Remote Indicating Magnetic Compass — Page 226, Lesson 266

Let’s pick this up right where the transmitter rotor sits, because that’s the heart of the whole 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 main gyro is driving, that rotor’s orientation is the heading to be transmitted. Think of the rotor as the moving part that carries the heading information. Now, that rotor is fed with a constant primary excitation — an AC voltage. That AC excitation induces a magnetic field in the stators around it. The stators are the stationary windings. Here’s the clever part: those stators are connected directly, 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 radio link, no data bus — just three wires carrying the field pattern. Now let’s look at the repeater side. The repeater has its own 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 then to a motor. The motor turns a shaft — the shaft on which the repeater rotor is mounted. The repeater shaft keeps turning until no further voltage is detected. At that point, the repeater rotor is perpendicular to the field, and the system is in balance. So the repeater shaft follows any heading change in the main gyro drive shaft. If the gyro heading changes, the field changes, the repeater rotor senses it, the motor drives the shaft until it’s perpendicular again — and the repeater has faithfully reproduced the new heading. Now let me give you the summary, because it ties the whole system together. The gyro-magnetic compass system overcomes the weaknesses of the direct reading compass. Those weaknesses are: turning and acceleration errors, the magnetic element being too close to sources of deviation, and no feed to other equipment. It also overcomes the weakness of the directional gyro, which is that it has no magnetic monitoring — it drifts over time with no reference to the earth’s field. 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 steady, accurate heading over seconds and minutes; the earth’s magnetic field keeps you honest over hours. Now, the question at the end asks what a gyro-magnetic compass, or magnetic heading reference unit, always consists of. The four components listed are: a directional gyro, a vertical axis gyro, an earth’s magnetic field detector, and an azimuth control. That’s the full assembly — the gyro for short-term stability, the vertical axis gyro for the reference plane, the earth’s field detector for the magnetic reference, and the azimuth control to manage the heading output. That’s the complete picture of how the remote indicating magnetic compass works.