View Full Version : IRS? what is it really?

2nd Oct 2005, 10:02
Your common, every day IRS. i'm sure those of you who operate an aircraft using these handy little navigation aids all know what they do in some sense.

i remember a while back being taught what they are made up of, kind of. but as time has gone by i have taken this little wonders of aviation for granted.

i vaguely remember something about lasers, gyros, tilting mechanical tables or i'm i talking rubbish...?
i mean, i just know they do the job requierd but would like to know more.

replies appreciated and pardon my stupidity:O

2nd Oct 2005, 22:00
i vaguely remember something about lasers, gyros, tilting mechanical tables
Could be any of those, depending on the vintage of the IRS. Even our 747 Classics have ring laser gyros now.

Old Smokey
3rd Oct 2005, 00:51

I was waiting for one of the technocrats to come up with something edifying here, but in there absence will give a rough explanation (Now, watch the technocrats attack this one like bees to a honey pot).

Laser Ring Gyros, the primary component of the IRS, detect movement by measurement of the frequency shift of a laser emission as the aircraft moves. Light moves at a fixed speed, it is not subject to the vector sum of it's own speed and that of the originating source, that is, if light moves at 186000 miles per second, and is transmitted by an aircraft moving at 500 knots, it still moves at 186000 miles per second, NOT 186000 miles per second plus 500 knots. Observation of the laser transmission will therefore be obvious to a moving object (the aircraft) as a frequency shift from the constant speed light source (much like Doppler). This can be measured, and converted mathematically to relative speed.

Laser Ring Gyros typically fire laser pulses over a triangular course (contained within the aircraft of course), thus, longitudinal, vertical, and lateral components of movement can be measured. When this movement is referenced to the original position, the current horizontal and vertical position, speed, etc. are known.

Various spectrums of 'Normal' light, and other radio frequencies, as we know them, do not move at exactly the same speed, there is a slight variation in speed between frequencies, thus the astronomical observation of 'red shift' of light from receding galaxies (Red light moves faster). For an effective IRS, the speed must be very stable and predictable, and this is offered by the very pure and frequency stable laser emission.

There are no gyros, tilting mechanical tables etc. contained within an IRS, it has no moving parts, that description belongs to INS. Laser gyros are not gyros per se, but the equivelant thereof.

That's my two bob's worth, now, flak jacket on, awaiting incoming from the technocrats.


Old Smokey

3rd Oct 2005, 11:14

Coming down to SIN for a test and ferry mid October , care to meet up for a pint down at Robertson Quay for some tech lessons . . . !

By the way bang on on the IRS bit !



3rd Oct 2005, 12:31
Old Smokey,

A nice explanation of RLGs, however I'm not sure if you're correct in relation to what they actually measure.

A typical Intertial Reference System comprises, among other things:
- 3 Ring Laser Gyroscopes, as you described, orientated fwd-aft, left-right and about the normal axis such that all three planes of motion are covered
- 3 accelerometers (typically quartz-based, although others can be used) aligned along the same axes as the RLGs

Ring Laser Gyroscopes measure ROTATION (ie. radians(or deg)/sec). Rotating a RLG in its plane of orientation will cause the clockwise laser beam (say) to arrive at the sensor/origin earlier than the counterclockwise beam, thus generating an interference ('beat') pattern proportional to the rate of rotation. RLGs do not measure linear acceleration.

After initialising (levelling) the RLG system, it can therefore be seen that combining the outputs of the three RLGs enables a computer, after double integrating, to keep constant track of aircraft attitude (after all, changes in aircraft attitude are always a rotation about at least one of the three RLG axes).

The three accelerometers, mentioned above, are used to measure LINEAR accelerations in their planes of orientation. After perfoming the neccessary integrations, a computer combines the outputs from the RLGs and the linear accelerometers, and determines the component values of acceleration, velocity (first integration) and position (second integration) in the relevant planes (longitudinal, lateral, normal (aka. vert speed)).

Here's a simplistic example.

A B744 is climbing with an attitude of 10 deg nose up. It is also accelerating at 1m/sec/sec in nil wind. Assuming no lateral accelerations, the following is taking place:

1. Longitudinal RLG measures rotation as a/c pitches up from 0deg to 10deg nose up. Performs integrations, and outputs the new sensed attitude of 10deg.
2. Longitudinal accelerometer senses 1 m/sec/sec acceleration (but remember this accelerometer is tilted 10 deg nose up!)
3. IRS computer combines outputs from the two, and splits the sensed acceleration into true longitudinal acceleration (ie. towards the horizon) and true vertical acceleration. The true longitudinal acceleration is integrated to find change in Ground Speed (i'm simplifying this a bit), and then integrated again to find change in position (eg. lat/long). True vertical acceleration is integrated once to find IRS vertical speed.

Note this is highly simplistic, but should illustrate the point. I don't intend to cover Earth Rate, correction, wander, drift, Schuler loops etc, only because I just don't know enough about them ;)

Hope this helps! IORRA

Edit. Here's an example of a manufacturer's overview of their IRS system. Note that both accelerometers and RLGs are listed, among other things.


Old Smokey
4th Oct 2005, 13:18

Very nice explanation indeed. Just the sort of thing needed for the "technical" response required from the original poster. My very simplistic post was, as stated, a 'fill-in' until a more qualified technocrat came along.

P.S. Thanks for not shooting down my simplistic 'fill-in' too hard, seems my flak jacket wasn't needed:O

airfranz, send me a PM as the time gets nearer, sounds good to me, but the rostering demons may have their say! I look forward to learning something:ok:


Old Smokey