Captain H. Peacock ---Who says that Pprune has 'gone down' in quality?
That was a very learned and interesting discourse!!!
there's so much offered on this site its unbelievable!!
Most of it however was wrong!!
I will try to deal with them item by item.
The laser ring gyro is a ‘true gyro’ in that it measures angular displacement (turning to you and me).
Two errors here. First. The RLG is in fact
not a gyro at all - it has none of the properties of a gyro (precession and rigidity). It is in fact a laser interferometer. Secondly. The RLG is a rate device, it measures the
rate of angular displacement. Once the angle has changed and the heading/attitude/etc is constant there is no RLG output (dither oscillation excluded). Rate outputs provide much higher accuracy because they can be tuned such that small rates of change give large outputs.
At the equator, the angular motion is in a ‘tipping’ sense, ie although not rotating in the normal plane, the laser ring ‘tips’ toward the rising Sun – eastwards – at 15˚ per hour.
The maths is correct but The RLG can only sense rate of rotation in one plane. For that reason there are in fact 3 RLG's in a typical inertial reference computer. These will be orientated such that they sense movement about the a/c lateral, longitudinal and normal axis.
In the northern hemisphere for instance, the direction of true North will move westward by 15sin(latitude) per hour, and if your laser ring moved 10˚ in 1 hr, then the latitude would be 42˚N. Hence, if we had input 42˚N into the IRS at ALIGN, then the laser ring could deduce the direction of true North.
An IRS can quite happily calculate latitude and the true North-South axis without any operator input. What it can't resolve is the ambiguity. Is it north or south of the equator and which direction on the axis is north? For that it needs to know its longitude and either N or S wrt latitude.
In the old days of spinning iron gyroscopes and analogue computers, a platform could take 30 minutes to align, not least because a conventional gyroscope measures angular acceleration which needs to be integrated to provide an angular displacement.
As previously stated the RLG is a rate device. It measures angular
velocity. Integration of its output is still required to calculate angular displacement.
A laser ring gyro can accurately sense a 1˚ change in angular displacement directly, and hence 7 minutes at mid-latitudes is sufficient to divine the direction of true North.
Try 0.01 degrees. These things are very sensitive.
Rather fascinatingly, and one of the things that really impresses me about Mother Nature, is that the IRS platform behaves like a huge pendulum with its bob at the centre of the Earth. If the pendulum is displaced, it will swing with a period of about 85 minutes, a phenomenon called the ‘Schuler Loop’.
Not so much a mother nature phenomenon as a man made one. In order to be accurate IRS (and INS) platforms need to be "Schuler tuned". This is so that they ignore angular movements due to the curvature of the earth (in effect they are what we call "tied" to the earth). Older gymbal based platforms were tied mechanically via pendulous sensors that precessed the gyros and kept the platform level. Modern strapdown systems are tied electronically with speed, altitude and other parameters used to calculate the expected drift and then cancel it out. This Schuler tuning gives rise to the schuler period phenomenon. Failure of the schuler circuits can often lead to schuler period oscillations.