The DGI needs to be reset at regular intervals by reference to the magnetic compass. The instrument is suffering from the following errors:

• Gimballing error – produced due to the shape of the gimbal system.
• Random wander – change of direction in space of the rotor’s spin axis due to manufacturing imperfections.
• Apparent wander due to Earth’s rotation – the gyro is initially aligned with a given meridian, but over time this meridian has moved due to Earth’s rotation, and the gyro is not correctly aligned anymore.
• Rotor speed variations – which will be sensed and corrected by streaming more air to the bucket vanes of the rotor.
• Transport wander – apparent wander due to the change of aircraft position.

Apparent wander due to Earth’s rotation can be calculated using the following formula:

Apparent wander (°/h) = 15 x sin (latitude)