strapdownINS

Strapdown Inertial Navigation System (strapdown INS) is an inertial navigation system in which the inertial measurement unit (IMU) is rigidly attached to the host vehicle. The system estimates attitude, velocity, and position by integrating the IMU’s measurements of angular rate and specific force directly in the body frame, with transformations to a navigation frame performed using the current attitude estimate. This approach contrasts with gimballed INS, which uses a stabilized platform to hold sensors level.

Principle and operation: The IMU provides angular rates from gyroscopes and linear accelerations from accelerometers. Attitude is updated by integrating the angular rates, typically represented as quaternions or a rotation matrix. Accelerations are transformed from the body frame to the navigation frame and gravitation is subtracted to obtain specific force. The resulting velocity and position are obtained by double integrating the acceleration in the navigation frame. Because attitude errors couple into all measurements, Strapdown INS commonly uses an error-state estimator such as an extended Kalman filter to jointly estimate sensor biases, scale factors, misalignments, and navigation states.

Architecture and components: A typical strapdown INS comprises an IMU, a processor or embedded computer implementing real-time navigation algorithms, and software for attitude estimation and state propagation. Many systems are augmented with aiding sensors such as GNSS, barometers, or magnetometers to constrain drift. IMUs can range from MEMS devices in consumer equipment to high-precision fiber-optic or ring-laser gyros in aerospace and defense applications.

Performance, challenges, and applications: Strapdown INS are compact and rugged but suffer from drift and accumulation of small biases over time, requiring periodic aiding or calibration. They are widely used in aircraft, missiles, submarines, unmanned vehicles, and portable navigation devices, where their lack of moving parts offers robustness and reduced maintenance compared with gimballed systems.