What distinguishes a strapdown INS from a gimbaled INS?

• A. Gimbaled INS uses rotating platforms to mechanically stabilize sensors.
• B. Gimbaled INS uses fixed sensors on the body to directly measure attitude.
• C. Strapdown uses fixed sensors on the body and processes rotations to compute the navigation state.
• D. Strapdown INS is identical to a gimbaled system.

Answer: (C)

The essential idea is how the sensors are mounted and how attitude is obtained. In a strapdown INS, the gyros and accelerometers are fixed to the vehicle body, with no moving platform. The navigation state is computed entirely from processing the body-frame measurements—integrating the body angular rates and transforming accelerations through rotation calculations to the navigation frame. In a gimbaled INS, the sensors sit on a stabilized platform that can rotate on gimbals to keep the sensors oriented in inertial space, so attitude is tied to that moving platform rather than directly to the body.

So, the description that strapdown uses fixed sensors on the body and processes rotations to compute the navigation state matches the correct distinction. The other ideas describe mechanical stabilization of a platform or claim the two are identical, which isn’t accurate.