Gyro Magnetic Compass

Wing Commander Rajagopal

Direction Measurement using Gyro Magnetic Compass

A gyro-magnetic compass combines the stability of a gyroscope and accuracy of compass using the Earth’s magnetic field. It reduces errors caused by acceleration, turning and magnetic dip. This makes a GMC more reliable for maintaining precise heading during flight.

Introduction to the Gyro Magnetic Compass

  • A remote-indicating compass reduces many of the limitations of a direct-reading magnetic compass.
  • It minimises heading errors that occur during aircraft acceleration and turns.
  • It reduces magnetic deviation caused by locating the sensing element inside the cockpit.
  • It allows magnetic heading information to be transmitted to other aircraft instruments.
  • A gyro magnetic compass combines a directional gyro with a magnetic compass system.
  • The directional gyro is continuously monitored and corrected by a magnetic detector.
  • The gyro is corrected using a slow precession rate of approximately 3° per minute.
  • The slow correction rate filters out temporary errors from the magnetic detector.

Construction of the Gyro Magnetic Compass

  • The Gyro Magnetic Compass is also known as the Magnetic Heading Reference System (MHRS).
  • It combines the stability of a gyroscope with the directional reference of a magnetic compass.
  • The system consists of a horizontal directional gyro and a magnetic flux valve detector.
  • A compass dial displays heading through a precession amplifier and drive motor.
  • Both manual and automatic rapid synchronisation systems are provided.
  • A rapid synchronisation button quickly aligns the directional gyro with the magnetic flux valve.

Magnetic Flux Valve

  • The magnetic flux valve senses the direction of magnetic north.
  • A three-spoke flux valve is usually installed beneath a wing tip or inside the tail fin.
  • This location minimises interference from electrical equipment.
  • The flux valve is suspended pendulously using a Hooke’s joint.
  • The Hooke’s joint allows the detector to remain level during aircraft pitch and roll.

Magnetic Flux Detector

Principle of the Magnetic Flux Valve

  • The magnetic flux valve operates on Faraday’s Law of Electromagnetic Induction.
  • An electromotive force (EMF) is induced whenever the magnetic flux linking a conductor changes.
  • The induced EMF is proportional to the rate of change of magnetic flux.
  • The detector consists of a central excitation coil surrounded by three sensing spokes.
  • Alternating current in the central coil creates equal magnetic fields in the surrounding spokes.
  • Without an external magnetic field, the resulting output is zero.
  • The Earth’s magnetic field changes the magnetic saturation of the spokes.
  • This produces an output voltage proportional to the direction of magnetic north.

Transmission of Magnetic Heading

  • A Selsyn (Synchro) system transmits heading information between transmitting and receiving units.
  • The transmitter rotor rotates with the heading drive shaft.
  • The rotor position represents the aircraft heading.
  • The rotor induces an equivalent magnetic field in the stator windings.
  • The receiving synchro develops currents that rotate its shaft to the same position.
  • An error signal exists whenever the transmitter and receiver are not aligned.
  • The repeater continues rotating until the error voltage becomes zero, indicating the correct heading.

Signal Processing to the Compass Dial

  • The Selsyn system produces an error signal whenever the displayed heading is incorrect.
  • The error signal is amplified, phase detected, and converted into direct current (DC).
  • The DC output drives the precession motor that rotates the compass dial.
  • The displayed heading is the directional gyro heading corrected by the magnetic flux valve.
  • The compass card is read against a fixed lubber line.
  • A heading bug allows the pilot to preset a desired heading.
  • A warning flag indicates failure of the magnetic flux detector or associated circuitry.

Control Panel of the Gyro Magnetic Compass

  • A FREE/SLAVE selector chooses between gyro-only and magnetically slaved operation.
  • Slave Mode is the normal operating mode.
  • In Slave Mode, the directional gyro is continuously corrected by the magnetic flux valve.
  • An annunciator indicates that magnetic slaving is operating.
  • Free Mode displays gyro heading without magnetic correction.
  • Free Mode is commonly used near the magnetic poles where magnetic dip is excessive.
  • CW and CCW controls are provided to manually synchronise the initial heading.

Gyro Wander

  • Gyro wander consists of gyro drift and gyro topple.
  • Drift is minimised by continuously slaving the directional gyro to the magnetic flux valve.
  • Topple is corrected using mercury levelling switches and torque motors.
  • These correction systems slowly precess the gyro back to the horizontal reference.
  • The slow precession rate prevents unnecessary corrections caused by temporary disturbances.
  • Temporary disturbances include aircraft turns and acceleration errors.
  • The Selsyn (Synchro) system also distributes heading information to instruments such as the Radio Magnetic Indicator (RMI), Horizontal Situation Indicator (HSI), and Autopilot.