I don't know what's in the ATPL syllabus, but I'll give you my simplistic Engineer's approach to this.. .. .(1) Take a lump of metal. Hold it in some mounts. .(2) Whenever you move the lump of metal, there will be stresses in the mounts.. .(3) Strain gauge the mounts, feed the output into a computer.. .(4) The computer calculates the accelerations in the lump of metal, from the strain gauge output.. .(5) Going back to high school physics, speed = acceleration x time. So, the computer can work out how fast the lump of metal is going (assuming it started off still).. .(6) Ditto, distance = speed x time, so the computer can work out not just how fast the lump of metal is going, but where it has got to (assuming that it knew where you were in the first place).. .. .That's the basic principle, but to really work, you need.... .. .(1) Strain gauges in 3-dimensions, because the aeroplane moves in 3 dimensions.. .(2) Gyroscopes and damping to keep the metal from swinging about and messing up the strain gauge readings.. .(3) The computer also needs, to know where you really are, to compensate for the fact that the earth wobbles in a sine wave of about 3/4nm at 78 minutes (this is called the Schuler Cycle).. .(4) Incredibly accurate measurement devices.. .(5) Some quite difficult sums to work from three accelerations, to a 3d position in space.. .. .The modern versions use what's called a "Ring Laser Gyro", which does the same thing but uses a laser, travelling around a loop. Because the speed of light is fixed, a very accurate measurement of the time it takes the laser to travel around the loop does the same thing.. .. .You also have a personal built-in INS, which is the canals in your inner ear, it's what gives you a sense of balance - although your brain isn't quite clever enough to work out your position from this, just which way up you are (most of the time !).. .. .Hope this helps,. .. .G