What characterizes a Strapdown system?

• A. An inertial navigation system with accelerometers and gyros permanently aligned with the three axes of the aircraft.
• B. An inertial navigation system with gimbals that hold the sensing units.
• C. A GPS plus INS integrated with a platform.
• D. An inertial system that measures altitude only.

Answer: (A)

Strapdown systems mount the sensing elements directly on the aircraft, so the accelerometers and gyros are fixed to the aircraft’s own axes. There are no gimbals or stabilized platforms holding the sensors; everything moves with the airplane. Because the sensors measure motion in the body axes, the system must use onboard computations to rotate those measurements into a navigation frame and to integrate angular rates and accelerations to determine attitude, velocity, and position.

This approach explains why the description of a strapdown system is “an inertial navigation system with accelerometers and gyros permanently aligned with the three axes of the aircraft.” The other concepts describe different architectures: a gimbaled or stabilized platform, which is not strapdown; GPS–INS integration on a platform, which adds GPS but isn’t the strapdown characteristic; and an inertial system that measures altitude only, which is not accurate for an inertial navigation system.