Gyroscopes — Page 148, Lesson 171

We're starting a new part of the gyroscope chapter now, and this is where we separate the two big families of wander: real and apparent. Let's get into it. First, remember that all gyro wander—whether it's drift or topple—can be split into two categories. We have real wander and apparent wander. Let's start with real wander. In real wander, the axis of the gyro physically moves with respect to inertial space. That's the key phrase: inertial space. It means the gyro's axis is actually departing from its original orientation in the universe, not just appearing to. What causes this? Manufacturing imperfections. Things like uneven rotor bearing friction, gimbal friction, imbalance in the mass of the rotor, and unbalanced gimbals. These are physical flaws in the hardware. Now, can we fix it? Yes, to a degree. Real wander can be reduced by higher quality engineering and manufacturing. But here's the practical catch: depending on the application, the cost of reducing real wander may not be justified by the level of accuracy required. So just like instrument error, a gyro needs to be only as accurate as the need of the application in use. Over-engineering costs extra money, and that's not always worth it. One more name for real wander: it's also known as 'random' wander, because it's unpredictable and varies from unit to unit. Now, here's the fascinating part. Even if we eliminated all real wander—even if the gyro remained perfectly fixed in space, say pointing at a distant star—the direction it indicates would still change. That's apparent wander. The gyro hasn't moved at all, but the observer's frame of reference has changed. And there are two causes of apparent wander. One is the rotation of the earth, called Earth Rate. The other is caused by flight east or west at latitudes other than the equator, and that's called Transport Wander. Let's look at Earth Rate with a concrete example. Imagine an aircraft parked on the equator, with a gyro axis pointing to true north, aligned with the local meridian. As the earth rotates from position 1 to position 2, the gyro axis remains fixed in space. But here's the beautiful part: it also remains aligned with the local meridian. Why? Because at the equator, the local meridian itself is rotating with the earth, and the gyro axis is staying with it. So there's no horizontal component of earth rate at the equator. The gyro doesn't appear to drift at all. That's the core idea: real wander is a physical defect we can engineer away, but apparent wander is a geometric consequence of our moving frame of reference. We'll see next how that changes when we move away from the equator.