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.
http://www.nsd.es.northropgrumman.co...TN-90-100.html