Inertial Navigation Basic Principles: INS, IRS, and Error Behaviour

022-05-01Pablo Asensio Martínez2026-03-273 min

Inertial Navigation System (INS)

An Inertial Navigation System (INS) is a self-contained navigation system that provides continuous information on the aircraft's position, velocity, and attitude without relying on external references (like radio aids or GPS).

Fundamental Operation

The core principle of an INS is the measurement and integration of acceleration:

  1. Accelerometers measure the aircraft's acceleration.
  2. First Integration: Integrating acceleration over time gives velocity (Groundspeed).
  3. Second Integration: Integrating velocity over time gives distance travelled and position relative to a known starting point.

Inputs and Outputs

  • Initial Inputs: The system requires an accurate initial position (Latitude/Longitude) before the flight.
  • Air Data Input: The INS requires True Airspeed (TAS) input from the Air Data Computer (ADC) to calculate Wind Velocity (by solving the triangle of velocities: Groundspeed vector vs. TAS vector).
  • Outputs: Position, Track, True Heading, Groundspeed, Wind Direction/Speed, and Attitude (Pitch/Roll).

System Types

Stable Platform (INS)

In traditional INS (1st generation), accelerometers are mounted on a gyro-stabilised platform.

  • Mechanism: The platform is mounted in gimbals and isolated from aircraft manoeuvres.
  • Orientation: Torque motors keep the platform level (perpendicular to local vertical) and aligned with True North.
  • Sensors:
    • Accelerometers (2): Oriented North-South and East-West.
    • Gyroscopes: Detect platform movement to drive the torque motors and maintain alignment.

Strap-down System (IRS)

Modern aircraft use an Inertial Reference System (IRS), which is a "strap-down" system.

  • Mechanism: Sensors are rigidly fixed ("strapped down") to the aircraft's airframe. There are no gimbals.
  • Sensors:
    • 3 Accelerometers: Measure acceleration along the aircraft's body axes (Longitudinal, Lateral, Vertical).
    • 3 Ring Laser Gyros (RLG): Measure angular rates of rotation (Pitch, Roll, Yaw).
  • Operation: A computer mathematically creates a "stable platform" by processing the rate data to resolve body-axis accelerations into North/South and East/West components.
  • Advantages: Greater reliability (no moving parts/solid-state), higher accuracy, less maintenance, instant "spin-up" (laser light).

Errors

INS errors are categorized into bounded and unbounded errors.

Bounded Errors (Schuler Tuning)

  • Behavior: These errors oscillate and do not grow continuously over time. They tend to return to zero over a cycle.
  • Schuler Period: The oscillation period is 84.4 minutes.
  • Causes: Platform tilt, initial misalignment, integration errors in the first stage.

Unbounded Errors (Cumulative)

  • Behavior: These errors grow primarily with time.
  • Inertial Drift: The radial position error generally increases at a rate of:
    • 1 - 2 NM/h for older mechanical INS.
    • Significantly less for modern Laser IRS.
  • Causes:
    • Track/Position Errors: Azimuth misalignment, gyro drift (wander).
    • Distance Errors: Levelling gyro wander, second stage integrator errors.

Comparison table

Feature Stable Platform (INS) Strap-down (IRS)
Mounting Gimballed platform isolated from aircraft Fixed to airframe
Gyros Mechanical rate-integrating gyros Ring Laser Gyros (RLG) - Solid State
Alignment Mechanical leveling and torquing Mathematical calculation
Maintenance High (moving parts) Low (Solid state)
Accuracy Good Excellent