the instruments and autofright. It was the first self-contained navigation.

IRU = Inertial Reference Unit. Same as INU, only newer and better.

IRS = Internal Revenue Service - the tax collectors in the US.

RNAV = Random and/or Area NAVigation, with inputs from various sensors, optionally including INU/IRU.

FMS = Fright Management System, as first seen on the 767. It added extensive vertical guidance to the lateral guidance for the first time.

3+3T or 2+2T error limit - as decreed by the US Dept. of Defense. All the inertial navigators were capable of much tighter limits, but error was required, especially perhaps for foreign carriers, due to some munitions export control.

GB