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Flashcards in IRS Deck (16):

What is an IRS?

Inertial reference system and is a solid state accelerometer.

While INS is the 1st gen model, IRS is the 2nd.


How long does it take for an IRS to align?

8-10 mins

And like INS, must be aligned on the ground/when stationary.


What is the main mechanical difference between IRS and INS?

IRS use a strap down system i.e. IRS moves with the A/C

IRS is not a standalone system and is often integrated with other systems to provide more qty and accuracy of data.

E.g. ADC will feed the IRS with TAS info to retrieve wind, Mach no etc.

The whole point of IRS is, when you have an instrument measuring e.g. Vertical movement, why do you need a separate instrument for VSI and AI? IRS combine all data with other computers to get the required info so that there is no duplication. The only thing that is duplicated is the IRS unit itself for fail safe purposes.


What are the main differences (given data) between an IRS and INS?

IRS gives altitude, VSI and if fitted with GPS, MN, TAS.

Position error/accuracy of IRS 0.1nm vs INS 1.5nm


What are RLG?

Ring Laser gyros (LRG) are used in IRS.

LRG works like a gyro but is not actually a gyro but 2 beams travelling through a rotor in opposite directions. It uses the interference principal to detect acceleration.

E.g. We know that when light travels at same speed, thus the freq is the same. But if you rotate the 'gyro' then 1 beam will travel further than the other. Because of this the beams will now have different freq and so difference interference patterns are measured.

By measuring freq, even the slightest movement will be detected making this instrument far more accurate.

However RLG still suffer from random drift i.e. Mechanical imperfections, and the size of an LRG is larger than the standard (actual) gyro.


What is laser lock?

Laser lock occurs in LSG.

This is where the LSG unit rotates too slowly and the 2 beams become 'locked' /in phase and so cancel each other out.

To solve this problem dithering is used via a dithering motor, which just vibrates the LSG unit.


How long does it take to align a GPSADIRS/GNADIRS (global nav)?

3 to 17 mins on the ground.

In the air:
10-15 for ATT mode
15 to 30 mins for NAV mode
That's total 15 to 40 mins


What is an ADIRS

Sensor fusion of an ADC and IRS.

There are often 3 ADIRU (units) in a ADIRS. So often 2 out of 3 data is selected for calcs use.

ADC are inaccurate short term (e.g. due to lag readings from static/pitot ports) and

IRS are inaccurate long term due to unbounded errors.


In a RLG, interference is proportional to....

Interference is proportional to the rate of rotation.



GPS, ADC and IRS system combined.

If GN then global nav data has been stored in the system. It still has a GPS, ADC and IRS combined system.


An IRS is aligned to allow the system to build a platform mathematically. What is this called?

Computed trihedron with respect to the earth.


A laser gyro consists of 3 electrodes, what are they called?

Anodes and cathodes


The output data of an IRS include:

• True angle
• Drift angle
• GS


What are the difference(s) between the control switch of an INS and IRS system?


IRS: Off - NAV - ATT
(Usually a Boeing config)


WHen EASA refer to "sensors" of an INS measure, they mean INS accelerometers.

Accelerometers measure linear accelerations and rate.

Integrating (or laser) gyros measure angular accelerations.


Inertial systems:

By integrating speed and time you can get distance travelled.

To get distance from acceleration it must be integrated twice.

But only if system knows:

start from (position) and
Initial speed