Irs

[sr562]

Hi,

I wondered if anyone could enlighten me as to how the IRS works. I know it is independant from both radar and sattelite nav systems. What i want to know is how it knows where it is, and why is it so accurate.

Answres as simple as possible will be appreciated as i am thick.

[Mad (Flt) Scientist]

The IRS uses a means of sensing accelerations. (Sometimes laser based, as in a laser gyro, more often a mechanical system).

From that it can integrate what speed the IRS is being moved at, and in which direction, and then integrates speed to determine location.

It's really just very accurate dead reckoning - if I go north at 300kts for an hour, I am 300nm north.

Because of the integration thing, small errors do eventually build up - often called 'drift' and it's best to zero-out the errors at a known position (called 'alignment') or by cross referring to another nav system occassionally for alignment.

[QAVION]

"The IRS uses a means of sensing accelerations. (Sometimes laser based, as in a laser gyro, more often a mechanical system). "

Wouldn't acceleration be measured by the accelerometers in the IRU's? Laser gyro's simply sense pitch, roll and yaw (angular movement).

"Sr562"... Accelerometers are basically weights (masses) on springs. Mass has inertia and has a tendency to stay where it is, and if an aircraft starts moving (accelerating), the mass's inertia causes the spring to stretch. However, the IRU's try to keep the mass in the same position relative to the aircraft with electromagnetism. The power required to keep the mass (relatively) motionless during acceleration is measured and converted to a digital signal for acceleration (and ultimately distance) computations. Basic High School physics formulae apply: e.g. "s=ut+1/2 at^2". "s" = Distance from starting position (Starting position lat/long is given to the IRS by the pilots). They enter this via the keyboard on a Control Display Unit (CDU) in the cockpit).

I believe all commercial airline IRS gyro's are laser-based. The old INS had mechanical gyros.

Regards.
Q.