View Full Version : Ins & Irs

24th Aug 2004, 11:18
Could somebody tell me the main differences of INS and IRS?

24th Aug 2004, 11:24
INS is mechanical (revolving gyros, accelerometers)

IRS is solid state (no moving parts)

That is it simply. If more is needed search the net!!

24th Aug 2004, 14:07
Don't know about all IRS's, but the one in the 767's still use basically the same accelerometers as the old style INS's do.
That is, an arm that hangs out and is held in place with an electric motor, and there's a couple of light sensors that detect movement, which then feed back to a box that sends power to the electric motor to keep the arm in place. The amount of current used is related to the G-force being acted upon the arm.

FE Hoppy
24th Aug 2004, 14:19
The spelling!!!!!




the old ins had a nav computer built in whereas an irs provides data to other nav systems.

24th Aug 2004, 16:07
the old ins had a nav computer built in whereas an irs provides data to other nav systems.

Not entirely convinced about that.
I currently fly a 747 Classic that uses regular Delco INS's, and they run through a regular Honeywell FMS, as fitted to IRS equipped 747's.

24th Aug 2004, 18:13
I'm pretty sure this has been done to death before on Pprune (try 'search'?) but my understanding (as FE H's) is that

IRS = Inertial REFERENCE system ie pitch/bank/yaw (used as part of an..........

INS = Inertial NAVIGATION system ie where are you etc.

Both INS and IRS can be based on EITHER Solid State or mechanical gyro platforms.

oxford blue
24th Aug 2004, 23:02
In broad terms, the difference between INS and IRS is about 30 years of technological advance.

INS emerged in military equipments in the early sixties and in civil systems in the late sixties/early seventies. IRS emerged in the nineties as part of a bigger system - the FMS.

There are 3 main differences between INS and IRS:

1) A far greater degree of system integration. In the sixties, you probably already had a B707 or two in your fleet. It was fitted with VOR and DME. Maybe it even carried a flight navigator and a Doppler system. You bought a bolt-on INS and it did the navigation for you. Sure, it had all these wonderful gyros in it, but you still carried on using your gyro-magnetic compass and your ADI because it would have cost too much to re-design the system in retrospect.

IRS is different. The systems engineer is brought in at the design stage of the aircraft, along with the airframe designer and the engine designer. Now you use IRS gyros as your primary reference source for EFIS, for stabilising the AWR scanner, you use IRS velocities in your maxaret braking system and your FADEC, you have a vertical channel for your VSI to replace those old-fashioned dashpots, and attitude, accelerations, velocities and positions are all fed to the FDR. It's now all part of one big system.

2) The advent of ring laser gyros. Thirty years ago you had lumps of rotating metal in your INS. Now we shoot photons through a prism and measure the path difference. There are no moving parts. With INS the mean time between failures (MTBF) was of the order of 1000 hours. With IRS there is just nothing to go wrong. No electric motors, no servos, no gyro bearings, no heavy electrical engineering. The MTBF has become virtually irrelevant. People bang on about the increased accuracy of IRS, but they miss the point. Sure, it is more accurate but, far more important, it's more reliable and cheaper. It hardly ever fails.

3) The vastly increased power of modern computing. In the old days you had to have a gyro-stabilised platform to keep the accelerometers level. The platform stayed level and north-orientated whilst the aircraft manoeuvred round it. This was expensive and it tended to fail every so often. Now, with real-time computing you bolt the accelerometers and lasers onto the airframe and they manoeuvre with the aircraft. However, the computer knows which way is north and which way is up (or down) and resolves these accelerations by matrix algebra and axis transformation into north, east, and down.

There are lots of other differences as well, but that's enough to be going on with.

26th Aug 2004, 02:25

No doubt you are aware of stand alone INS navigation.
When the FMS fails, those Delco INS's can still be used, in a stand alone function.


26th Aug 2004, 09:12
A fairly good answer Blue, and broadly speaking correct. The INS system you described would have been the old LITTON INS. The Delco carousel INS actually had the componants mounted on a rotating platform to reduce standing errors.

The one glaring error is the assumption that there are no motors or moving parts in an IRS which uses laser gyros. In fact there are 3. These are known as dither motors and are mounted on the individual laser gyros to prevent a phenominum known as laser lock, where a rotational movement will in fact not produce a change in the interference fringe pattern.

From my old days studying this was a favourite CAA "catch you out" question.

Apart from that there are infact modern INS's the main difference is as has been stated that an INS includes a navigation computer, whereas the IRS does not (the navigation is done by the FMC). It is again wrong to suggest that the INS gyro outputs were not interfaced with a/c systems the older 707/727/747 and some 737's picked off INS platform attitude and used it to drive the ADI's and HSI's

26th Aug 2004, 21:31
In most simple terms

INS knows where it isn't and using interpolation tells the operator where it must be by process of elimination.

IRS constantly tracks its known position and reports that.

INS can not be updated in flight although the computer can be updated to provide an adjusted assumed position (vor/dme/tacan updating) up until the error correction region limit is exceeded. Then it dumps.

IRS can be corrected but not reinitialized in flight. There is no limit to the amount of error correction so dumps without an accelerometer failure are very rare.

INS knows where it isn't
IRS knows where it is
DME/GPS removes the error in the Navigation computer which today is most commonly the FMS but used to be (not so long ago) in the INS.

Don't confuse FMS with INS/IRS
FMS uses what it's told to navigate with.

You guys are really making me feel my gray hair.:}

3rd Oct 2004, 11:38

you actually forgot 3 more moving parts.

Apart from the 3 dithering motors (one for each axis, or is it one omtor for all), there are for each axis a small servo controlled mirror to align the lasers. It moves only on alignment, if ever, and the movement is a fraction of a degree.

The IRS (the laser one) is actually a beam of light from a laser sent into a triangle bade up by mirrors and a prism. the light is deflected both ways around in the triangle, then it meets again in the prism. The small servo operated mirror corrects the light so that the two counterrotating beams are exactly syncronized when they meet again.

The syncronization is only possible because LASER light is not only only one frequency, but also in phase (like the output from an AC generator). The result is a so called "fringe pattern" which will be steady when the IRS is kept calm, but it starts moving as soon as the IRS is turned. Just like a music record player of the type with dashes painted on the disc, where a light powered by AC (a stroboscope) makes the lines appear steady when the correct speed is matched.

So, IRS still need accelerometers, as the laser part of it is only the angular displacement, not the acceleration.

Cheaper systems use fibre optics instead of the mirrors. The are a lot cheaper, but not as precise over time. I would imagine this is OK for missiles which in nature need not fly for prolonged times, and they usually need only fly once :eek: boom

The original question as to what is INS vs IRS has many confused so both terms are used on either laser or stabilized platforms. Just like terms like FMS/FMC/CDU are all used to describe the same thing.