Gridded Charts — Page 471, Lesson 465

I want to walk you through the three gyro errors that matter when we steer by gyro in high latitudes — Real Drift, Earth Rate, and Transport Wander. These are the reasons we can't always trust a magnetic compass up there, and why we switch to gyro steering in the first place. Let me set the scene. Near the poles, the Earth's magnetic field is too weak to be useful — the horizontal component is too weak, and in some places the rate of magnetic variation changes too rapidly to be usable. So we steer by gyro instead. But a gyro isn't perfect. It has three errors we have to understand and correct for. First, Real Drift — also called Random Wander. In an INS-quality gyro, this drift is on the order of one-hundredth of a degree per hour. That's tiny. But here's the practical point: if an INS is fitted, you usually don't need Grid at all, because most INSs are polar-programmed and can keep operating in True up to very high latitudes. However, if no INS is fitted, a typical modern airline-quality Slaved Gyro Compass will still drift only about one-tenth of a degree per hour when operating in DG mode — that's Directional Gyro mode. So Real Drift is low enough to be insignificant in practice. Second, Earth Rate. This happens because the gyro is aligned with True North at some particular meridian. The aircraft rotates with the Earth, along with that meridian, but the gyro stays aligned to the point in space where it was originally aligned. So the gyro appears to drift relative to True North. The correction factor is 15 × sine latitude degrees per hour. Many modern Slaved Gyro Compasses have a Latitude Correction control — a knob the crew turns to set the local latitude, and that correction is fed to the gyro. The crew keeps resetting the latitude as it changes. Third, Transport Wander — and this is just another name for meridian convergence. Again, the gyro is aligned with True North at some particular meridian. Then, as the aircraft travels east or west, the gyro stays aligned to the original direction in space, but the direction of the meridians — and therefore the local direction of True North — changes underneath it. That's the convergence of the meridians showing up as an apparent wander of the gyro. So to tie it together: Real Drift is the gyro's own internal wander, Earth Rate is the gyro holding its alignment in space while the Earth rotates, and Transport Wander is the meridians converging as you move east or west. Each one is small on its own, but at high latitudes, where the magnetic compass fails, these are the errors we correct for when steering by gyro.