In the INS navigation equations, velocity update relies on what?

• A. The cross product of angular rate with velocity.
• B. Gravity only.
• C. Specific force transformed to the navigation frame minus gravity.
• D. Latitude corrections only.

Answer: (C)

In an INS, the velocity update comes from the accelerometers’ reading of specific force, not raw acceleration. Specific force is what you get when you subtract gravity from the actual acceleration. To update velocity, you first rotate the measured specific force from the body frame into the navigation frame, then subtract the gravity vector expressed in the navigation frame. What remains is the net non-gravitational acceleration that changes the vehicle’s velocity, and integrating that over time yields the velocity update. Gravity is handled separately from the measured force because the accelerometers respond to gravity as part of their weight sensation, and you need to remove it to obtain the motion-driven acceleration. The other options don’t reflect this essential step: cross terms with angular rate relate to frame rotation, gravity alone doesn’t drive the velocity change, and latitude corrections aren’t the fundamental driver of the velocity update.