Strapdown

Strapdown refers to a design approach for inertial sensing and navigation in which the inertial measurement units (gyroscopes and accelerometers) are rigidly mounted to the vehicle itself, and attitude, velocity, and position estimates are computed entirely in the body frame by transforming sensor measurements into the navigation frame through mathematical rotation representations.

Because strapdown systems lack gimbaled platforms, they rely on high-rate digital processing to integrate angular rates and accelerations. Attitude is updated by integrating gyroscope data, while velocity and position are obtained by integrating accelerometer measurements, with corrections applied by estimators such as Kalman filters that fuse sensor data with external aiding signals (for example GPS or magnetometers).

Historically, inertial navigation often used mechanically stabilized gimbals to keep sensors level in inertial space. Strapdown technology emerged with advances in digital computation and compact, solid-state sensors (notably MEMS) in the late 20th century, offering reduced moving parts, lower cost, and lighter weight, at the cost of increased demands on processing power and sensor quality to maintain accuracy under dynamic motion.

Common implementations use representations such as rotation matrices (direction cosine matrices) or quaternions to avoid singularities and reduce computational error. Systems require careful initial alignment and calibration of biases, scale factors, and misalignments, and may rely on aiding sensors to constrain drift over time.

Applications span aerospace, defense, and consumer devices, including aircraft, missiles, spacecraft, unmanned vehicles, and smartphones, where compact, robust, and cost-effective inertial sensing is essential.