The laser ring gyro is a ‘true gyro’ in that it measures angular displacement (turning to you and me). The laser ring has no idea that it’s on the Earth’s surface, and it simply reports that angle that it has turned through to the IRS systems.
If you tell the IRS where it is on the Earth’s surface, then as the Earth turns, the laser ring senses a turning motion.
The Earth turns 360˚ in 24 hrs, that’s 15˚ an hour.
At the North Pole, the laser ring would sense 15˚ of turn every hour in the normal (horizontal) plane, because the axis of rotation passes through the pole. 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.
If you know the position of the laser ring at a given time, at a certain time later, you can say that some of the angular displacement has been from normal rotation, and some from tipping. If you know what your latitude is, then it’s just a matter of doing the sums. 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.
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. 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.
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’. Hence if the initial position input to the IRS is wrong, the heading and attitude outputs from the IRS will wobble about every hour and a half.
Isn’t physics great fun?
