Ring Laser Gyroscope

A ring laser gyroscope is a sensor with no spinning parts that tells a rocket or spacecraft how fast it is turning. It uses two beams of laser light racing in opposite directions to measure rotation with remarkable precision.

Quick facts

• What it measures: angular velocity, the rate at which something rotates about one axis.
• Operating principle: the Sagnac effect (an optical effect, with no spinning mass).
• Industry standard: Honeywell’s GG1320 family of ring laser gyroscopes.
• Reference specs (Honeywell GG1320AN): bias stability about 0.0035 degrees per hour (typical); scale factor of 1,164,352 counts per revolution (1.113065 arc-seconds per count); a triangular Zerodur glass-ceramic block with roughly a 2.0 inch (5 cm) optical path per leg; about 450 grams; roughly 1.77 in (4.5 cm) tall and 3.45 in (8.8 cm) in diameter.
• Operating range: -65 degrees F to +185 degrees F; runs on +15 V and +5 V.
• Reliability: high-grade units achieve better than 0.01 degrees per hour bias uncertainty; the GG1320 fleet has logged on the order of 8 billion flight hours, with commercial-aircraft mean time between failures estimated above 400,000 hours.

What it is and how it works

Inside a sealed, triangular cavity made of zero-expansion glass, a single-frequency laser beam is split so that two beams travel around the ring in opposite directions, clockwise and counter-clockwise, bouncing off mirrors at each corner. When the gyroscope sits still, both beams complete the loop in exactly the same time.

When the device rotates about the axis perpendicular to the ring, something subtle happens: one beam’s effective path lengthens while the other’s shortens. This is the Sagnac effect. The result is a tiny frequency difference between the two beams. Combining them produces an interference pattern (called a beat pattern) whose frequency is directly proportional to how fast the gyro is turning, and whose direction reveals which way it is turning. Counting the interference fringes adds up the rotation into a precise angle.

Think of two runners on a circular track moving in opposite directions. If the track itself slowly rotates, one runner has slightly farther to go and the other slightly less. The mismatch in their finishing times reveals the track is turning, and how fast.

At very low rotation rates, a problem called lock-in causes the two beam frequencies to couple together and falsely read zero, a kind of dead band. To escape it, the cavity is mechanically vibrated, or “dithered,” at about 400 Hz. Because there is no spinning rotor, the device has essentially no friction-induced drift, starts instantly, and stays highly stable.

Why it matters

Three ring laser gyroscopes, one for each axis (X, Y, and Z), combined with three accelerometers (sensors that measure changes in speed), form an Inertial Measurement Unit, or IMU. That unit is the heart of an Inertial Navigation System, which lets a vehicle work out its own attitude (orientation), velocity, and position using only onboard sensing, with no external reference like GPS.

That self-contained ability is essential for launch vehicles and spacecraft, which must steer accurately through ascent, stage separation, orbit insertion, and entry, descent, and landing, often where GPS is unavailable, jammed, or out of range, such as deep space, the far side of the Moon, or Mars. Ring laser gyroscopes replaced bulky, drift-prone spinning-mass mechanical gyros because they have no moving parts to wear out, are compact and light, start immediately, and shrug off the shock and vibration of launch. In safety- and mission-critical roles they are often arranged redundantly for fault tolerance.

Where it is used and notable examples

• Atlas V launch vehicle: its Inertial Navigation and Control Assembly uses five single-axis Honeywell ring laser gyros plus five QA3000 accelerometers for primary guidance and flight control, with heritage across 58-plus Atlas V missions.
• Mars 2020 / Perseverance: Honeywell’s radiation-hardened Miniature Inertial Measurement Unit, which is ring laser gyro based, handled navigation during cruise and the “seven minutes of terror” entry, descent, and landing.
• Rosetta: ESA’s comet mission to 67P/Churyumov-Gerasimenko used Honeywell laser-gyro-based packages for attitude control and guidance.
• Aircraft: Honeywell ring laser gyro based inertial reference units fly on the Airbus A320, Boeing 777 and 757, and fighters such as the F-16 and F-15E.
• Missiles, ships, and the ISS: these systems serve on strategic missiles such as Trident, naval vessels and submarines, and the International Space Station.