A ring laser gyro is a rotation sensor used in aircraft inertial navigation systems. It measures how fast an aircraft is turning without relying on moving mechanical parts. Instead of spinning wheels or gimbals, it uses light traveling around a closed path.
This makes it rugged, accurate, and useful when GPS is unavailable or unreliable.
Inside the gyro, two laser beams travel in opposite directions around a triangular or square optical cavity. When the aircraft rotates, the path effectively becomes slightly longer for one beam and shorter for the other, causing a measurable frequency difference called the Sagnac effect. Electronics read this beat frequency and convert it into angular velocity.
By combining three gyros aligned along different axes, an aircraft can track roll, pitch, and yaw for solid-state inertial navigation.
Key Facts
- A ring laser gyro measures angular velocity, usually in rad/s or deg/s.
- The Sagnac effect makes counter-rotating light beams have different travel times when the ring rotates.
- Beat frequency is the difference between the two laser frequencies: fbeat = |fCW - fCCW|.
- For a ring laser gyro, the Sagnac frequency shift is proportional to rotation rate: Δf = 4AΩ/(λP).
- In Δf = 4AΩ/(λP), A is enclosed area, Ω is angular velocity, λ is laser wavelength, and P is perimeter.
- Three gyros mounted at right angles measure rotation about the aircraft roll, pitch, and yaw axes.
Vocabulary
- Ring laser gyro
- A sensor that measures rotation by comparing two laser beams traveling in opposite directions around a closed optical path.
- Sagnac effect
- The change in travel time or frequency between counter-rotating light beams caused by rotation of the optical path.
- Beat frequency
- The frequency difference produced when two light waves with slightly different frequencies are combined.
- Inertial navigation system
- A navigation system that estimates position, velocity, and orientation using motion sensors instead of external signals.
- Angular velocity
- The rate at which an object rotates, commonly measured in radians per second or degrees per second.
Common Mistakes to Avoid
- Thinking the laser beams push the aircraft, which is wrong because the gyro only senses rotation and does not create useful thrust.
- Assuming both beams always take exactly the same time, which is wrong because rotation changes their effective path times in opposite directions.
- Confusing angular velocity with linear speed, which is wrong because a gyro measures turning rate, not how fast the aircraft moves forward.
- Ignoring the cavity area and perimeter, which is wrong because the sensitivity depends on geometry through Δf = 4AΩ/(λP).
Practice Questions
- 1 A ring laser gyro has A = 0.010 m^2, P = 0.60 m, and λ = 633 nm. If Ω = 0.20 rad/s, calculate the Sagnac frequency shift Δf using Δf = 4AΩ/(λP).
- 2 A gyro reports a beat frequency of 42 Hz. If its scale factor is 210 Hz per rad/s, what angular velocity is it measuring in rad/s?
- 3 Explain why a ring laser gyro can be more reliable than a mechanical spinning gyro in an aircraft navigation system.