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Inertial Reference System
By webmaster | May 16, 2008
New universal heading reference systems are continually being developed and improved to meet new and existing requirements, with an all weather capability, for military and commercial aircraft. The Attitude and heading reference system provides an accurate determination of aircraft heading relative to true north. This is accomplished by use of fluxgate magnetometers, MEMS accelerometers and/or angular rate gyros, depending on the region of operation. Engineers and other personnel engaged in testing and evaluating aircraft systems have developed certain procedures of testing over a long period of time. These procedures, properly used, can aid in determining the acceptability of heading reference systems for an intended use.
In short, here’s how it works. First, you turn on the sitting motionless on the ramp. The inertial system senses angular motion (and it knows that this motion can only be in a east-west direction). This happens at a certain rate, that’s because the earth turns at around 15.04 degress per hour in space (the result of the 24 hour rotation and the annual rotation around the sun). At the equator, you have travel 21600 nm divided by 24 = 900 nm in space every hour by sitting on the ramp. If you go north and follow e.g. the 75 degree latitude around the earth and measure the mileage (or do the math), it falls far short of the 21600 nm at the equator. Therefore the rotational speed is much less until it reduces to practically zero at the poles (neglecting the rotation around the sun).
While it may take as little as 2 - 3 minutes to complete the alignment at the equator, it takes an easy ten minutes around the 70 degree north latitude. Further north and the system might not be able to align correctly before it starts to drift and might be unable to enter NAV mode because it doesn’t come up with useful numbers to satisfy its criteria for accuracy. It just gets more difficult to do exact calculations since the starting values are much smaller further towards the poles.
Topics: Engineering |