Remote Indicating Magnetic Compass — Page 226, Lesson 266

Let’s start with the heart of the system — the transmitter rotor. In the remote indicating magnetic compass, the rotor of the transmitter is physically attached to the heading drive shaft, and it rotates with that shaft. So whatever heading the shaft represents, the rotor’s orientation is exactly that heading — the heading that is to be transmitted onward. Now, that rotor is fed with a constant primary excitation AC voltage. That AC voltage induces a magnetic field in the stators — the stationary windings around the rotor. So the rotor’s position creates a field pattern in the stators that encodes the heading. Here’s the clever part: those stators are directly connected by a 3-strand wire to the three stator arms of the repeater. So the exact same magnetic field that exists in the transmitter stators is reproduced in the repeater stators. No electronics in between — just three wires carrying the field information. Now, inside the repeater there’s another rotor. If that repeater rotor is not perpendicular to the field in the repeater stators, an AC voltage is induced in it. That voltage is passed to an amplifier, and the amplifier drives a motor. The motor turns the shaft on which the repeater rotor is mounted. The shaft keeps turning until no further voltage is detected — meaning the rotor has reached the perpendicular position where no voltage is induced. At that point, the repeater shaft has matched the heading of the main gyro drive shaft. So the repeater shaft continuously follows any heading change from the main gyro. Let me make sure the principle is clear: the transmitter rotor sets up a field; the repeater rotor senses when it’s not aligned with that field; the amplifier and motor drive it until it is aligned; and that alignment means the repeater shaft tracks the heading. Now the summary — this is important because it ties the whole system together. The gyro-magnetic compass system overcomes the weaknesses of two earlier devices. First, the direct reading compass, which had turning and acceleration errors, had its magnetic element close to sources of deviation, and had no feed to other equipment. Second, the directional gyro, which had no magnetic monitoring — it would drift over time with nothing to correct it. So the gyro-magnetic compass combines the short-term stability of a gyroscope with the long-term directional stability of the earth’s magnetism. The gyro gives you stable heading over short periods; the earth’s magnetic field gives you a reliable reference over the long term. Together, they correct each other’s weaknesses. Now, the question at the end asks what a gyro-magnetic compass — or magnetic heading reference unit — always consists of. The four items listed are: a directional gyro, a vertical axis gyro, an earth’s magnetic field detector, and an azimuth control. That’s the assembly definition — those are the components that make up the unit. Let me pause here