An inertial measurement unit (IMU) measures angular rate with gyroscopes and specific force with accelerometers along three axes. The flight controller uses it at high rate to stabilise the vehicle. An inertial navigation system (INS) adds processing, and usually GNSS aiding, to output attitude, heading, velocity and position.
Technologies and grades
Grade
Typical technology
Gyro bias stability (order of magnitude)
Typical use
Consumer / industrial
MEMS
Tens of °/h or more
Small drones, autopilot internal IMUs
Tactical
High-end MEMS, FOG
Around 1 °/h or better
Professional UAS, gimbals, mapping
Navigation
FOG, ring laser gyro
Around 0.01 °/h
Long GNSS-denied navigation, large UAS
MEMS sensors are small, cheap and robust. Fiber-optic gyroscopes (FOG) and ring laser gyroscopes are larger and more expensive, but drift much less, which matters when GNSS is lost.
Key specifications
Gyro bias instability and angle random walk: how fast the heading and attitude estimate drifts.
Accelerometer bias stability and velocity random walk.
Measurement range: maximum angular rate and acceleration, which must cover manoeuvres and shocks.
Bandwidth and output rate.
Temperature calibration and performance across the operating range.
Vibration rejection, critical on aircraft with engines or large rotors.
For an INS: GNSS aiding (single or dual antenna for heading), RTK support, and position drift without GNSS.
Export classification, size, weight and power.
Choosing
Match the grade to how long the vehicle must navigate without GNSS and how precise the payload pointing or mapping must be. For a multirotor that always has GNSS, a good MEMS unit is enough; for long flights in contested areas, a tactical or navigation-grade INS combined with visual navigation is the safer choice.
An IMU only measures rates and accelerations. An INS processes those measurements, usually together with GNSS, to output attitude, velocity and position.
What does gyro bias instability mean?
The smallest drift of the gyroscope output that cannot be calibrated out, usually expressed in degrees per hour. The lower it is, the longer the system keeps an accurate heading and attitude on its own.
Is a fiber-optic gyro better than MEMS?
FOG units drift much less and are preferred for long GNSS-denied navigation, but they are larger, heavier and more expensive. High-end MEMS now cover many tactical applications at lower SWaP and cost.
Why does a dual-antenna GNSS help an INS?
Two antennas a known distance apart give a heading measurement that does not depend on the magnetometer or on the vehicle moving, which is useful for hovering drones and near magnetic disturbances.
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