Why do you have on big jets 3 IRS positions (left, middle, right) on your FMC, but only 2 GPS positions?

Question

And does it mean that you have two different GPS-antennaes, and 3 different IRS-antennaes?

Answer 1

An IRS does not have an antenna.  An IRS, or Inertial Reference System, is completely self-contained.  Once the system is initialized and knows its initial location, it senses acceleration in all three axis to determine where it is.

Since it is completely self-contained, there is no reliability check of the unit from the outside.  By having three independent IRS units, if one fails or gives inaccurate readings, the other two still match so you can tell which one has gone bad.  You can't do that if you only have two units: if one was giving bad information, you can't tell which one is accurate and which is bad.

A GPS uses satellite signals and is self-monitoring for integrity.  If there is any kind of failure, it will tell you.  The second GPS is purely for backup, not for "testing" the first unit.

Answer 2

Wow, thanks i know the question is true but am not quite sure with the answer thanks to you and the answerer coffeesomething. Let me also share; In A340 these IRS are all in 3 ADIRU or Air Data/Inertial Reference Unit

Answer 3

Three sources of attitude and position are mandatory for CAT III  landings. And they are self contained and does not depend on external data.

GPS satellites may be powered down (they actually were just b4 gulf war started) and signal might be lost. GPS is not primary source for CAT3 landings.

If one of the three IRS computers fails in any way, will drop to basic mode and pilots will read a "NO LAND 3" mesagge in EICAS or ECAM (depending on aicraft type).

Autoflight is still usable, but not Autoland.

Regarding antenna-related question, IRS does not use antenna. It uses a Very special type of accelerometer.

Cheers