February 19, 1993: F-15E Ring Laser Gyro Inertial Navigation Set Testing

  • Published
  • Air Force Flight Test Center

A three-year evaluation of the F-15E Ring Laser Gyro Inertial Navigation Set, conducted at the Air Force Flight Test Center and at Eglin Air Force Base, Florida, was completed.

The Ring Laser Gyros (RLG) can be used as the stable elements (for one degree of freedom each) in an inertial guidance system. The advantage of using a RLG is that there are no moving parts. Compared to the conventional spinning gyro, this means there is no friction, which in turn means there will be no inherent drift terms. Additionally, the entire unit is compact, lightweight and virtually indestructable, meaning it can be used in aircraft.

The basic principle of operation is that a single RLG can measure any rotation about its sensitive axis. This implies that the orientation in inertial space will be known at all times. The elements that measure actual accelerations can therefore be resolved into the appropriate directions.

Here's how a RLG can measure rotation about its sensitive axis:  The input laser beam is split into two beams that travel the same path but in opposite directions: one clockwise and the other counter-clockwise.  The beams are recombined and sent to the output detector. In the absence or rotation, the path lengths will be the same and the output will be the total constructive interference of the two beams.  If the apparatus rotates, there will be a difference (to be shown later) in the path lengths travelled by the two beams, resulting in a net phase difference and destructive interference. The net signal will vary in amplitude depending on the phase shift, therefore the resulting amplitude is a measurement of the phase shift, and consequently, the rotation rate.

A certain rate of rotation induces a small difference between the time it takes light to traverse the ring in the two directions according to the Sagnac effect. This introduces a tiny separation between the frequencies of the counter-propagating beams, a motion of the standing wave pattern within the ring, and thus a beat pattern when those two beams are interfered outside the ring. Therefore, the net shift of that interference pattern follows the rotation of the unit in the plane of the ring.  RLGs, while more accurate than mechanical gyroscopes, suffer from an effect known as "lock-in" at very slow rotation rates. When the ring laser is hardly rotating, the frequencies of the counter-propagating laser modes become almost identical. In this case, crosstalk between the counter-propagating beams can allow for injection locking so that the standing wave "gets stuck" in a preferred phase, thus locking the frequency of each beam to that of the other, rather than responding to gradual rotation.  Forced dithering can largely overcome this problem. The ring laser cavity is rotated clockwise and anti-clockwise about its axis using a mechanical spring driven at its resonance frequency. This ensures that the angular velocity of the system is usually far from the lock-in threshold. Typical rates are 400 Hz, with a peak dither velocity of the order of 1 degree per second. Dither does not fix the lock-in problem completely, as each time the direction of rotation is reversed, a short time interval exists in which the rotation rate is near zero and lock-in can briefly occur. If a pure frequency oscillation is maintained, these small lock-in intervals can accumulate. This was remedied by introducing noise to the 400 Hz vibration.

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