Lesson 11 - 16/9/25

IRS / INS Differences

• IRS has better system integration - provides better info to other systems

• IRS has better computing power - able to utilise strapdown platform, and computing ability to resolve direction

• Doesn’t conduct navigation - only forms data and gives it to FMS to navigate

INS - Stand-alone mechanical system

IRS - Integrated with FMS aircraft with greater computing power

Ring Laser Gyro

Uses monochromatic lasers (of known frequency and wavelength) to measure angular rotation

Angular rotation causes the apparent change in path of the light - change in fringe pattern

Movement of the platform causes a change in frequency and a change in fringe pattern

Peaks are counted by photodiodes

Ring laser gyro advantages/disadvantages

Advantages -

• reduced alignment times

• Better accuracy

• More reliable

• Small and lighter

• Lower power requirements

Disadvantages -

• Dither - Small rates of rotation, tendency for beams to lock onto the same frequency giving zero input, so the gyro is oscillated about its axis (dithering), as the dithering is is equal in both directions, sum over time is zero

• Real Wander - even with few mechanical parts for thermal expansion to occur, it still occurs, this can be compensated for

MEMs Accelerometers

Piezoelectrical material = change of shape induces a voltage

Proof weight acts like a pendulum

Quartz beam bends

Changes shape of Piezoelectric strain gauge

Change of shape proportional to acceleration

Induced voltage is proportional to acceleration

IRS Operation

Measures acceleration along 3 axis (aircraft axis) with 3 accelerometers - strapdown system

This is related to longitude/latitude and earths vertical

IRS Initialisation

When stationary on ground

All accelerometers are used to measure gravity and reference the vertical, meaning changes of attitude against a space reference must be coming from earths rotation

The RLGs can use earths rotation to establish true north which is stored in the computer

IRS will then use earths rotation to estimate latitude

Pilot inputs latitude/longitude into FMS/IRS to establish a start point

Initialisation is known as alignment but happens theoretically

IRS alignment takes 5 mins at equator, 10 mins at 70 north, and 17 mins above that

FMS Aircraft Positioning

Methods -

• IRS

• GNSS

• DME

• VOR

• Localiser

All positions are put through Kalman filter which filters out inaccurate data

If stationary, GS Should be zero in relation to waypoints but may display 1 due to small errors. Big numbers represent a problem

IRS Errors

• Coriolis Errors

• Earth Rotation Wander

• Transport Wander

• Schuler error - present even with no physical platform

IRS MSU

Does not have a standby function due to the gyro not needing to spin up

Lights -

• Align - (Steady = Alignment in progress, Flashing = alignment can’t be completed, Unlit = alignment completed)

• Fault

• On DC - Using battery power

• DC Fail - No battery power available, only generated AC Power available

ISDU Function

Only used when a FMS failure happens

IRS doesn’t navigate, the FMS does so when it fails, the ISDU is used

IRS Output

Accuracy of IRS = 0.5 NM per hour compared to 1.5 NM per hour for INS

Cannot be interfered from the ground - conflict areas

IRS/INS Differences/Similarities

Air data inertial reference unit - ADIRU

IRS Combined with a ADC - More modern