> The new guidance platform also added a gyrocompass under the alignment block, a special compass that could precisely align itself to North by precessing against the Earth's rotation. At first, the gyrocompass was used as a backup check against the autocollimator, but eventually the gyrocompass became the primary alignment. For calibration, the alignment block also includes electrolytic bubble levels to position the stable platform in known orientations with respect to local gravity.<p>Had never heard of gyrocompasses before. I worked on a small robot in the past and remember having to calibrate the magnetic compass, which was not very accurate (similar to smartphone compasses). I never thought about how they’d get super precise headings for ICBMs.<p>The Encyclopedia Britannica article on gyrocompasses is really good. Here it explains why you can’t use a gyrocompass on a vehicle on fast aircraft (and I guess small robots that are jostled around a lot):<p>> A major contribution by Schuler was the discovery that, when the period of oscillation is 2π√(Earth radius/gravity), the heading precession of the gyroscope spin-axis due to acceleration is exactly the rate of change of the angle between the apparent and true meridians seen on a moving vehicle. The gyrocompass will then read true north at all times if its indicating reference is offset by the angle between these two meridians. The angle, at ship speeds, is a direct function of the north-south speed and is easily set into the system. The need for accurate speed measurement for this offset is the main reason why a gyrocompass is not practical for use in aircraft.<p><a href="https://www.britannica.com/technology/gyrocompass" rel="nofollow">https://www.britannica.com/technology/gyrocompass</a><p>Love this article!